Friday, 9 April 2010
The physical origins of the morphology-density relation: evidence for gas stripping from the SDSS
This figure by van der Wel et al. shows how the axis ratio of quiescent galaxies from SDSS depends on the halo mass and stellar mass. The left panel shows that at lower halo masses, quiescent galaxies with stellar mass (5-10)x10^10 Msun tend to be round, but at higher halo masses quiescent galaxies show a wider range of axis ratios. Most of the quiescent galaxies in high-mass halos are satellites, and so this additional satellite population (which is not present at lower halo masses) tends to have higher ellipticities.
The authors go on to present a simple model that shows how this extra population of quiescent satellites has an ellipticity distribution that is indistinguishable from ~L* spiral galaxies. If I understood it correctly, this model has no free parameters... which makes the agreement with data pretty impressive. So the interpretation is that the satellite galaxies may have been typical field spirals, which had their star formation shut off through some environmental process that did not affect the structural properties. The natural physical explanation is the gradual stripping of gas in the satellite galaxies and their (sub-) halos.
Which all sounds fine to me. But note that the difference in ellipticities only holds over an intermediate range of stellar mass, as shown in the right panel. My first thought is that perhaps the more massive galaxies were already quiescent and already had round profiles before they were accreted. And that at lower masses, all of the quiescent galaxies are satellites that became quiescent through environmental processes which operate with the same efficiency even in lower mass halos.