Will No-One Rid Me Of This Turbulent Sphere ?
Most detected neutral atomic hydrogen HI is found in close association with optically bright galaxies. However, a handful of HI clouds are known which appear to be optically dark and have no nearby potential progenitor galaxies, making tidal debris an unlikely explanation. In particular, 6 clouds identified by the Arecibo Galaxy Environment Survey are interesting due to the combination of their small size, isolation, and especially their broad line widths. A recent suggestion is that these clouds exist in pressure equilibrium with the intracluster medium, with the line width arising from turbulent internal motions. Here we explore that possibility by using the FLASH code to perform a series of 3D hydro simulations. Our clouds are modelled using spherical Gaussian density profiles, which are embedded in a hot, low-density gas representing the intracluster medium. The simulations account for heating and cooling of the gas, and we explore the effects of varying the structure and strength of their internal motions. We create synthetic HI spectra, and find that none of our simulations reproduce the observed cloud parameters for longer than ~100 Myr : the clouds either collapse, disperse, or experience rapid heating which would cause ionisation and render them undetectable to HI surveys. While the turbulent motions required to explain the high line widths generate structures which appear to be inherently unstable, making this an unlikely explanation for the observed clouds, these simulations demonstrate the importance of including the intracluster medium in any model seeking to explain the existence of these objects.
Includes an acknowledgement to king Henry II via Robert Minchin.
Wednesday, 8 November 2017
Monday, 6 November 2017
Lecture 2/4 : Nothing Will Come Of Nothing
Second post from my lecture course on galaxy evolution. This one covers two major practical techniques : optical photometry (measuring brightness and other parameters) and HI spectroscopy (measuring the gas content from radio telescopes). For the full details see the complete transcript of the 90 minute lecture.
In this much shorter post I'll skip the gritty equations and just summarise the major methods, their uncertainties, and why the subjective aspect isn't as bad as you might think. This post is focused on methodology rather than galaxy evolutionary theory.
Optical photometry
There are several major parameters we can get very easily that can tell us some important information : brightness, colour, and size. There's no end to how difficult we can make these measurements, if we want to get sophisticated, so here's the most basic approach possible.
When we have an optical image of our galaxy, we can define an aperture around it in using specialised software. This is much like drawing a polygon in standard drawing programs. Unlike ordinary jpeg or png images though, we can manipulate the displayed data range in more complex ways so as to reveal fainter features. This often involves a lot of interacting with the data, trying out different ranges to see what works best. When we think we can see the faintest emission, we draw our aperture.
You can see in the example there are also some boxes with dashed outlines. These are what we have decided is background noise, while the circle with the red strikethrough is a masked region. When we get the software to make the measurement, it will sum up all the emission within the green ellipse, excluding any present within the mask. Then it will estimate the average background value using the boxes and subtract this. We keep the boxes close to the target galaxy as the background level can vary in complex ways (in this example it's about as flat as it ever gets), so we need similar values to what would be present at the location of the galaxy itself.
From this simple procedure we can directly measure the apparent brightness of the galaxy and also its angular diameter. If we know the galaxy's distance, we can easily convert these to absolute magnitude (i.e. how much energy the galaxy is emitting, or its stellar mass) and physical size.
Unlike images from a smartphone, in astronomy we save the data from different wavelengths separately. The image above uses one wavelength range, but if we want to measure the galaxy's colour we have to repeat the procedure using another wavelength range. Colour is defined as the difference of the measurements in the two wavebands (usually only slightly different from each other).
You might wonder if the subjective size of the aperture is a problem. For brightness this isn't really a big deal. As long as our data quality is high, then it doesn't really matter if we make the aperture too big : the sum of the noise beyond the edge of the galaxy will be close to zero. It's a lot more important when it comes to measuring size, but more on that shortly.
The above is an ideal case. In reality we often have to deal with bright foreground stars, clouds of dust in our own Galaxy, the difficulties of defining where interacting galaxies end, and instrumental effects that can make the noise look really weird. Some of these effects we can mitigate, but if they're too bad we have to admit defeat and discard those observations from our analysis.
We can also get the morphology of the galaxy by inspection of the images. Sometimes a classification will already be available in existing catalogues. If not, we have to decide for ourselves. This is really just a judgement call. We can check if the galaxy has specific features but there's no (good) automatic solution to this.
The photometric measurements of a single galaxy are usually incredibly boring. But if we have a large sample we can already start to do very useful science. For example, we can construct a luminosity function, which shows the distribution of galaxies of given (absolute) luminosities.
This shows three examples of a particular function (called a Schecter function, which is often used as it's a good fit to real data), each with a slightly different slope of the faint end (α). In all cases, there are clearly far more galaxies of low luminosity than there are bright galaxies. Indeed, above a certain threshold the number drops very rapidly. The slope of the linear, faint-end part is controversial. Cosmological simulations have predicted it should be a lot steeper than is observed : that is, there should be more faint galaxies than we actually detect. This boring-looking graph reveals one the biggest problems in modern cosmology !
We can also plot a colour-magnitude diagram like we saw last time. The key, take-home message from this post is that it's not so much about what data we have, it's all about what comparisons we make. Having a bunch of data, even really good data, means diddly-squat by itself. But divide the sample cleverly and trends can be revealed that can tell us what galaxies are up to.
This example shows galaxies in the Virgo cluster. LTGs are blue squares whereas ETGs are small triangles. LTGs which are strongly deficient in gas (more later) are green blobs. The paints quite a convincing picture of gas loss driving morphology evolution, where the lack of gas from a LTG prevents further star formation and eventually transforms it in an ETG, slowly moving up the colour-magnitude diagram.
The by-eye methods described are not the only ones available. It is possible to be much more objective, it's just more difficult. The aperture photometry gives us the total amount of light coming from a galaxy, but if we divide it into smaller regions we can construct a surface brightness profile. This shows how the amount of light emitted per unit area varies radially throughout the galaxy.
Different types of galaxy have different shapes of profile, so this is one way to supplement our subjective judgement on morphology with something more rigorous and quantifiable. Surface brightness profiles sound like a fantastic improvement on aperture photometry, but they have limitations besides being harder to construct. They are impractical for large galaxy samples, still involve some amount of subjective decision-making, and there are many galaxies for which they simply can't be done : those which are very small or have weird morphologies, for example. So aperture photometry still has an important role to play.
The shape of the surface brightness profiles hints at the final issue for optical data - deciding on a measurement of the galaxy's size. There are different conventions adopted, not all of which are appropriate for every galaxy. We could measure the size at some fixed surface brightness level (the isophotal radius, e.g. the Holmberg radius), or we could find some particular part of the profile (e.g. where it begins to drop rapidly), or extrapolate it further based on the best-fit function. One popular measure is the half-light radius, also known as the effective radius. This is the radius enclosing half of the galaxy's light. Because the profile can be very much steeper in the centre, the effective radius can be much smaller than the isophotal radius. For example the effective radius of the Milky Way is estimated at around 3.6 kpc, whereas the isophotal value is more like 15 kpc.
Measuring the atomic gas
Virtually all problems in extragalactic science revolve around star formation. We know that gaseous clouds can collapse under their own gravity, but almost all the details of this are controversial. The rate of collapse is affected by the chemistry of the gas, which determines how quickly it radiates away heat that would otherwise exert an outward pressure. Chemistry is affected by star formation, with massive stars converting light elements into heavier ones. And the gas itself can be in different phases : hot, ionised gas (with temperatures > 1 million Kelvin and emitting X-rays), warm neutral atomic gas (at 10,000 K) and cold molecular gas (typically, say, ~500 K).
