Day 933

Tuesday.


1507.06996
Intrinsic alignments of galaxies in the EAGLE and cosmo-OWLS simulations
Velliscig, Cacciato, Schaye, Hoekstra, ... et al

Report results for the alignments of galaxies in the EAGLE and cosmos-OWLS simulations as a function of galaxy separation and halo mass.  The combination of these hydro-cosmo sims enables to span four orders of magnitude in halo mass (10.7<log10[M200/(Msun/h)]<15) and a large range of separations (-1<log10(r/[Mpc/h]<2).  Focus on two classes of alignments: the orientations of galaxies with respect to either the directions to, or the orientations of, surrounding galaxies.  Find that the strength of the alignment is a strongly decreasing function of the distance between galaxies.  The orientation-direction alignment can remain significant up to ~100 Mpc, for galaxies hosted by the most massive haloes in the simulations.  Galaxies hosted by more massive sub haloes show stronger alignment.  At a fixed halo mass, more aspherical or prolate galaxies exhibit stronger alignments.  The spatial distribution of satellites is anisotropic and significantly aligned with the major axis of the main host halo.  The major axis of satellite galaxies, when all stars are considered, are preferentially aligned towards the center of the main host halo.  The predicted projected direction-orientation alignment, epsilon_g+(r_p), is in broad agreement with recent observations when only stars within the typical observable extent of galaxy are used to define galaxy orientations.  Find that the orientation-orientation alignment is weaker than the orientation-direction alignment on all scales.  Overall, the strength of galaxy alignments depends strongly on the subset of stars that are used to measure the orientations of galaxies and it is always weaker than the alignment of the DM haloes.  Thus, alignment models that use halo orientation as a direct proxy for galaxy orientation will overestimate the impact of intrinsic alignments on weak lensing analyses.


1507.07340
The mass distribution in an assembling super galaxy group at $z=0.37$
Smit, Schrabback, Valender, Kuijken, Gonzalez, Moustakas, Tran

Present a WL analysis of supergroup SG1120-1202, consisting of four distinct X-ray luminous groups, that will merge to form a cluster comparable in mass to Coma at z=0.  These groups lie within a projected separation of 1 to 4 Mpc and within Delta nu=550 km/s and form a unique  protocluster to study the matter distribution in a coalescing system.  Using high-resolution HST/ACS imaging, combined with an extensive spectroscopic and imaging data set, study the WL distortion of background galaxy images by the matter distribution in the supergroup.  Compare the reconstructed projected density field with the distribution of galaxies and hot X-ray emitting gas in the system and derive halo parameters for the individual density peaks.  Show that the projected mass distribution closely follows the locations of the X-ray peaks and associated brightest group galaxies.  One of the groups that lies at slightly lower redshift (z~0.35) than the other 3 groups (z~0.37) is X-ray luminous, but is barely detected in the WL signal.  The other 3 groups show a significant detection (up to 5 sigma in mass), with velocity dispersions between 355±60 and 530±50 km/s and masses between 0.8±0.4e14 and 1.6±0.5e15 Msun/h, consistent with independent measurements.  These groups are associated with peaks in the galaxy and gas density in a relatively straightforward manner.  Since the groups show no visible signs of interaction, this supports the picture that these are groups just before they merge into a cluster.