Friday, 12 September 2008

Do Sub-mm Galaxies Really Trace The Most Massive Dark Matter Halos?

Chapman et al.

This paper presents evidence of a strong over-density of sub-mm galaxies at z=1.99 in GOODS-N. The interesting thing about it is that there is also an over-density of the more typical blue star-forming galaxies at the same redshift, but that the density contrast for the blue galaxies is much weaker than for the sub-mm galaxies.

The authors suggest that this is a cluster in the first stages of formation. The strength of the over-density of sub-mm galaxies is due to the numerous ongoing mergers, and thus it is not representative of the overall matter over-density, which would be much weaker. The fact that the masses and SFRs of the blue star-forming galaxies in the redshift spike is similar to the values for galaxies outside of the spike supports this argument, since you would expect most galaxies in a cluster to be older and more massive (even at these high redshifts).

The biggest caveat that the authors note is that there may be a large population of quiescent galaxies in this redshift spike that have not been observed spectroscopically. If this were the case, the true matter over-density would be closer to the over-density of sub-mm galaxies, and the merger argument wouldn't be necessary. The same caveat applies for blue star-forming galaxies that don't have spectroscopic redshifts. So without having a good idea of the statistical significance of these results, I would say that the main conclusion is a bit of a stretch... although not any more so than some other recent claims in the literature.

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