Atomic hydrogen gas (HI, pronounced H one) is sometimes described as the reservoir of fuel for star formation, though the current thinking is that the gas has to cool to molecular densities before it can form stars. The atomic gas is probably only involved more indirectly, though it still has a role to play. But observationally, the atomic gas has a number of advantages : it's easier to measure and usually more massive and extended compared to the molecular gas.
In some cases we can map the HI directly (shown in red in the above figure), just like making an optical map. Unlike the optical images we can also directly measure the line of sight velocity of the gas, allowing us to construct a rotation curve. This shows how fast the galaxy is rotating at different points. The disagreement between the theoretical prediction, based on the mass of the visible stars and gas, and the observed shape of the curve is one of the most important discoveries in radio astronomy, and is one of the main reasons we infer the presence of dark matter.
Unfortunately we cannot usually map the structure of the gas to this level of precision : that requires the galaxy be quite close and/or we use an interferometer, a linked array of radio telescopes that's extremely powerful but also highly complex. Most HI measurements come from single-dish telescopes, which don't have great resolution but are much, much easier to use - and also hundreds of times more sensitive. What we typically get from such a telescope is a spectrum. This shows the brightness of the HI (from the whole galaxy) as a function of velocity. The shape and size of this spectrum tell us about the mass of gas and how quickly it's rotating.
Here, the central velocity (about 1,000 km/s) indicates the systemic velocity of the galaxy. We can use Hubble's Law to estimate its distance based on this. We see that emission is detected over the velocity range ~800 - 1,200 km/s, giving us an indication of the rotation speed (it would be half this line width, after we correct for the galaxy's orientation with respect to us).
This examples shows a very distinct double-horn (a.k.a. "Batman") profile, typical of spiral galaxies (deviations from this can indicate the galaxy might be interacting with something). If you look back to the rotation curve figure, you see it's mainly flat, with most of the gas moving at a single velocity relative to us. Of course, since the galaxy is rotating, about half the gas is moving away from us and half is moving towards us. Hence we get these two brighter "horns" representing the majority of gas at this single rotation velocity on different sides of the galaxy respective to us.
The total area under the curve can be measured, and that gives us the total HI flux. It's not that dissimilar to the optical measurements : we choose which part of the spectrum to measure, and the software does the integration and background subtraction for us. And once we've got the flux, we can convert it into HI mass if we know the distance.
As mentioned, the line width has to be corrected for the viewing angle to get the rotation. If we assume the galaxy is actually circular it becomes easy to calculate this - we just have to measure its major and minor axial diameter from the optical data.
Finding gas
Modern HI surveys typically map large areas of the sky, but at low resolution. Somehow we have to turn data like this :
https://www.youtube.com/watch?time_continue=1&=&v=XmpGTGkaFg4
... into a nice catalogue of galaxies. The animation shows a series of slices through a data set. Each elongated bright blob is the HI component of a galaxy.
I won't cover the extraction procedures in detail, but it boils down to someone going through frame by frame (or rather, channel by channel) and deciding where they think the HI is present. This is obviously imperfect. The observer might find something that looks real but isn't, or miss real sources. Fortunately, afterwards we can do simple follow-up observations to quickly confirm or disprove the candidate sources in a nice objective way. And this leads to two key parameters of our HI catalogue :
Completeness is defined as the fraction of real sources present that are in your catalogue. If you should somehow find all of them then your catalogue is 100% complete.
Reliability is defined as the fraction of sources in your catalogue which are real. If all your sources are real then your catalogue is 100% reliable, but it would be very unlikely to also be 100% complete.
Why reliability is easy to measure through repeat observations, completeness is much, much more difficult - but more on that next time. Knowing these parameters can be useful in everyday life : think about what someone really means if they say a procedure is 90% reliable !
Putting it all together
With all this optical and radio data, and knowing some basic statistical parameters to check, we can do quite a lot. We have stellar mas, colour, size, morphology, gas mass, and rotation speed. We can also get HI deficiency alluded to earlier. Studies on large numbers of galaxies have revealed that in the field, knowing the size and morphology of a galaxy enables a fairly accurate prediction of its HI mass. By comparing the actual mass of a galaxy with its prediction, we can work out if it has more or less gas than expected. Typically, galaxies in clusters have significantly less gas - by a factor 10 or more - than comparable field galaxies. This deficiency measurement gives us another parameter we can use to divide up our sample.
We'd also like to have the total dynamical mass of a galaxy. The rotation curves indicate how much dark matter is present, but we can also get a crude estimate of this from the line width. For this we need to know the radius of the HI, which we can't measure directly. Fortunately all those field galaxy studies have found that the HI is typically extended by around a factor of 1.7 compared to the optical size of a galaxy, so that gives us something reasonable to work with. Not perfect, but decent.
Let's finish off with two take-home messages :
In this much shorter post I'll skip the gritty equations and just summarise the major methods, their uncertainties, and why the subjective aspect isn't as bad as you might think. This post is focused on methodology rather than galaxy evolutionary theory.
Optical photometry
There are several major parameters we can get very easily that can tell us some important information : brightness, colour, and size. There's no end to how difficult we can make these measurements, if we want to get sophisticated, so here's the most basic approach possible.
When we have an optical image of our galaxy, we can define an aperture around it in using specialised software. This is much like drawing a polygon in standard drawing programs. Unlike ordinary jpeg or png images though, we can manipulate the displayed data range in more complex ways so as to reveal fainter features. This often involves a lot of interacting with the data, trying out different ranges to see what works best. When we think we can see the faintest emission, we draw our aperture.
 |
| Aperture photometry using the popular ds9 software. |
You can see in the example there are also some boxes with dashed outlines. These are what we have decided is background noise, while the circle with the red strikethrough is a masked region. When we get the software to make the measurement, it will sum up all the emission within the green ellipse, excluding any present within the mask. Then it will estimate the average background value using the boxes and subtract this. We keep the boxes close to the target galaxy as the background level can vary in complex ways (in this example it's about as flat as it ever gets), so we need similar values to what would be present at the location of the galaxy itself.
From this simple procedure we can directly measure the apparent brightness of the galaxy and also its angular diameter. If we know the galaxy's distance, we can easily convert these to absolute magnitude (i.e. how much energy the galaxy is emitting, or its stellar mass) and physical size.
Unlike images from a smartphone, in astronomy we save the data from different wavelengths separately. The image above uses one wavelength range, but if we want to measure the galaxy's colour we have to repeat the procedure using another wavelength range. Colour is defined as the difference of the measurements in the two wavebands (usually only slightly different from each other).
You might wonder if the subjective size of the aperture is a problem. For brightness this isn't really a big deal. As long as our data quality is high, then it doesn't really matter if we make the aperture too big : the sum of the noise beyond the edge of the galaxy will be close to zero. It's a lot more important when it comes to measuring size, but more on that shortly.
