Day 873

Thursday.


1504.05578
On the physical origin of galactic conformity
Hearin, Behroozi, van den Bosch

Correlation between the SFRs of nearby galaxies (so-called galactic conformity) have been observed for projected separaitons up to 4 Mpc, an effect not predicted by current SAM.  Investigate correlations between the mass accretion rates (dMvir/dt) of nearby haloes as a potential physical origin for this effect.  Find that pairs of host haloes "know about" each others' assembly histories even when their present-day separation is greater than thirty times the viral radius of either halo.  These distances are far too large for direct interaction between the halos to explain the correlation in their dMvir/dt.  Instead, halo pairs at these distances reside in the same large-scale tidal environment, which regulates dMvir/dt for both haloes.  Larger haloes are less affected by external forces, which naturally gives rise to a mass dependence of the halo conformity signal.  SDSS measurements of galactic conformity exhibit a qualitatively similar dependence on stellar mass, including how the signal varies with distance.  Based on the expectation that halo accretion and galaxy SFR are correlated, predict the scale-, mass- and z-dependence of large-scale galactic conformity, finding that the signal should drop to undetectable levels by z>1.  These predictions are testable with current surveys to z~1; confirmation would establish a strong correlation between DM halo accretion rate and central galaxy SFR.


1504.05591
The splash back radius as a physical halo boundary and the growth of halo mass
More, Diemer, Kravtsov

The boundaries of CDM haloes are commonly defined to enclose a density contrast Delta relative to a reference (mean or critical) density.  Argue that a more physical boundary of haloes is the radius at which accreted matter reaches its first orbital epicenter after turnaround.  This splash back radius, Rsp, manifests itself as a sharp density drop in the halo outskirts, at a location that depends upon the mass accretion rate.  Present calibrations of Rsp and the enclosed mass, Msp, as a function of the accretion rate and alternatively peak height.  Find that Rsp varies between ~0.8-1 R200m for rapidly accreting haloes and ~1.5 R200m for slowly accreting haloes.  The extent of a halo and its associated environmental effects can thus extend well beyond the conventionally defined "viral" radius.  Show that Msp and Rsp evolve relatively strongly compared to other commonly used definitions.  In particular, Msp evolves significantly even for the smallest dwarf-sized haloes at z=0.  Also contrast Msp with the mass enclosed within 4 scale radii of the halo density profile, M<4rs, which characterizes the inner halo.  During the early stages of halo assembly, Msp and M<4rs evolve similarly, but in the late stages M<4rs stops increasing while Msp continues to grow significantly.  This illustrates that haloes at low z can have "quiet" interiors while continuing to accrete mass in their outskirts.  Discuss potential observational estimates of the splash back radius and show that it may already have been detected in galaxy clusters.


1504.05598
Cross-correlation of Planck CMB Lensing and CFHTLenS galaxy weak lensing maps
Liu, Hill

CFHT: probes LSS at z~0.9, cross correlate with WL maps using Planck 2013 and 2015 data.  Forecast a S/N of ~4.6.  Noise as expected, but signal low, with A=0.48 or 0.44 for 2013 and 2015, respectively (A=1 corresponds to fiducial Planck 2015 LCDM prediction); detection significance is moderate (~2 sigma).  WMAP9 parameter does slightly better, A-0.56/0.52.  Systematic effects: photo-z uncertainty, contamination by IA, masking of galaxy clusters have ~10% each effect to resolve the tension.  An overall multiplicative bias in CFHTLenS can also play a role.  Close with forecasts for measurement of this cross-correlation using ongoing and future WL surveys, which will definitively test the significance of the tension in the results with respect to LCDM.


1504.05672
Probing cosmology with weak lensing selected clusters I: halo approach and all-sky simulations
Shirasaki, Hamana, Yoshida

Explore a variety of statistics of clusters selected with cosmic shear measurement by utilizing both analytic models and large numerical sims.  First develop a halo model to predict the abundance and the clustering of WL selected clusters.  Observational effects such as galaxy shape noise are included in the model.  Then generate realistic mock WL catalogs to test the accuracy of the analytic model.  To this end, perform full-sky ray-tracing simulations that allow multiple realizations of a large continuous area.  Model the masked regions on the sky using the actual positions of bright stars, and generate 200 mock WL catalogs with sky coverage of ~1000 sq deg.  Utilize the large set of mock catalogs to evaluate the covariance matrices between the local and non-local statistics.  Show that the theoretical model agrees well with the ensemble average of statistics and their covariances calculated directly from the mock catalogues.  With a typical selection threshold, ignoring shape noise correction causes overestimation of the clustering of WL selected clusters with a level of about 10%, and shape noise correction boosts the cluster abundance by a factor of a few.  Calculate the cross-covariances using the halo model with accounting for the effective reduction of the survey area due to masks.  The covariance of the cosmic shear auto power spectrum is affected by the mode-coupling effect that originates from sky masking.  Model and the results can be readily used for a cosmo analysis with ongoing and future WL surveys.