This paper claims there could be a huge population of as-yet undiscovered very faint galaxies. The idea is that they have been previously missed in existing surveys because of problems with identification techniques and survey sensitivity limits.
I know (to varying degrees) all three of the authors so I was privy to an earlier draft of this last year. I already sent the first author some detailed comments many months ago, but the only response I had was, "thanks for your thoughtful contribution". As far as I can tell, my comments haven't been included in this updated version. I'm not sure whether it's worth the effort of sending a second email or not.
There are two serious problems here I want to point out :
1) The idea that neutral hydrogen (HI) surveys have missed a huge number of dark galaxies because of poor identification procedures
This can be falsified using the author's own numbers. Firstly, they claim that, for instance, the ALFALFA HI survey identify optical galaxies as much as 2 arcminutes away from the coordinates from the HI measurements. While true, the median offset is just 23 arcseconds ! So most of the time the identifications are reliable.
Secondly, the authors state the minimum physical separation between the HI and optical coordinates should be < 13 kpc for a secure identification. I looked at the numbers on this, and for the ALFALFA survey within 50 Mpc of the Milky Way just 0.6% of their detections have larger separations. So it cannot possibly have missed a large number of dark galaxies in the nearby universe.
2) Optical surveys have missed a huge population of very faint galaxies
They even give reasons why CCDs can't detect them. It's a little agonising to read because a year or so ago this might have been interesting. The trouble is that these ultra-diffuse galaxies are a hot topic at the moment with papers coming out regularly. We know they exist, and in large numbers.
What's particularly baffling is that the first author was the PhD supervisor of my PhD supervisor (thus making him my grand-supervisor), who's done a lot of work on this himself. They work in the same building. Yet apparently the one is not aware of the other. Oh deary deary me...
So, large optically faint galaxies do exist. They are definitely a thing, with certainty. But large gas-rich optically dark galaxies do not exist, with an extremely high degree of confidence. However, smaller gas-rich dark galaxies may well exist. But that's another story, aka a 26 page paper with > 200 simulations, currently in preparation. :)
http://arxiv.org/abs/1603.02590
Thursday, 10 March 2016
Monday, 7 March 2016
Do you feel lucky ?
"As people dredge the unknown, they are engaging in a highly creative act. What an inventor “finds” is always an expression of him- or herself. Martin Chalfie, who won a Nobel Prize for his work connected with green fluorescent protein — the stuff that makes jellyfish glow green — told me that he and several other Nobel Prize winners benefited from a chain of accidents and chance encounters on the way to their revelations. Some scientists even embrace a kind of “free jazz” method, he said, improvising as they go along: “I’ve heard of people getting good results after accidentally dropping their experimental preparations on the floor, picking them up, and working on them nonetheless,” he added."
Observational science is rather different from the hypothesis->experiement->verify->refine/disprove cycle taught in schools. It makes sense to have some idea you want the data to test, but planning anything that's more than vaguely defined is (as a rule of thumb) foolish. Too often the data will just show something you didn't expect at all. Better to approach the data with as few preconceptions as possible and try and determinine what it's telling you without bias. You could try speculating about as many different scenarios as possible but it's usually a waste of effort that ends up being castles in the air. So in that sense, observational astronomy is a great example of an inherently serendipitous process.
Of course, there are exceptions. If you're building a new instrument you need to have some idea as to what it should detect - you can't go spending millions of currency units on some new-fangled telescope unless you're pretty sure it's going to detect something. But more often than not, once you've established that you'll detect something, you let the data tell you about the universe, rather than trying to test a specific hypothesis. Hypothesis testing is useful and has its place, but it is not the be-all and end-all of science.
Which is a very roundabout way of saying, "I don't like writing telescope proposals, they're silly".