The above is an ideal case. In reality we often have to deal with bright foreground stars, clouds of dust in our own Galaxy, the difficulties of defining where interacting galaxies end, and instrumental effects that can make the noise look really weird. Some of these effects we can mitigate, but if they're too bad we have to admit defeat and discard those observations from our analysis.
 |
| Example of a problematic foreground star. |
 |
| Example of faint dust clouds (left) which cause galaxies to appear redder than they really are, which we can correct for, and much thicker clouds (right) which can completely hide background galaxies and prevent any measurements at all. |
 |
| The same patch of sky viewed through two filters, one of which cause horrendous fringing. |
We can also get the morphology of the galaxy by inspection of the images. Sometimes a classification will already be available in existing catalogues. If not, we have to decide for ourselves. This is really just a judgement call. We can check if the galaxy has specific features but there's no (good) automatic solution to this.
The photometric measurements of a single galaxy are usually incredibly boring. But if we have a large sample we can already start to do very useful science. For example, we can construct a luminosity function, which shows the distribution of galaxies of given (absolute) luminosities.
 |
| Distribution functions are a lot like histograms, except with many bins. |
We can also plot a colour-magnitude diagram like we saw last time. The key, take-home message from this post is that it's not so much about what data we have, it's all about what comparisons we make. Having a bunch of data, even really good data, means diddly-squat by itself. But divide the sample cleverly and trends can be revealed that can tell us what galaxies are up to.
The by-eye methods described are not the only ones available. It is possible to be much more objective, it's just more difficult. The aperture photometry gives us the total amount of light coming from a galaxy, but if we divide it into smaller regions we can construct a surface brightness profile. This shows how the amount of light emitted per unit area varies radially throughout the galaxy.
 |
| Examples of typical surface brightness profiles. |
The shape of the surface brightness profiles hints at the final issue for optical data - deciding on a measurement of the galaxy's size. There are different conventions adopted, not all of which are appropriate for every galaxy. We could measure the size at some fixed surface brightness level (the isophotal radius, e.g. the Holmberg radius), or we could find some particular part of the profile (e.g. where it begins to drop rapidly), or extrapolate it further based on the best-fit function. One popular measure is the half-light radius, also known as the effective radius. This is the radius enclosing half of the galaxy's light. Because the profile can be very much steeper in the centre, the effective radius can be much smaller than the isophotal radius. For example the effective radius of the Milky Way is estimated at around 3.6 kpc, whereas the isophotal value is more like 15 kpc.
Measuring the atomic gas
Virtually all problems in extragalactic science revolve around star formation. We know that gaseous clouds can collapse under their own gravity, but almost all the details of this are controversial. The rate of collapse is affected by the chemistry of the gas, which determines how quickly it radiates away heat that would otherwise exert an outward pressure. Chemistry is affected by star formation, with massive stars converting light elements into heavier ones. And the gas itself can be in different phases : hot, ionised gas (with temperatures > 1 million Kelvin and emitting X-rays), warm neutral atomic gas (at 10,000 K) and cold molecular gas (typically, say, ~500 K).
Atomic hydrogen gas (HI, pronounced H one) is sometimes described as the reservoir of fuel for star formation, though the current thinking is that the gas has to cool to molecular densities before it can form stars. The atomic gas is probably only involved more indirectly, though it still has a role to play. But observationally, the atomic gas has a number of advantages : it's easier to measure and usually more massive and extended compared to the molecular gas.
 |
| NGC 628 from the optical SDSS and the HI survey THINGS. |
 |
| Pretty much all galaxies show curves like this. Unless the galaxies are actually exploding, the only way to explain these flat rotation curves is the presence of a substantial amount of undetected mass. |
 |
| A classic HI spectral profile. |
This examples shows a very distinct double-horn (a.k.a. "Batman") profile, typical of spiral galaxies (deviations from this can indicate the galaxy might be interacting with something). If you look back to the rotation curve figure, you see it's mainly flat, with most of the gas moving at a single velocity relative to us. Of course, since the galaxy is rotating, about half the gas is moving away from us and half is moving towards us. Hence we get these two brighter "horns" representing the majority of gas at this single rotation velocity on different sides of the galaxy respective to us.
The total area under the curve can be measured, and that gives us the total HI flux. It's not that dissimilar to the optical measurements : we choose which part of the spectrum to measure, and the software does the integration and background subtraction for us. And once we've got the flux, we can convert it into HI mass if we know the distance.
As mentioned, the line width has to be corrected for the viewing angle to get the rotation. If we assume the galaxy is actually circular it becomes easy to calculate this - we just have to measure its major and minor axial diameter from the optical data.
Finding gas
Modern HI surveys typically map large areas of the sky, but at low resolution. Somehow we have to turn data like this :
https://www.youtube.com/watch?time_continue=1&=&v=XmpGTGkaFg4
... into a nice catalogue of galaxies. The animation shows a series of slices through a data set. Each elongated bright blob is the HI component of a galaxy.
I won't cover the extraction procedures in detail, but it boils down to someone going through frame by frame (or rather, channel by channel) and deciding where they think the HI is present. This is obviously imperfect. The observer might find something that looks real but isn't, or miss real sources. Fortunately, afterwards we can do simple follow-up observations to quickly confirm or disprove the candidate sources in a nice objective way. And this leads to two key parameters of our HI catalogue :
Completeness is defined as the fraction of real sources present that are in your catalogue. If you should somehow find all of them then your catalogue is 100% complete.
Reliability is defined as the fraction of sources in your catalogue which are real. If all your sources are real then your catalogue is 100% reliable, but it would be very unlikely to also be 100% complete.
Why reliability is easy to measure through repeat observations, completeness is much, much more difficult - but more on that next time. Knowing these parameters can be useful in everyday life : think about what someone really means if they say a procedure is 90% reliable !
Putting it all together
With all this optical and radio data, and knowing some basic statistical parameters to check, we can do quite a lot. We have stellar mas, colour, size, morphology, gas mass, and rotation speed. We can also get HI deficiency alluded to earlier. Studies on large numbers of galaxies have revealed that in the field, knowing the size and morphology of a galaxy enables a fairly accurate prediction of its HI mass. By comparing the actual mass of a galaxy with its prediction, we can work out if it has more or less gas than expected. Typically, galaxies in clusters have significantly less gas - by a factor 10 or more - than comparable field galaxies. This deficiency measurement gives us another parameter we can use to divide up our sample.
We'd also like to have the total dynamical mass of a galaxy. The rotation curves indicate how much dark matter is present, but we can also get a crude estimate of this from the line width. For this we need to know the radius of the HI, which we can't measure directly. Fortunately all those field galaxy studies have found that the HI is typically extended by around a factor of 1.7 compared to the optical size of a galaxy, so that gives us something reasonable to work with. Not perfect, but decent.
Let's finish off with two take-home messages :
- Comparisons are king. It's nice to have really good data, but far more important to have a good sample and clever ways to divide it. Sometimes we can see nice dramatic examples of galaxies undergoing change, but to work out whether this is important overall we need good statistical data.
- The procedures we use are often messy. They aren't black-and-white cases where we can ever be truly certain about our precision, and some where objective measurements are actually inferior to subjective judgements. But this subjectivity is limited : you can't really accidentally measure colours so badly that you don't see a red or blue sequence.
In the next two posts we'll look at the importance of statistics and subjective/objective measurements in a lot more detail.
Tuesday, 31 October 2017
Observing proposal accepted !
