Day 984

Thursday.


1510.01733
Detecting direct collapse black holes: making the case for CR7
Agarwal, Johnson, Zackrisson, Labbe, van den Bosch, Natarajan, Khochfar

Propose that one of the sources in the recently detected system CR7 by Sobral+2015 through spectro-photometric measurements at z=6.6 harbors a direct collapse black hole (DCBH).  Argue that the LW radiation field required for direct collapse in source A is provided by sources B and C.  By tracing the LW production history and SFR over cosmic time for the halo hosting CR7 in a LCDM universe, demonstrate that a DCBH could have formed at z~20.  The spectrum of source A is well fit by nebular emission from primordial gas around a BH with MBH~4.4e6 Msun accreting at a 40% of the Eddington rate, which strongly supports the interpretation of the data.  Combining these lines of evidence, argue that CR7 might well be the first DCBH candidate.


1510.01737
Quantifying the color-dependent stochasticity of large-scale structure
Patej, Eisenstein

Address the question of weather massive red and blue galaxies trace the same LSS at z~0.6 using the CMASS sample of galaxies from SDSS DR12.  After splitting the catalog into subsamples of red ad blue galaxies using a simple color cut, measure the clustering of both subsamples and construct the correlation coefficient, r, using two statistics.  The correlation coefficient quantifies the stochasticity between the two subsamples, which is examined over intermediate scales (20<R<100 Mpc/h).  Find that on these intermediate scales, the correlation coefficient is consistent with 1; in particular, find r>0.95 taking into account both statistics and r>0.974 using the favored statistic.


1510.01740
Large covariance matrices: smooth models from the 2-point correlation function
O'Connell, Eisenstein, Vargas, Ho, Padmanabhan

Introduce a new method for estimating the covariance matrix for the galaxy correlation function in surveys of LSS.  The method combines simple theoretical results with a realistic characterization of the survey to dramatically reduce noise in the covariance matrix.  For example, an investment of only ~1000 CPU hours can produce a model covariance matrix with noise levels that would otherwise require ~35000 mocks.  Non-Gaussian contributions to the model are calibrated against mock catalogs, after which the model covariance is found to be in impressive agreement with the mock covariance matrix.  Since calibration of this method requires fewer mocks than brute force approaches, believe that it could dramatically reduce the number of mocks required to analyse future surveys.


1510.01745
Detection of enhancement in Number densities of background galaxies due to magnification by massive galaxy clusters
Chiu, Dietrich, Mohr, Applegate, ... et al

Present a detection of the enhancement of the number densities of background galaxies induced from lensing magnification and use it to test the SZE inferred masses in a sample of 19 galaxy clusters with median redshift z~0.42 selected from the SPT-SZ survey.  Two background galaxy populations are selected for tis study through their photometric colors; they have median redshifts z_median~0.9 (low-z background) and z_median~1.8 (high-z background).  Stacking these populations, detect the magnification bias effect at 3.3 sigma and 1.3 sigma for the low- and high-z backgrounds, respectively.  Fit NFW models simultaneously to all observed magnification bias profiles to estimate the multiplicative factor eta that describes the ratio of the weak lensening mass to the mass inferred from the SZE observable-mass relation.  Further quantify systematic uncertainties in eta resulting from the photometric noise and bias, the cluster galaxy contamination and the estimations of the background properties.  The resulting eta for the combined background populations with 1 sigma uncertainties is 0.83±0.24 (stat)±0.074(sys), indicating good consistency between the lensing and the SZE-inferred masses.  Use the best-fit eta to predict the weak lensing shear profiles and compare these predictions with observations, showing agreement between the magnification and shear mass constraints.  This work demonstrate the promise of using the magnification as a complementary method to estimate cluster masses in large surveys.


1510.01961
The galaxy cluster concentration-mass scaling relation
Groener, Goldberg, Sereno

Scaling relation of clusters make clusters important cosmo probes of structure formation.  In this work, present a comprehensive study of the relation between two profile observables, c_vir and M_vir.  Collect the largest known sample of measurements from lituerature which make use of one or more or the following reconstruction methods: WL, SL, WL+SL, Caustic method (CM), LoS velocity dispersion (LOSVD), and X-ray.  Find that the c-M relation is highly variable depending upon the reconstruction technique used.  Also find concentrations derived from DM only sims (at approximately Mvir~1e14 Msun) to be inconsistent with the WL and WL+SL relations at the 1 sigma level, even after the projection of triaxial haloes is taken into account.  However, to fully determine consistency between simulations and observations, a volume-limited sample of clusters is required, as selection effects become increasingly more important in answering this.  Also find evidence for a steeper WL+SL relation as compared to WL alone, a result which could perhaps be caused by the varying shape of cluster isodensities, though most likely reflects differences in selection effects caused by these two techniques.  Lastly, compare concentration and mass measurements of individual clusters made using more than one technique, highlighting the magnitude of the potential bias which could exist in such observational samples.