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.

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.

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.

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.