Remember back when I had fifty million things to do all at once instead of one huge, all-consuming series of PowerPoint presentations to prepare ? One of which being a proposal to the IRAM 30m telescope in Spain, which we wrote (somehow) in two days, from scratch. Well we got the time ! Even better, we got 36 hours when we only asked for 21. Best of all, the scheduling letter is written in comic sans... say what you will, it's a lovely font. :P (No idea why we got the extra time, they only sent the scheduling letter and not the comments on the proposal yet)
We're going to be observing some ultra-diffuse galaxies in the CO line to see if they have molecular gas. With that we hope to understand why the things are so damn bad at forming stars.
And now back to my lord and master, PowerPoint...
http://www.iram-institute.org/EN/30-meter-telescope.php
We're going to be observing some ultra-diffuse galaxies in the CO line to see if they have molecular gas. With that we hope to understand why the things are so damn bad at forming stars.
And now back to my lord and master, PowerPoint...
http://www.iram-institute.org/EN/30-meter-telescope.php
Monday, 30 October 2017
Lecture 1/4 : The Cosmic Community
I gave my first lecture course back in the autumn of 2017. It was a short, six hour course consisting of four 90 minute lectures. Preparing this turned out to be such an enormous amount of work that after each lecture I recorded the transcripts, more-or-less verbatim. You can find the original PowerPoint slides to the first lecture here and the transcript here. While reading the transcript is a hell of a lot faster than speaking it aloud, it's still far too long for a general audience. So here's a summary that will be closer to 9 minutes than 90, which will at least give you the gist of what was said.
This course focuses on galaxy evolution in different, nearby environments, as well as the unseen components of galaxies that don't always make the pretty pictures of press releases. I don't look much at what was going on in the early Universe when conditions were quite different and information is scarce. Even with the highly detailed observations we have of nearby galaxies, understanding the processes at work is an often controversial area.
A brief history of galaxy observations
Galaxies were first recorded in a systematic way at least as far back as the 18th century through the efforts of Charles Messier, who was determined to stop confusing these annoying fuzzy blobs with his real passion : comets. With the equipment of the time, no-one knew what they were, but speculation was already starting that they were "island universes" - separate discs of stars set apart from our own Milky Way. Others thought that they were simply gaseous nebulae within our own Galaxy.
The issue was finally settled in 1928, when Edwin Hubble measured the distance to Andromeda and showed that it was extremely distant. Hubble also began classifying the galaxies according to structure, noting that some were complex (he called these "late type") and others were structurally smooth (which he deemed "early type"; contrary to popular belief, he did not think that early types evolved into late types). Later classification schemes became more and more elaborate, eventually getting to the point of being ridiculously grandiose, but today most people adopt a simplified, linear version of Hubble's original.
He also discovered what's now known as Hubble's Law : the further away a galaxy is, the faster it's moving away from us. The Universe, it seems, is expanding. This view was challenged by Fred Hoyle and others who advocated a "Steady State" idea where the Universe was constantly being replenished, so it had looked much as it does now for all eternity. This was disproved by the discovery of the Cosmic Microwave Background, which was predicted by the Big Bang theory and all but impossible to explain in a Steady State model.
By the 1980s it became possible to map the structure of galaxies on a very large scale indeed, revealing that they were not distributed uniformly but in an intricate network of filaments and voids. It was also possible to measure their gas content, and it was found that galaxies in clusters tend to lack gas and so have reduced star formation activity compared to galaxies in other environments. Their morphology also seems to be affected, with galaxies in clusters tending to be dominated by early-types whereas those elsewhere (the "field") being generally late-types.
Towards the end of the century, problems in the general view of galaxies began to emerge. Observations from the 1970s onwards gave indirect but extremely strong evidence for the presence of enormous amounts of unseen "dark matter". This worked well in reproducing the large-scale structures in computer simulations, but failed badly on the scale of individual galaxies. Far more small satellite galaxies were predicted around galaxies as massive as the Milky Way than are actually observed. The struggle to resolve this problem continues to this day.
The nature of galaxies
Before tackling the problems extragalactic studies have raised, we should review what we know about them with confidence. Professional astronomers often measure galaxies by their brightness rather than physical sizes (for reasons we'll cover in part two), but for the sake of getting an intuitive feel it's worth seeing how they compare to each other.
Galaxies cover a huge range of sizes. Our Milky Way is somewhere in the middle of this, being pretty massive for a spiral (but by no means the largest) but small compared to many elliptical galaxies. As mentioned, morphologies are generally divided into two classes, but the above chart also shows that several key features can't be seen at optical wavelengths. While only a few galaxies possess massive plasma jets, many contain large amounts of gas (sometimes they can have more gas than stars) that's often much more extended than their stellar emission. Both jets and gas can sometimes be seen at optical wavelengths, but usually require a radio telescope to reveal their full extent.
Galaxies show two distinct sequences if you plot their colour as a function of brightness. ETGs are mainly red whereas LTGs are mainly blue. There is also a transition region between the two sequences which is not entirely unpopulated, and there are exceptions to all classes : both blue ETGs and red LTGs are known to exist, though they are rare.
Colour is a good indication of star formation activity which in turn indicates gas content. Since gas can collide with itself, a gas-rich galaxy is more prone both to star formation (where the gas density is high) and forming internal structures. The most short-lived stars are by far the brightest and short-lived. So this, at a very basic level, is why LTGs are blue and structured : their structures originate from their gas which also drives star formation. When the gas is exhausted, the short-lived blue stars all die and the remaining red stars slowly disperse, erasing the structure. The galaxy eventually, it's thought, becomes an ETG.
Galaxy environments
There are two ways to classify where galaxies live. On the large scale they tend to be found either in filaments or in voids (see figure below). On the smaller scale their local density can vary. A galaxy in a filament may be isolated, but it might also be in a member of a pair, or in a group or cluster. The isolated criteria refers only to the local density, not the larger-scale environment. A void galaxy may well be isolated, but being in a void doesn't guarantee that it will be. The void environment is much less dense than the filamentary environment, but there are still some galaxies there. A few of them have even been observed interacting with each other.
Galaxy pairs consist, unsurprisingly, of two relatively massive galaxies plus a host of much smaller attendant satellite galaxies. Groups consist of a few (or perhaps a few tens) of massive galaxies, while clusters contain hundreds or even thousands of massive galaxies. Groups, pairs and clusters are all gravitationally bound structures, whereas the much larger filaments of galaxies are not.
Galaxies in different environments experience different processes that affect them. In groups this is dominated by gravitational encounters with other galaxies. This is particularly dangerous for galaxies in groups. There, given the small mass because there are only a few galaxies present, they move relatively slowly. And that means a close encounter can be a long, drawn-out affair, giving gravity a great deal of time to act and perturb the stars and gas from their stable orbits.
While both individual galaxies and groups of galaxies may possess an envelope of hot, low density gas (usually very difficult to detect), in clusters this becomes much more important. The density of this intracluster medium can be substantially higher than in groups, and because of the high mass of a cluster, galaxies move through it at much higher velocities. This means both that tidal encounters are less important (because each interaction is brief and so doesn't do much damage) and that galaxies are more vulnerable to ram pressure stripping. Their high speed as they move through the ICM causes a pressure build-up which can be enough to completely strip their own gas content, if they have any.
