Surveying the Universe
Archive for June, 2008
Galaxy Zoo: the independence of morphology and colour
Jun 3rd
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
Jun 3rd
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.
I live in York and I'm a research fellow in 
