1702.01121
Halo histories vs. Galaxy properties at z=0 II: large-scale galactic conformity
Tinker, et al
Using group catalogs from SDSS DR7, attempt to measure galactic conformity in the local universe. Measure the quenched fraction of neighbor galaxies around isolated primary galaxies, dividing the isolated sample into SF and quiescent objects. Restrict the measurements to scales >1Mpc to probe the correlations between the formation histories of distinct halos. Over the stellar mass range 1e9.7<M*/Msun<1e10.9, find minimal statistical evidence for conformity. Further compare these data to predictions of the halo age-matching model, in which the oldest galaxies are associated with the oldest haloes at fixed M*. For models with strong correlations between halo and stellar age, the conformity signal is too large to be consistent with the data. For weaker implementations of age-matching, galactic conformity is not a sensitive diagnostic of halo assembly bias, and would not produce a detectable signal in SDSS data. Reproduce the results of Kauffmann+ 2013, in which the SFR of neighbor galaxies are significantly reduced around primary galaxies when the primaries are themselves low star formers. However, find this result is mainly driven by contamination in the isolation criterion, when using the group catalog to remove the small fraction of satellite galaxies in the sample, the conformity signal largely goes away. Lastly, show that small conformity signals, i.e., 2-5% differences in the quenched fractions of neighbor galaxies, can be produced by mechanisms other than halo assembly bias. For example, if passive galaxies occupy more massive haloes than SF galaxies of the same stellar mass, a conformity signal that is consistent with recent measurements from PRIMUS (Berti+2016) can be produced.
1702.01682
Assembly bias and splash back in galaxy clusters
Busch, White
Use publicly available data for Millennium Sim to explore the implications of the recent detection of assembly bias and splash back signatures in a large sample of galaxy clusters. These were identified in the SDSS/DR8 photometric data by the redMaPPer algorithm and split into high- and low-concentration subsamples based on the projected positions of cluster members. Use simplified versions of these procedures to build cluster samples of similar size from the sim data. These match the observed samples quite well and show similar assembly bias and splash back signals. Previous theoretical work has found the logarithmic slope of halo density profiles to have a well-defined minimum whose depth decreases and whose radius increases with halo concentration. Projected profiles for the observed and simulated cluster samples show trends with concentration which are opposite to these predictions. In addition, for high-concentration clusters the minimum slope occurs at significantly smaller radius than predicted. Show that these discrepancies all reflect confusion between splash back features and features imposed on the profiles by the cluster identification and concentration estimation procedures. The strong apparent assembly bias is not reflected in the 3D distribution of matter around clusters. Rather it is a consequence of the preferential contamination of low-concentration clusters by foreground or background groups.
1702.01700
Measuring cosmic shear and birefringence using resolved radio sources
Whittaker, Battye, Brown
Develop a new method of extracting simultaneous measurements of WL shear and a local rotation of the plane of polarization using observations of resolved radio sources. The bias of the method is an assumption that the direction of the polarization is statistically linked with that of the gradient of the total intensity field. Using a number of sources spread over the sky, this method will allow constraints to be placed on cosmic shear and birefringence, and it can be applied to any resolved radio sources for which such a correlation exists. Assuming that the rotation and shear are constant across the source, use this relationship to construct a quadratic estimator and investigate its properties using simulated observations. Develop a calibration scheme using sims based on the observed images to mitigate a bias which occurs in the presence of measurement errors and an astrophysical scatter on the polarization. The method is applied directly to archival data of radio galaxies where a mean rotation signal of -2.02±0.75 degrees and an average shear compatible to zero using 30 reliable sources is measured. This level of constraint on an overall rotation is comparable with current leading constraints from CMB experiments and is expected to increase by at least an order of magnitude with future high precision radio surveys, such as this performed by the SKA. Also measure the shear and rotation 2-pt correlation functions and estimate the number of sources require to detect shear and rotation correlations in future surveys.
1702.01722
The halo boundary of galaxy clusters in the SDSS
Baxter, et al
Mass around DM halos can be divided into "infalling" material and "collapsed" material that has passed through at least one pericenter. Analytical models and sims predict a rapid drop in the halo density profile associated with the transition between these two regimes. Using data from SDSS, explore the evidence for such a feature in the density profiles of galaxy clusters and investigate the connection between this feature and a possible phase space boundary. First estimate the steepening of the outer galaxy density profile around clusters: the profiles show an abrupt steepening, providing evidence for truncation of the halo profile. Next, measure the galaxy density profile around clusters using 2 sets of galaxies selected based on color. Find evidence of an abrupt change in the galaxy colors that coincides with the location of the steepening of the density profile. Since galaxies are likely to be quenched of SF and turn red inside of clusters, this change in the galaxy color distribution can be interpreted as the transition from an infalling regime to a collapsed regime. Also measure this transition using a model comparison approach which has been used recently in studies of the "splash back" phenomenon, but find that this approach is not a robust way to quantify the significance of detecting a splashback-like feature. Finally, perform measurements using an independent cluster catalog to test for potential systematic errors associated with cluster selection. Identify several avenues for future work: improved understanding of the small-scale galaxy profile, lensing measurements, identification of proxies for the halo accretion rate, and other tests. With upcoming data from DES, KiDS and HSC surveys, can expect significant improvements n the study of halo boundaries.
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