Friday, 7 March 2008

The evolution of submillimetre galaxies: two populations and a redshift cut-off

Wall, Pope, & Scott 2008, MNRAS, 383, 435

These authors find reasonably secure counterparts for 35/38 SCUBA sub-mm sources in GOODS-N. This plot shows the relationship between redshift (combination of spectroscopic and photometric redshifts) and rest-frame 850 micron luminosity (which is also illustrated by the size of the circular plotting symbols). The black symbols are those below the median luminosity, and the blue symbols are above.  The curves show the detection limits; since the noise varies strongly over the SCUBA map, a different completeness curve is shown for the location on the map of each detected galaxy.  The fact that these curves flatten at high redshift is due to the negative K-correction.

One notes an apparent lack of high-luminosity sources at z<~1.5 and of low-luminosity sources at z>~2.5.  This is interpreted as an aspect of the generic downsizing trend of star-formation.  The authors split the sample in half by luminosity, and model the evolution of the luminosity function of the two samples independently as a function of redshift.  But the qualitative conclusions remain the same, and based on the data you'd have to be pretty brave to conclude very much more than that.  I'll post a few more thoughts in the comments, but those will probably only be relevant to those who have actually taken a look at this paper.

1 comment:

Ryan Quadri said...

In this figure, it looks like all of the blue curves lie above the black curves. I assume that each of the blue curves each corresponds to a blue symbol, but this would suggest that only bright galaxies were found in noisy parts of the image, and only faint galaxies in the deeper parts... which would suggest a problem with the data. Although this isn't a problem if the deeper parts are only a small fraction of the image. But it is also weird that some of the blue circles lie below the lowest blue curve (unless there are other lower blue curves that are covered by black curves). Or am I missing something obvious?

The authors try a few different ways of modeling the luminosity function. They focus on a broken power law (although, as far as I can tell, they don't require that the power laws match up at the break...but maybe I just didn't look carefully enough). They find that the power law for the more luminous galaxies is steeper than for the fainter galaxies, and that it evolves faster with redshift. They go on to claim that this is evidence for "two distinct populations." That isn't quite how I would phrase it though. If the LF does steepen towards higher luminosities, well that just means that a power law isn't a good approximation, and that something along the lines of a Schechter function would be better. And if the luminous part does evolve differently, rather than saying that this is evidence of two distinct populations, let's just call it evidence of downsizing.