Day 642

Tuesday.

1404.6524
Beyond halo mass: galactic conformity as a smoking gun of central galaxy assembly bias
Hearin, Watson, van den Bosch

Quenched central galaxies tend to reside in a preferentially quenched large-scale environment, a phenomenon that has been dubbed galactic conformity.  This tendency persists out to scales far larger than the virial radius of the halo hosting the central.  Therefore, conformity manifestly violates the widely adopted assumption that the DM halo mass Mvir exclusively governs galaxy occupation statistics.  This paper is the first in the series studying the implications of the observed conformity signal for the galaxy-CM connection.  Show that recent measurements of conformity on scales R~1-5 Mpc imply that central galaxy quenching statistics cannot be correctly predicted with the knowledge of Mvir alone.  Also demonstrate that ejected (or 'backsplash') satellites cannot give rise to the signal. Then invoke the age matching model, which is predicated on the coevolution of galaxies and haloes.  Find that this model pro dues a strong signal, and that central galaxies are solely responsible.  Conclude that large-scale '2-halo' conformity represents a smoking gun of central galaxy assembly bias, and indicates that contemporary models of satellite quenching have systematically over-estimated the influence of post-infall processes.

1404.6527
Some like it triaxial: the universality of dark matter halo shapes and their evolution along the cosmic time
Despali, Giocoli, Tormen

Present a detailed analysis of DM halo shapes, studying how the distributions of ellipticity, prolateness and axial ratios evolve as a function of time and mass.  With this purpose in mind, analyzed the results of three cosmological simulations, running an ellipsoidal halo finder to measure triaxial halo shapes.  The simulations have different scales, mass limits and cosmological parameters, which allows to ensure a good resolution and statistics in a wide mass range, and to investigate the dependence of halo properties on the cosmological model.  Confirm the tendency of haloes to be prolate at all times, even if they become more triaxial going to higher z.  Regarding the dependence on mass, more massive haloes are also less spherical at all z, since they are the most recent forming systems and so still retain memory of their original shape at the moment of collapse.  Then propose a rescaling of the shape-mass relations, using the variable nu=delta_c/sigma to represent the mass, which absorbs the dependence on both cosmology and time, allowing to find universal relations between halo masses and shape parameters (ellipticity, prolateness and the axial ratios) which hold at any redshift.  This may be very useful to determine prior distributions of halo shapes for observational studies.


1404.6828
Galaxy and mass assembly (GAMA): the halo mass of galaxy groups from maximum-likelihood weak lensing
Han, Eke, Frnek, Mandelbaum, Norberg, Schneider, Peacock, … et al

Present maximum-likelihood WL analysis of the mass distribution in optically selected spectroscopic Galaxy Groups in GAMA, using SDSS photometric galaxies as sources.  The scaling of halo mass, M_h, with various group observables is investigated.  Main results: 1) the measured relations of halo mass with group luminosity, virial volume and central galaxy stellar mass, M*, agree very well with predictions from mock group catalogues constructed from a GALFORM SAM implemented in Millennium LCDM N-body sim, 2) the measured relations of halo mass with velocity dispersion and projected half-abundance radius show weak tension with mock predictions, hinting at problems in the mock galaxy dynamics and their small scale distribution; 3) the median Mh|M* measured from WL depends more sensitively on the dispersion in M* at fixed Mh than it does on the median M*|Mh.  Measurements suggest an intrinsic dispersion of sigma_log(M*) ~ 0.15; 4) Comparing mass estimates with those in the catalogue, find that the group catalog mass can give biased results when used to select subsets of the group sample.  Of the various new halo mass estimators that are calibrated using WL measurements, group luminosity is the best single-proxy estimator of group mass.