Surveying the Universe
Posts tagged UKIDSS
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.
Evolution of Schechter function ... so?
Apr 4th
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!
UKIDSS at ESO
Dec 20th
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
UKIDSS poster
Dec 10th
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.
I live in York and I'm a research fellow in 