In short, the two main environmental processes are tidal encounters, which dominate in small groups, and ram pressure stripping, which dominates in clusters. Galaxies also experience internal effects as the winds and supernovae of their hot young stars (if they have any) redistribute their gas or even remove it completely - how important this is depends on the mass of the galaxy. And as well as losing gas they can also acquire more, either via other galaxies (during close encounters and mergers) or directly from the intergalactic medium - this is called accretion.
We can observe many of these process directly : we can see huge "supershells" of gas expelled by star clusters, galaxies exchanging material in tidal tails, and great stripped wakes of material from ram pressure stripping. Yet the relative importance of internal versus external process is still not at all clear. It's even still disputed as to if and how galaxies can change morphology. To understand why this is such a difficult problem, in part two I'll explain some of the basic techniques of data analysis.
This course focuses on galaxy evolution in different, nearby environments, as well as the unseen components of galaxies that don't always make the pretty pictures of press releases. I don't look much at what was going on in the early Universe when conditions were quite different and information is scarce. Even with the highly detailed observations we have of nearby galaxies, understanding the processes at work is an often controversial area.
A brief history of galaxy observations
Galaxies were first recorded in a systematic way at least as far back as the 18th century through the efforts of Charles Messier, who was determined to stop confusing these annoying fuzzy blobs with his real passion : comets. With the equipment of the time, no-one knew what they were, but speculation was already starting that they were "island universes" - separate discs of stars set apart from our own Milky Way. Others thought that they were simply gaseous nebulae within our own Galaxy.
The issue was finally settled in 1928, when Edwin Hubble measured the distance to Andromeda and showed that it was extremely distant. Hubble also began classifying the galaxies according to structure, noting that some were complex (he called these "late type") and others were structurally smooth (which he deemed "early type"; contrary to popular belief, he did not think that early types evolved into late types). Later classification schemes became more and more elaborate, eventually getting to the point of being ridiculously grandiose, but today most people adopt a simplified, linear version of Hubble's original.
 |
| Early-type galaxies (ETGs, left) include spheroidals, ellipticals and lenticulars. Late-type galaxies (LTGs) include various types of spiral (middle) and irregulars (right). |
He also discovered what's now known as Hubble's Law : the further away a galaxy is, the faster it's moving away from us. The Universe, it seems, is expanding. This view was challenged by Fred Hoyle and others who advocated a "Steady State" idea where the Universe was constantly being replenished, so it had looked much as it does now for all eternity. This was disproved by the discovery of the Cosmic Microwave Background, which was predicted by the Big Bang theory and all but impossible to explain in a Steady State model.
By the 1980s it became possible to map the structure of galaxies on a very large scale indeed, revealing that they were not distributed uniformly but in an intricate network of filaments and voids. It was also possible to measure their gas content, and it was found that galaxies in clusters tend to lack gas and so have reduced star formation activity compared to galaxies in other environments. Their morphology also seems to be affected, with galaxies in clusters tending to be dominated by early-types whereas those elsewhere (the "field") being generally late-types.
Towards the end of the century, problems in the general view of galaxies began to emerge. Observations from the 1970s onwards gave indirect but extremely strong evidence for the presence of enormous amounts of unseen "dark matter". This worked well in reproducing the large-scale structures in computer simulations, but failed badly on the scale of individual galaxies. Far more small satellite galaxies were predicted around galaxies as massive as the Milky Way than are actually observed. The struggle to resolve this problem continues to this day.
The nature of galaxies
Before tackling the problems extragalactic studies have raised, we should review what we know about them with confidence. Professional astronomers often measure galaxies by their brightness rather than physical sizes (for reasons we'll cover in part two), but for the sake of getting an intuitive feel it's worth seeing how they compare to each other.
Galaxies cover a huge range of sizes. Our Milky Way is somewhere in the middle of this, being pretty massive for a spiral (but by no means the largest) but small compared to many elliptical galaxies. As mentioned, morphologies are generally divided into two classes, but the above chart also shows that several key features can't be seen at optical wavelengths. While only a few galaxies possess massive plasma jets, many contain large amounts of gas (sometimes they can have more gas than stars) that's often much more extended than their stellar emission. Both jets and gas can sometimes be seen at optical wavelengths, but usually require a radio telescope to reveal their full extent.
 |
| The Magellanic Clouds seen at optical wavelengths with radio (21 cm) emission overlaid in yellow. |
 |
| Brightness is shown on the horizontal axis with colour on the vertical (more positive means redder, more negative means bluer). Red points are ETGs, blue are LTGs, while green are LTGs with a strong bulge (usually consisting of old red stars, so these galaxies are somewhere in between ETGs and LTGs). |
Colour is a good indication of star formation activity which in turn indicates gas content. Since gas can collide with itself, a gas-rich galaxy is more prone both to star formation (where the gas density is high) and forming internal structures. The most short-lived stars are by far the brightest and short-lived. So this, at a very basic level, is why LTGs are blue and structured : their structures originate from their gas which also drives star formation. When the gas is exhausted, the short-lived blue stars all die and the remaining red stars slowly disperse, erasing the structure. The galaxy eventually, it's thought, becomes an ETG.
Galaxy environments
There are two ways to classify where galaxies live. On the large scale they tend to be found either in filaments or in voids (see figure below). On the smaller scale their local density can vary. A galaxy in a filament may be isolated, but it might also be in a member of a pair, or in a group or cluster. The isolated criteria refers only to the local density, not the larger-scale environment. A void galaxy may well be isolated, but being in a void doesn't guarantee that it will be. The void environment is much less dense than the filamentary environment, but there are still some galaxies there. A few of them have even been observed interacting with each other.
 |
| Galaxy distribution on the large scale. Red ellipses mark the position of clusters. |
Galaxy pairs consist, unsurprisingly, of two relatively massive galaxies plus a host of much smaller attendant satellite galaxies. Groups consist of a few (or perhaps a few tens) of massive galaxies, while clusters contain hundreds or even thousands of massive galaxies. Groups, pairs and clusters are all gravitationally bound structures, whereas the much larger filaments of galaxies are not.
Galaxies in different environments experience different processes that affect them. In groups this is dominated by gravitational encounters with other galaxies. This is particularly dangerous for galaxies in groups. There, given the small mass because there are only a few galaxies present, they move relatively slowly. And that means a close encounter can be a long, drawn-out affair, giving gravity a great deal of time to act and perturb the stars and gas from their stable orbits.
While both individual galaxies and groups of galaxies may possess an envelope of hot, low density gas (usually very difficult to detect), in clusters this becomes much more important. The density of this intracluster medium can be substantially higher than in groups, and because of the high mass of a cluster, galaxies move through it at much higher velocities. This means both that tidal encounters are less important (because each interaction is brief and so doesn't do much damage) and that galaxies are more vulnerable to ram pressure stripping. Their high speed as they move through the ICM causes a pressure build-up which can be enough to completely strip their own gas content, if they have any.
In short, the two main environmental processes are tidal encounters, which dominate in small groups, and ram pressure stripping, which dominates in clusters. Galaxies also experience internal effects as the winds and supernovae of their hot young stars (if they have any) redistribute their gas or even remove it completely - how important this is depends on the mass of the galaxy. And as well as losing gas they can also acquire more, either via other galaxies (during close encounters and mergers) or directly from the intergalactic medium - this is called accretion.
We can observe many of these process directly : we can see huge "supershells" of gas expelled by star clusters, galaxies exchanging material in tidal tails, and great stripped wakes of material from ram pressure stripping. Yet the relative importance of internal versus external process is still not at all clear. It's even still disputed as to if and how galaxies can change morphology. To understand why this is such a difficult problem, in part two I'll explain some of the basic techniques of data analysis.
