Friday, 23 April 2010

An x-ray detected cluster at z=1.62

Tanaka et al. have independently identified the cluster at z=1.62 that I blogged about recently, and have confirmed the redshift using NIR spectroscopy.  They have also detected it in the x-ray, thereby upgrading its status from "protocluster" to "cluster" (according to the common usage of those terms), and making it the most distant cluster known.

Dust in DLAs

Frank & Peroux ( has done a study of dust attenuation in damped-Lya absorbers from the SDSS DR7. Using 676 absorbers and a comparison sample of QSOs they manage to put stringent constraints on the amount of dust attenuation in the Lya absorbers and find that on average they have <0.01 in E(B-V). What is nice is that they show that if they use the same selection techniques as others have used in the past, they do recover their detections - so their conclusion is that the average DLA has very little dust but that some specially selected subsamples do show some.

Friday, 16 April 2010

Peng et al (2010, take a data-driven approach to study how star-formation is turned off in different environments/different mass scales using SDSS and zCOSMOS. They combine a number of results but one of the fundaments for their work is the plot above which shows the relative quenching efficiency as a function of local density in several mass bins (top) and as a function of mass in several local density bins (bottom) - this plot is for the SDSS.

They define quenching here to be the number of objects on the red sequence that would have been on the blue sequence in the lowest density environment - so it is all done relative to the lowest density environment.

Their conclusion is that the effect of environment and mass seem to be decoupled and can be treated separately - ie. that there is one source of quenching that depends on stellar mass and one that depends on local density but that they do not interact significantly. They also find the same for zCosmos at z~0.8 and postulate that it extends somewhat higher in redshift.

They are able to fit a range of observables with a very simple model and quantify the relative importance of what they call 'mass quenching' and 'environmental quenching' for galaxies of different mass at different redshifts.

Thursday, 15 April 2010

A z=1.82 Analog of Local Ultra-massive Elliptical Galaxies

This figure by Onodera et al. shows a velocity dispersion (red-filled circle) of a massive galaxy at z=1.82 in comparison to others (i.e. SDDS, Cappellari et al. 2009, van Dokkum et al. 2009). The velocity dispersion was measured from a 4.7 hours spectrum obtained with MOIRCS on Subaru. They find that their observations (morphology, size, and velocity dispersion) are fully consistent with those expected for passively evolving progenitors of today's giant ellipticals. I particularly like their conclusion: "It is clear that many more observations of similar galaxies are required to establish which kind of ETG is commonest at high redshift: either the compact/high-velocity dispersion objects like those found by van Dokkum et al. (2009), or the apparently normal, low- velocity dispersion objects presented in this paper."

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.