EDIT : I was also going to add that with regards to fostering an environment that promotes serendipitous discoveries, I've already written about that here : http://astrorhysy.blogspot.cz/2015/11/when-worlds-collide-science-in-society.html. Relevant quote :
These similarities mean that sometimes the process of doing both science and the arts can be very similar. Both require large amounts of time to do nothing but thinking (and in the case of science at least, an awful lot of background reading). Inspiration can't be forced - you cannot make people have new ideas. You can, however, encourage them. Science and art are both sometimes highly elaborate forms of play, to explore the question, "what if I did it this way...", or better yet, "what does this button do ?" Such thinking intrinsically demands a liberal, reasonably informal atmosphere. Insisting that people are at their desks during some particular set of hours and only talk to each other during scheduled meetings makes absolutely no damn sense whatsoever.
Except that I have a nice personal counter-example. I recently came up with a way to view volumetric data in spherical polar coordinates directly in Blender with no need for Cartesian gridding... during a long and extremely boring meeting to which I was not paying any attention. So perhaps seemingly pointless meetings have their uses after all.
http://www.nytimes.com/2016/01/03/opinion/how-to-cultivate-the-art-of-serendipity.html?_r=0
Observational science is rather different from the hypothesis->experiement->verify->refine/disprove cycle taught in schools. It makes sense to have some idea you want the data to test, but planning anything that's more than vaguely defined is (as a rule of thumb) foolish. Too often the data will just show something you didn't expect at all. Better to approach the data with as few preconceptions as possible and try and determinine what it's telling you without bias. You could try speculating about as many different scenarios as possible but it's usually a waste of effort that ends up being castles in the air. So in that sense, observational astronomy is a great example of an inherently serendipitous process.
Of course, there are exceptions. If you're building a new instrument you need to have some idea as to what it should detect - you can't go spending millions of currency units on some new-fangled telescope unless you're pretty sure it's going to detect something. But more often than not, once you've established that you'll detect something, you let the data tell you about the universe, rather than trying to test a specific hypothesis. Hypothesis testing is useful and has its place, but it is not the be-all and end-all of science.
Which is a very roundabout way of saying, "I don't like writing telescope proposals, they're silly".
EDIT : I was also going to add that with regards to fostering an environment that promotes serendipitous discoveries, I've already written about that here : http://astrorhysy.blogspot.cz/2015/11/when-worlds-collide-science-in-society.html. Relevant quote :
These similarities mean that sometimes the process of doing both science and the arts can be very similar. Both require large amounts of time to do nothing but thinking (and in the case of science at least, an awful lot of background reading). Inspiration can't be forced - you cannot make people have new ideas. You can, however, encourage them. Science and art are both sometimes highly elaborate forms of play, to explore the question, "what if I did it this way...", or better yet, "what does this button do ?" Such thinking intrinsically demands a liberal, reasonably informal atmosphere. Insisting that people are at their desks during some particular set of hours and only talk to each other during scheduled meetings makes absolutely no damn sense whatsoever.
Except that I have a nice personal counter-example. I recently came up with a way to view volumetric data in spherical polar coordinates directly in Blender with no need for Cartesian gridding... during a long and extremely boring meeting to which I was not paying any attention. So perhaps seemingly pointless meetings have their uses after all.
http://www.nytimes.com/2016/01/03/opinion/how-to-cultivate-the-art-of-serendipity.html?_r=0
Thursday, 3 March 2016
The ALFALFA Sky II : The Thrilling Sequel
Got my colour issues all sorted. For those who missed the earlier posts, this shows the visible light component of 22,935 galaxies from the ALFALFA hydrogen survey - using the actual image of each galaxy, from the Sloan Digital Sky Survey. Galaxy sizes have been exaggerated by a factor of forty (not five as I earlier reported). More detail in the link below... it's a post from 2013, but I've updated it to include the new data/techniques. The previous version had a poxy 11,710 galaxies, so this one is like, waaay better.
Go straight to the full HD video here, partly in 3D if you've got any red-blue glasses hanging about :
Go straight to the full HD video here, partly in 3D if you've got any red-blue glasses hanging about :
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