Thursday, 26 October 2017
Dammit people, what the hell are you doing ?
OK, so following up on the earlier post, here's another claim of galaxy satellite planes that frankly borders on just plain silly. The Milky Way plane of satellites is definite. The M31 plane looks to be marginal. The other major claim is that there are two planes in the Centaurus A group, shown below (data from https://arxiv.org/abs/1503.05599).
But this one looks to be completely mad. Viewed without colour coding, it looks for all the world like an isotropic halo with small number statistics. So how do they decide which ones are members of the two planes (red and blue; green indicates no plane membership) ? Apparently just by making stuff up. They don't say anywhere in the paper how they decide on membership. It's completely bonkers.
I just don't understand how anyone could look at this data and decide, "I have discovered something worth reporting". Let alone when that person is the famous Brent Tully (I met him a couple of times, he seems nice). As with the M31 plane, there's no way I'd let this one past through peer review. Mad as clams, the lot of 'em.
Is it a bird ? Because it isn't a plane
Ahead of my fourth and final lecture, a crude but useful rendering of the so-called "Great Plane of Andromeda". With all these lectures I'm picking topics I already know about (obviously), but I also go back to the source literature to make sure I've got the details right. In this case this has resulted in a "the hell ?" moment.
Ibata et al. 2013 claim they've discovered a bunch of dwarf galaxies around Andromeda which are rotating in a narrow plane. Such planes would be a challenge to standard models, which predict that satellite galaxies should be in roughly spherical clouds. Such a plane definitely exists around the Milky Way, with certainty. Here they claim there's a similar plane around Andromeda with 99.998% confidence (more on this in a future post). Doubts had already been sown for me by a seminar last year by one of the leading experts on satellite planes, which had the unintended consequence of persuading me that the Andromeda plane is not as astonishing as is sometimes claimed (the speaker was trying to do the opposite !), but when I finally got around to looking at this in detail.... urrgh.
The problem is the way they select the galaxies in the plane is bloody weird. For the Milky Way, all the galaxies are in a plane, so there's no need for any selection effects. Here, they select galaxies which look like they'd be in a narrow band across the sky as seen from Andromeda, without initial reference to the true 3D position. So they select things based on projected position, not true position. Worse, they do so explicitly by searching for the narrowest structure the statistical algorithm can find. It's hardly surprising that they found something !!!
EDIT : It would be far more convincing if they selected by some other, physical parameter (luminosity, for example) and then demonstrated that, say, all the brightest galaxies are in a plane. As it is, what they've done is pretty much equivalent to saying, "all the galaxies in a plane are in a plane".
The gif shows first the entire satellite galaxy population (grey) around Andromeda (white), then the galaxies in the supposed plane (red) and out of the plane (blue). I defy anyone to claim they can spot a plane without this colour-coding. Yet true 3D position is what's physically meaningful, not sky position.
They also claim that this structure shows evidence of rotation from the velocities of the galaxies (not shown here). That's a bit better - I'd buy the claim that the narrowest structure around Andromeda is rotating. But if you select by rotation, there are other galaxies that should be included as well, which would make the plane an awful lot thicker. As far as I can tell, there's absolutely no physical justification for selecting these particular galaxies at all - they do so solely because they form a thin plane. The fact that you can select a thin structure from an isotropic cloud doesn't really mean that such a structure has physical significance, because you could equally select many other such structures which have very similar physical parameters.
I have yet to read the more detailed follow-up paper but if I was refereeing the Ibata paper I'd have rejected it. I'm not wholly convinced that there isn't a plane of satellites around Andromeda, but I think the evidence is far too marginal to justify these very strong anti-standard model claims that these structures are often used for.
Right, that's my re-emergence for the day, now I submerge again into the world of PowerPoint....
Saturday, 14 October 2017
Wikipedia's science articles are too hard, says person on internet
The Wikipedia article for the electroweak force consists of a two-paragraph introduction that basically just says what I said above plus some fairly intimidating technical context. The rest of the article is almost entirely gnarly math equations. I have no idea who the article exists for because I'm not sure that person actually exists: someone with enough knowledge to comprehend dense physics formulations that doesn't also already understand the electroweak interaction or that doesn't already have, like, access to a textbook about it.
Probably a person who just wants to find one equation and can't be bothered to open a textbook. Ever since the academic world discovered that books don't have a Ctrl+F function, things have gotten worse.
Is this elitism? Is it just scientists writing like scientists? Have no doubt that a great many scientists are terrible at communication, but we can also imagine a world in which Wikipedia would attract the scientists that actually are good at communication. There have to be some that aren't otherwise occupied with writing books about string theory.
Wiki's articles cover the whole two dimensional parameter space of readability versus depth : it's not that there aren't any very good science articles, it's that there are too many of the extremes. The majority tend to be either so short as to be meaningless, or so technical as to be meaningless. The latter probably accounts for pretty close to 100% of mathematics articles (e.g. https://en.wikipedia.org/wiki/Pearson_correlation_coefficient), most of which I can't understand. The former makes up a high fraction of even slightly obscure astronomy articles (e.g. https://en.wikipedia.org/wiki/Jellyfish_galaxy). If you want a very broad overview of a big topic, say, cosmology, wiki tends to be very good. But try and dig deeper and it's like tunnelling through a field landmines.
No doubt yes, there are some good science communicators that aren't using wikipedia for whatever reason. But good review articles are hard to write from scratch. Small snippets which are scarcely longer than dictionary articles are easy, and so is adding a single equation. Planning the structure of a publically-readable review, however, takes a lot more time : it requires someone who's already very familiar with the topic, preferably an "insider" to have a general sense of which papers are well-accepted and which should be left out. Even then it involves a lot of careful reading of many different technical papers. Chances are, most of those who can do this are going to write independently of the anonymous wikipedia.
And before anyone asks, I'm going to be spending most of this weekend preparing a lecture course, so there. :P
https://motherboard.vice.com/amp/en_us/article/ne7xzq/wikipedias-science-articles-are-elitist
Probably a person who just wants to find one equation and can't be bothered to open a textbook. Ever since the academic world discovered that books don't have a Ctrl+F function, things have gotten worse.
Is this elitism? Is it just scientists writing like scientists? Have no doubt that a great many scientists are terrible at communication, but we can also imagine a world in which Wikipedia would attract the scientists that actually are good at communication. There have to be some that aren't otherwise occupied with writing books about string theory.
Wiki's articles cover the whole two dimensional parameter space of readability versus depth : it's not that there aren't any very good science articles, it's that there are too many of the extremes. The majority tend to be either so short as to be meaningless, or so technical as to be meaningless. The latter probably accounts for pretty close to 100% of mathematics articles (e.g. https://en.wikipedia.org/wiki/Pearson_correlation_coefficient), most of which I can't understand. The former makes up a high fraction of even slightly obscure astronomy articles (e.g. https://en.wikipedia.org/wiki/Jellyfish_galaxy). If you want a very broad overview of a big topic, say, cosmology, wiki tends to be very good. But try and dig deeper and it's like tunnelling through a field landmines.
