Day 1046

Tuesday.


1602.00002
The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters
van de Burg, Muezzin, Hoekstra

Recent observations have highlighted a significant population of faint but large (r_eff>1.5 kpc) galaxies in the Coma cluster.  The origin of these UDGs remains puzzling,as the interpretation of these observational results has been hindered by the subjective selection of UDGs, and the limited study of only the Coma (and some examples in the Virgo) Cluster.  In this paper, extend the study of UDGs using 8 clusters in 0.044<z<0.063 with deep g- and r-band imaging data taken with MegaCam at the CFHT.  Describe an automatic selection pipeline for a quantitative identification, tested for completeness using image simulations of these galaxies.  Find that the abundance of the UDGs increases with cluster mass, reaching ~200 in typical haloes of M200~1e15 Msun.  The cluster UDGs have colors consistent with the cluster red sequence, and have a steep size distribution that declines as n~r_eff^-3.4.  Their radial distribution is significantly steeper than NFW in the outskirts, and significantly shallower in the inner parts.  They follow the same radial distribution as the more massive quiescent galaxies in the clusters, except within the core region of r<0.15 x R200 (or <300 kpc).  Within this region the number density of UDGs drops and is consistent with zero.  These diffuse galaxies can only resist tidal forces down to this cluster-centric distance if they are completely DM dominated.  Moreover, this picture is consistent with the observation that the radial distribution of more compact dwarf galaxies (r_eff<1.0 kpc) with similar luminosities follows the same distribution as the UDGs, but they exist down to a smaller distance of 100 kpc from the cluster centre.  Although a number of scenarios can give rise to the UDG population, the results point to difference in the formation history as the most plausible explanation.


1602.00611
Cosmology with velocity diversion counts: an alternative to measuring cluster halo masses
Caldwell, McCarthy, Baldry, Collins, Schaye, Bird

[...]  Propose an alternative strategy: directly compare predicted and observed cluster counts as a function of the 1-d velocity dispersion of the cluster galaxies.  Argue that the velocity dispersion of groups/clusters can be theoretically predicted as robustly as mass but, unlike mass, it can also be directly observed, thus circumventing the main systematic bias in traditional cluster counts studies.  With the aid of the BAHAMAS suite of cosmo hydro sims, demonstrate the potential of the velocity diversion counts for discriminating been similar lambda CDM models.  These predictions can be compared with the results from z surveys such as the highly complete GAMA survey.