Day 889

Wednesday.


1505.04788
Revisiting the original morphology-density relation
Houghton

In light of recent findings from the kinematic morpholoty-density relation, investigate whether the same trends exist in the original morphology density relation, using the same data as Dressler.  Distribution of ellipticals in a cluster depends on the relative local density of galaxies in that cluster: equivalent rises in the elliptical fraction occur at higher local densities for clusters with higher average local densities.  This is not true for the late-type fraction, where the variation with local density within a cluster is independent of the average local density of galaxies in that cluster, and is as Dressler originally found.  Furthermore, the overall ratio of ellipticals to early-types in a cluster does not depend on the average density of galaxies in that cluster (unlike the ratio of lenticular to disk systems), and is fixed at around 30%.  In the paradigm of fast and slow rotators, show that such an elliptical fraction in the early-type population is consistent with a slow rotator fraction of 15% in the early-type population, using the statistics of the ATLAS3D survey.  Also find the scatter in the overall ratio of ellipticals to early-types is greatest for clusters with lower average densities, such that clusters with the highest elliptical fractions have the lowest average local densities.  Finally, show that average local projected density correlates well with global projected density, but the latter has difficulty in accurately characterizing the density of irregular cluster morphologies.


1505.04798
Assessing color-dependent occupation statistics inferred from galaxy group catalogues
Campbel, van den Bosch, Hearin, Padmanabhan, Berlind, Mo, Tinker, Yang

Investigate the ability of current implementations of galaxy group finders to recover color-dependent halo occupation statistics.  To test the fidelity of group catalog inferred statistics, run 3 different group finders used in the literature over a mock that includes galaxy colors in a realistic manner.  Overall, the resulting mock group catalogues are remarkably similar, and most colour-dependent statistics are recovered with reasonable accuracy.  However, it is also clear that certain systematic errors arise as a consequence of correlated errors in group membership determination, central/satellite designation, and halo mass assignment.  Introduce a new statistic, the halo transition probability (HTP), which captures the combined impact of all these errors.  As a rule of thumb, errors tend to equalize the properties of distinct galaxy populations (i.e., red vs. blue galaxies or centrals vs. satellites), and to result in inferred occupation statistics that are more accurate for red galaxies than for blue galaxies.  A statistic that is particularly poorly recovered from the group catalogues is the red fraction of central galaxies as a function of halo mass.  Group finders do a good job in recovering galactic conformity, but also have a tendency to introduce weak conformity when none is present.  Conclude that proper inference of color-dependent statistics from group catalogues is best achieved using forward modeling (i.e., running group finders over mock data), or by implementing a correction scheme based on the HTP, as long as the latter is not too strongly model-dependent.