No doubt yes, there are some good science communicators that aren't using wikipedia for whatever reason. But good review articles are hard to write from scratch. Small snippets which are scarcely longer than dictionary articles are easy, and so is adding a single equation. Planning the structure of a publically-readable review, however, takes a lot more time : it requires someone who's already very familiar with the topic, preferably an "insider" to have a general sense of which papers are well-accepted and which should be left out. Even then it involves a lot of careful reading of many different technical papers. Chances are, most of those who can do this are going to write independently of the anonymous wikipedia.
And before anyone asks, I'm going to be spending most of this weekend preparing a lecture course, so there. :P
https://motherboard.vice.com/amp/en_us/article/ne7xzq/wikipedias-science-articles-are-elitist
Thursday, 12 October 2017
An Ionised Gas Blob
This is an interesting little paper about a possible "dark galaxy" candidate. It contains the memorable line :
""In general it is very difficult to identify dark galaxies since they are extremely faint in the optical."
Yes, well, the thing about dark galaxies is....
Anyway, this team looked at this nice little group of galaxies and found there's a strange blob of warm ionised gas near one of the elliptical galaxies without anything visible at optical wavelengths. So it seems to be a star-free cloud of gas. And the velocity dispersion of the gas is quite high, implying quite a high dark matter content. Adorably, they've even given the blob a name (Totoro), which I think is a practise everyone should adopt.
But there are a number of problems with this. First, the cloud is so near the elliptical that it could be hard to see anything behind the galaxy. Second, there's another galaxy very close to the centre of the ionised blob - not exactly coincident with it but enough to make me go "hmmm". Third, although the ionised gas mass is pretty low, they only have upper limits on the atomic gas - potentially there's enough to greatly reduce the required dark matter content. Fourth, Totoro isn't completely detached from the elliptical galaxy - it's connected directly to the centre by at least one (and perhaps two) gas streams.
Fifth, we've shown extensively that small clouds with high velocity dispersions - embedded in streams - can be produced by interactions. Getting them to become separated from the streams is very, very hard indeed, but within streams they're common features in our simulations. The team here performed their own simulations to try and simulate Totoro, but these only included stars and virtually no other details are given - they don't even say how many simulations are given, let alone how many particles they used.
Another scenario they consider is that Totoro is the result of gas ejected by an active galactic nucleus in the elliptical galaxy. That ties in well with the stream connecting the two objects, and the complicated velocity structure of Totoro. Personally I think this is the most likely explanation. Although most elliptical galaxies don't have much gas, some do - occasionally large amounts of it.
I think it would be really difficult to pin down what's going on here. The group of galaxies is a very complex environment even just considering the gravitational fields. Add an AGN into the mix and things get really tricky. There isn't really anything inconsistent with it being a dark galaxy, but I think it would be very difficult indeed to establish that the other explanations don't work.
http://adsabs.harvard.edu/abs/2017ApJ...837...32L
""In general it is very difficult to identify dark galaxies since they are extremely faint in the optical."
Yes, well, the thing about dark galaxies is....
Anyway, this team looked at this nice little group of galaxies and found there's a strange blob of warm ionised gas near one of the elliptical galaxies without anything visible at optical wavelengths. So it seems to be a star-free cloud of gas. And the velocity dispersion of the gas is quite high, implying quite a high dark matter content. Adorably, they've even given the blob a name (Totoro), which I think is a practise everyone should adopt.
But there are a number of problems with this. First, the cloud is so near the elliptical that it could be hard to see anything behind the galaxy. Second, there's another galaxy very close to the centre of the ionised blob - not exactly coincident with it but enough to make me go "hmmm". Third, although the ionised gas mass is pretty low, they only have upper limits on the atomic gas - potentially there's enough to greatly reduce the required dark matter content. Fourth, Totoro isn't completely detached from the elliptical galaxy - it's connected directly to the centre by at least one (and perhaps two) gas streams.
Fifth, we've shown extensively that small clouds with high velocity dispersions - embedded in streams - can be produced by interactions. Getting them to become separated from the streams is very, very hard indeed, but within streams they're common features in our simulations. The team here performed their own simulations to try and simulate Totoro, but these only included stars and virtually no other details are given - they don't even say how many simulations are given, let alone how many particles they used.
Another scenario they consider is that Totoro is the result of gas ejected by an active galactic nucleus in the elliptical galaxy. That ties in well with the stream connecting the two objects, and the complicated velocity structure of Totoro. Personally I think this is the most likely explanation. Although most elliptical galaxies don't have much gas, some do - occasionally large amounts of it.
I think it would be really difficult to pin down what's going on here. The group of galaxies is a very complex environment even just considering the gravitational fields. Add an AGN into the mix and things get really tricky. There isn't really anything inconsistent with it being a dark galaxy, but I think it would be very difficult indeed to establish that the other explanations don't work.
http://adsabs.harvard.edu/abs/2017ApJ...837...32L
Wednesday, 11 October 2017
Some of our baryons aren't missing
The missing baryon problem is that Big Bang theory predicts there should be several times more normal (baryonic) matter in the Universe than is actually observed. Where could they be hiding ? The two leading contenders have been in hot, very thin gas in galactic halos, or in huge filaments between galaxies. Hot gas can be detected by X-ray telescopes if it's dense enough, as in the central regions of many galaxy clusters, but it it's too thin then our telescopes won't detect it.
Numerical simulations of the large-scale structure of the Universe have thus far been limited to purely dark matter, because the physics of baryonic matter is too computationally expensive to simulate on a large scale. Fortunately since dark matter dominates the mass, adding the baryons shouldn't be able to change the results regarding the overall structure - and indeed they're very successful at reproducing the complex network of filaments and voids of galaxies (the "cosmic web") that is actually observed. But of course it means we can't really determine from the simulations where that missing gas should be found : filaments or halos, it's anyone's guess.
There have been many, many claims over the years for the direct detection of this "cosmic web" at various wavelengths. Obviously we know the web itself exits, because it's very clearly visible in the distribution of galaxies. But does it contain any of the missing baryons ? Such claims include :
https://arxiv.org/abs/astro-ph/0501335
... from as far back as 2004, who detected hydrogen linking two galaxies, but two galaxies doth not a web make, and :
https://phys.org/news/2008-05-xmm-newton-universe.html
... who found a bridge of X-ray gas between two galaxy clusters, and in particular :
https://phys.org/news/2012-11-planck-filament-hot-gas-linking.html
... who found another, fairly spectacular bridge of hot gas linking two well-separated galaxy clusters. Again though, two clusters doth not a web make.
Then there are claims for the gas being in galaxy halos, such as this one :
https://phys.org/news/2012-09-chandra-milky-halo-hot-gas.html
... which claims there's a large halo of gas around our own Milky way
and this recent claim :
https://phys.org/news/2016-02-fast-radio-discovery-universe.html
... which used measurements of a fast radio burst in a distant galaxy to determine the properties of the intervening matter.
All of these claims are very interesting, but they all have a common problem : small number statistics. Most of them but huge extrapolations from single data points (especially the fast radio burst, which only probes a tiny line of sight) and make wholly unwarranted claims based on limited data ("we have found the missing baryons" - no, you've found some baryons, not all of them, and you don't even really know if they were missing or not).
The problem with these small number statistics is that any individual bridge of gas, either between galaxies or galaxy clusters, could simply be due to tidal interactions pulling out the gas in the galaxies/clusters. And we knew about that gas anyway - it wasn't really "missing" to begin with. What would be much more compelling would be to have evidence of these gas filaments in much greater numbers - if the gas really is primordial from the cosmic web, then it should be found in filaments which are (more or less) everywhere, not just in a few prominent examples.
