Anthony Smith's Research Blog
Surveying the Universe
The science of galaxy formation...
26 Aug 2008
...is the title of a provocative article by Gerry Gilmore(*) on today's astro-ph. There's a bit about the scientific method, such as:
The appropriate scientific methodology with which to address such questions is itself problematic: how does one apply what many consider the “traditional scientific method”, involving objective analysis of independent repeated experiments as a test of theory, when the Universe does not allow us to experiment, in the traditional laboratory physics sense; when we have no useful predictive theory for much of astrophysics; and when the nature of the Universe may restrict our observation to only a very small part of an unobservable larger whole? More specifically, is the observational test of prediction how science actually operates? Is that how astrophysics operates?
Good stuff. But the most cutting remarks come in his assessment of the current approach to modelling galaxy formation:
Such a long list of observations all inconsistent with apparently fundamental features of galaxy formation models suggests two approaches. In one approach, new complex physics (“feedback”) must be added, to “improve” agreement with observation. The appearances are to be saved. In another, common assumptions in the galaxy simulations could be examined further.
With the reference to the saving of appearances, the allusion is to Ptolemy's epicycles: making a misguided model seem more plausible by making it more contrived.
The specific problem Gilmore sees with cosmological simulations is the suppression of the "ultraviolet divergence", i.e., small-scale perturbations, by "numerical smoothing (‘finite resolution')": "It is unlikely that Nature does it that way." He suggests that many of the inconsistencies between galaxy formation models and observations could be a result of this poor handling of the small-scale power spectrum.
(*) Disclaimer: I will not be held responsible for any damage sustained to your eyes as a result of following links on this page.
Papers: your personal library of science
22 Jul 2008
Looking for a piece of software for your Mac that will allow you to:
- keep track of PDFs of academic papers,
- search for papers using Google Scholar, ADS, arXiv, ...,
- search your personal library in an instant,
- read papers full-screen,
- add notes to papers,
- organise the papers using collections and smart collections,
- interact with BibTeX databases and citation keys,
- and do all the above in something that looks and feels like iTunes?
Here it is: Papers by mekentosj.com.
Galaxy Zoo: the independence of morphology and colour
3 Jun 2008
Galaxies come in two types: red, elliptical galaxies that reside in high-density regions, and blue, spiral galaxies that reside in low-density regions. Right?
Actually, no.
At least, not according to this Galaxy Zoo paper, on the independence of morphology and colour (or here).
First of all, there's a sizeable population of galaxies that blatantly refuse to allow their colour to determine what shape they should be. There are red galaxies with beautiful spiral morphology and blue galaxies with plain old elliptical morphology.
Okay, but we know that red galaxies like to hang out in crowded places, and that elliptical galaxies are similarly gregarious, so clearly there's some connection between being red and being well-rounded?
Nope, wrong again!
The main reason that we see more red galaxies in dense environments is that the fraction of spiral galaxies that are red changes, and the fraction of elliptical galaxies that are blue changes. So in sparsely populated bits of the universe, most of the spiral galaxies are blue, but in densely populated regions, most of the spiral galaxies are red. It's similar for elliptical galaxies. In low-density regions, a large fraction (not quite half) of the elliptical galaxies are blue, whereas in dense environments the vast majority of elliptical galaxies are red.
So the morphology-density relation has really very little (directly) to do with the colour-density relation.
Moral: "elliptical/spiral" doesn't mean "red/blue"!
UKIDSS paper submitted
3 Jun 2008
Well, the deed has been done, and the paper has finally been submitted to MNRAS and to astro-ph. You can read it if you really want to: Luminosity and surface brightness distribution of K-band galaxies from the UKIDSS Large Area Survey. Here's a picture from the paper:
This is the K-band luminosity function: the number of galaxies per volume as a function of their luminosity, with low luminosity at the left and high luminosity at the right. It's far from perfect, but hopefully a step in the right direction. There's quite a bit of incompleteness (missing galaxies) and uncertainty (due to small numbers of galaxies and large-scale structure) at the faint end (left-hand side of the plot). But perhaps more interesting is the disagreement at the bright end (right-hand side). All of the previous results shown on the plot used 2MASS imaging, so this might explain the different results we have found. Specifically, it could be that (1) we use Petrosian magnitudes rather than Kron or total magnitudes, (2) UKIDSS photometry is better than 2MASS photometry, (3) the evolution corrections are different, (4) something else or (5) any combination of the above.
Evolution of Schechter function ... so?
4 Apr 2008
This is some work in progress: K-band luminosity function from the UKIDSS Large Area Survey (LAS, black dots), showing the number of galaxies per unit volume depending on the luminosity of the galaxies, from faint (left) to bright (right). I.e., there are lots more small galaxies than big galaxies.
I've fit several Schechter functions to the data. This is a convenient way of describing the luminosity function in terms of three numbers: the slope of the faint end (alpha), the luminosity brighter than which the number of galaxies drops off rapidly (M-star) and the number of galaxies per unit volume at M-star (phi-star). To fit the Schechter functions I've used only a portion of the data, as shown in the figure. For example, for the green curve, I've used only the black points brighter than (to the right of) absolute magnitude -21.
