Tracking down a critical halo mass for killing galaxies through the growth of the red-sequence

Figure 1 from Gilbank & Balogh (astro-ph 0801.1930)
Catption: The evolution of the Dwarf-to-Giant ratio (DGR) for various samples, taken from literature and converted to a uniform system, as described in the text. Filled circles and open diamonds denote cluster and field samples respectively from observational data, and lines show (1+z)^\beta fits. Horizontal error bars indicate the redshift range covered by the data. Naked error bars are measurements from individual clusters. Only cluster ensembles are included in the fit. The filled square shows the highest density regions in the SDSS, which agrees well with the cluster data. Asterisks and triangles show predictions from the Bower et al. (2006) semi-analytical model, connected with lines to guide the eye. The dot-dashed lines show the predicted evolution of the field DGR if isolated galaxies have a constant DGR=0 and the observed trend is due to the increasing abundance of groups (having the same DGR as clusters) above the labeled threshold mass, Mth, with cosmic time.

This plot shows that the DGR evolves with redshift, consistent with the down-sizing picture, in which the termination of star-formation progresses from the most massive to the least massive galaxies as the universe ages. Although individual cluster measurements of the DGR show considerable scatter, the ensemble averages of clusters show a clear trend. The two main results are that the DGR of field-galaxies is always lower than that of cluster galaxies at the same redshift, and the DGR of both samples evolves with redshift.
The semi-analytical model fails to reproduces these two effects. One possible explanation for this is the inclusion of "strangulation" in galaxy formation models, where all the hot gas is removed from a galaxy as soon as it enters the dark matter halo of a more massive galaxy and becomes a satellite. This process may be too effecient.