That seems to be what these independent teams have found. As in the case of that particularly spectacular example, they used data from the Planck telescope to measure the Sunyaev-Zel'dovich effect : the dimming of the cosmic microwave background by this hot, thin gas. But instead of looking for one or two extreme cases where it could be detected directly, they stacked a huge number (>100,000) of galaxy pairs believed to be connected by this cosmic web. Essentially they took the images of multiple galaxy pairs and re-oriented them so that the pairs of galaxies were at the same position, then they added them up. In this way any faint emission between the galaxies that's present but undetectable might reveal itself.
And it did. Convincingly. Stacking has a lot of caveats, but when it works it greatly increases sensitivity. What I particularly like was that they did tests to make sure that their claimed detection, which at 5.3sigma isn't particularly strong, isn't the result of some artifact introduced by this stacking procedure. As a comparison they tried choosing random pairs of galaxies that aren't in the same parts of the web and performing this same stacking procedure, which gave no evidence of the same detection. They also corrected for the fact that individual galaxies will have gaseous halos, but it seems that this signal really is attributable only (or largely) to the cosmic web. It's not likely to be tidal features either, because tidal features usually have tails extending in two opposite directions, which is not observed here.
There are some caveats. The most serious is this :
"the contributions from other types of galaxies, less massive galaxies and galaxies in higher redshift should be present at some level.
So I'd be a little worried that some of the gas in the filaments might be from other galaxies in the web which haven't been accounted for. Additionally, some of this gas is almost certainly from tidal and other interaction features : the problem with stacking is that it washes out all of the details. It will be interesting to see what happens using more limited samples, to see which particular types of galaxies/environments show this signal most clearly.
Of course this doesn't rule out that some of the missing baryons reside in galaxy halos too - again, stacking washes out the details. But it's a very cool result. Does this mean we can legitimately, just this once use that dastardly phrase, "mystery solved" ? I'd say : not quite, but very nearly. I'd like to see more explanations about subtracting the other galaxies present along the web, though these are likely to be too small to make a significant contribution. I think it's safe to stop worrying about the baryons being missing and start worrying instead about exactly where they are.
Rather nice press release here.
Numerical simulations of the large-scale structure of the Universe have thus far been limited to purely dark matter, because the physics of baryonic matter is too computationally expensive to simulate on a large scale. Fortunately since dark matter dominates the mass, adding the baryons shouldn't be able to change the results regarding the overall structure - and indeed they're very successful at reproducing the complex network of filaments and voids of galaxies (the "cosmic web") that is actually observed. But of course it means we can't really determine from the simulations where that missing gas should be found : filaments or halos, it's anyone's guess.
There have been many, many claims over the years for the direct detection of this "cosmic web" at various wavelengths. Obviously we know the web itself exits, because it's very clearly visible in the distribution of galaxies. But does it contain any of the missing baryons ? Such claims include :
https://arxiv.org/abs/astro-ph/0501335
... from as far back as 2004, who detected hydrogen linking two galaxies, but two galaxies doth not a web make, and :
https://phys.org/news/2008-05-xmm-newton-universe.html
... who found a bridge of X-ray gas between two galaxy clusters, and in particular :
https://phys.org/news/2012-11-planck-filament-hot-gas-linking.html
... who found another, fairly spectacular bridge of hot gas linking two well-separated galaxy clusters. Again though, two clusters doth not a web make.
Then there are claims for the gas being in galaxy halos, such as this one :
https://phys.org/news/2012-09-chandra-milky-halo-hot-gas.html
... which claims there's a large halo of gas around our own Milky way
and this recent claim :
https://phys.org/news/2016-02-fast-radio-discovery-universe.html
... which used measurements of a fast radio burst in a distant galaxy to determine the properties of the intervening matter.
All of these claims are very interesting, but they all have a common problem : small number statistics. Most of them but huge extrapolations from single data points (especially the fast radio burst, which only probes a tiny line of sight) and make wholly unwarranted claims based on limited data ("we have found the missing baryons" - no, you've found some baryons, not all of them, and you don't even really know if they were missing or not).
The problem with these small number statistics is that any individual bridge of gas, either between galaxies or galaxy clusters, could simply be due to tidal interactions pulling out the gas in the galaxies/clusters. And we knew about that gas anyway - it wasn't really "missing" to begin with. What would be much more compelling would be to have evidence of these gas filaments in much greater numbers - if the gas really is primordial from the cosmic web, then it should be found in filaments which are (more or less) everywhere, not just in a few prominent examples.
That seems to be what these independent teams have found. As in the case of that particularly spectacular example, they used data from the Planck telescope to measure the Sunyaev-Zel'dovich effect : the dimming of the cosmic microwave background by this hot, thin gas. But instead of looking for one or two extreme cases where it could be detected directly, they stacked a huge number (>100,000) of galaxy pairs believed to be connected by this cosmic web. Essentially they took the images of multiple galaxy pairs and re-oriented them so that the pairs of galaxies were at the same position, then they added them up. In this way any faint emission between the galaxies that's present but undetectable might reveal itself.
And it did. Convincingly. Stacking has a lot of caveats, but when it works it greatly increases sensitivity. What I particularly like was that they did tests to make sure that their claimed detection, which at 5.3sigma isn't particularly strong, isn't the result of some artifact introduced by this stacking procedure. As a comparison they tried choosing random pairs of galaxies that aren't in the same parts of the web and performing this same stacking procedure, which gave no evidence of the same detection. They also corrected for the fact that individual galaxies will have gaseous halos, but it seems that this signal really is attributable only (or largely) to the cosmic web. It's not likely to be tidal features either, because tidal features usually have tails extending in two opposite directions, which is not observed here.
There are some caveats. The most serious is this :
"the contributions from other types of galaxies, less massive galaxies and galaxies in higher redshift should be present at some level.
So I'd be a little worried that some of the gas in the filaments might be from other galaxies in the web which haven't been accounted for. Additionally, some of this gas is almost certainly from tidal and other interaction features : the problem with stacking is that it washes out all of the details. It will be interesting to see what happens using more limited samples, to see which particular types of galaxies/environments show this signal most clearly.
Of course this doesn't rule out that some of the missing baryons reside in galaxy halos too - again, stacking washes out the details. But it's a very cool result. Does this mean we can legitimately, just this once use that dastardly phrase, "mystery solved" ? I'd say : not quite, but very nearly. I'd like to see more explanations about subtracting the other galaxies present along the web, though these are likely to be too small to make a significant contribution. I think it's safe to stop worrying about the baryons being missing and start worrying instead about exactly where they are.
Rather nice press release here.
Sunday, 8 October 2017
Thanks, Google !
Oh, thank you Google translate for this gem :
Using numerical simulations, Dr. Taylor tried to investigate the possible mechanism of their origin and confirmed the popular notion that some clouds may be slapped by remnants from galaxies during their close intercourse.
I'm never going to look at those numerical simulations in the same way again.
Using numerical simulations, Dr. Taylor tried to investigate the possible mechanism of their origin and confirmed the popular notion that some clouds may be slapped by remnants from galaxies during their close intercourse.
I'm never going to look at those numerical simulations in the same way again.
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