Now here's the point. At high redshift, it is possible to see only the brightest galaxies. So we would be able to plot only the black points towards the right-hand side of the figure. But what effect would this have on the Schechter function? Even if we assume the luminosity function does not vary with redshift, our Schechter function fits would! In fact, if we relied on the Schechter function fit to tell us how the galaxy population varied with redshift (a silly thing to do, but people do it all the time), we would infer that the high-redshift galaxy population was (1) brighter (2) more dominated by small galaxies and (3) less abundant than the low-redshift galaxy population.
(Now (1) and (3) are probably true, but we don't need the Schechter function to tell us. Not so sure about (2).)
Moral: don't rely on the Schechter function!
A galaxy being emitted by a star
28 Feb 2008
Why is the universe so crowded? This kind of thing is really messing up my data!
Makes me want to work with simulations...
pIDLy: IDL within Python
31 Jan 2008
Now Python and IDL can talk to each other (okay, Python talks to IDL and IDL does what it's told), using pIDLy (pronounce as you please). I experimented with a few other solutions available online but couldn't get them to work. So I cobbled this one together with surprisingly little trouble, thanks largely to pexpect.
IDL code miscellany
28 Jan 2008
IDLdoc 3.0 (more info here) gives my badly-written bits of IDL the deceptive appearance of being well designed, useful and user-friendly. So I've made a few available here for your enjoyment.
UKIDSS at ESO
20 Dec 2007
Just back from my first visit to Garching (near Munich). ESO, to be more specific. The reason for the visit: a three-day workshop on Science from UKIDSS.
Here's the gist of it. Lots of good results already, lots of work in progress, and a sense that UKIDSS has come of age: the needle-in-a-haystack hunters now have enough hay (they hope!) to find some record-breaking needles (the smallest, nearest or furthest known luminiferous objects in the Universe) and the (Galactic or extra-Galactic) Gallup pollers have now canvassed enough individuals (stars or galaxies) to be reasonably confident about the views of the whole population.
I'm one of the extra-Galactic Gallup pollers. Some slides from the talk I gave on the final morning are on my (small but growing!) publications page.
Next tasks:
- Investigate the problem with deblending of large galaxies
- Write paper
- Write thesis
- Get job
Filtering astro-ph with CosmoCoffee
14 Dec 2007
One of the things mentioned in Sarah Bridle's talk at YAM last week was a filter for arXiv.org provided by CosmoCoffee. I decided to sample it this week.
After creating an account on CosmoCoffee, you will need to edit the keywords in your profile to reflect your interests (well, I did!). Then click on Arxiv new filter and you're off!
Here are my settings:
- Arxives in order of interest: astro-ph
- Arxiv New search key strings: galaxy (redshift )?survey, luminosity (function|density), surface brightness, UKIDSS, UKIRT, VISTA, SDSS, Sloan, WFCAM, near infrared, stellar mass, star formation (rate|history), galax, Bayes, redshift, astro-ph, ADS, extragalactic
And here are the results:
- Monday: 54 new on astro-ph, of which 21 made it through the filter. These were not only filtered but also sorted by CosmoCoffee so the most relevant were listed first. Very useful.
- Tuesday: 76 on astro-ph; 27 on CosmoCoffee. It missed The Future of Cosmology by George Efstathiou, which was a fun read. But I can't think of any way to adjust the CosmoCoffee filter to catch papers like this, without catching loads of other cosmology papers. But in the full astro-ph listings I skimmed over the paper on globular clusters and their host galaxies, which was ranked highly by CosmoCoffee.
- Wednesday: 42 on astro-ph; 14 on CosmoCoffee, filtered and sorted just right.
- Thursday: 52 on astro-ph; 22 on CosmoCoffee. Hmm, wish I knew more about dwarf galaxies.
- Friday: 30 on astro-ph; 7 on CosmoCoffee (must be getting near Christmas). Glad I skimmed through astro-ph, as my filter settings excluded this fascinating article on the history of dark energy. Apparently Newton thought of it (or something like it) 320 years ago!
Conclusion: based on this week's experience, I'm likely to miss interesting and relevant papers if I use either astro-ph or CosmoCoffee ... so I'll use both! Start each day adagio on CosmoCoffee, accelerando poco a poco, then prestissimo through astro-ph.
UKIDSS poster
10 Dec 2007
Last Friday was the RAS Young Astronomers Meeting up in Edinburgh. I presented a poster, A census of K-band galaxies from the UKIDSS Large Area Survey, which I've just put online on my (very short!) publications page.
Nothing worth reading
9 Dec 2007
This is the first entry in the new Research Blog, reserved for posts of a tedious, technical or boring nature related to my research in astronomy. See www.anthonysmith.me.uk for (slightly) more exciting posts. None of the information in this post is of any interest.
I live in York and I 
