1508.05388
Sussing merger trees: a proposed merger tree data format
Thomas et al
Propose a common terminology for use in describing both temporal merger trees and spatial structure trees for dark-matter haloes. Specify a unified data format in HDF5 and provide example I/O routines in C, FORTRAN and PYTHON.
1508.05393
Modeling galactic conformity with the color-halo age relation in the Illustris simulation
Bray et al
Comparisons between observational surveys and galaxy formation models find that the mass of DM haloes can largely explain galaxies' stellar mass. However, it remains uncertain whether additional environmental variables, generally referred to as assembly bias, are necessary to explain other galaxy properties. Use the Illustris Sim to investigate the role of assembly bias in producing galactic conformity by considering 18k galaxies with M*>2e9 Msun. Find a significant signal of galactic conformity: out to distances of about 10 Mpc, the mean red fraction of galaxies around redder galaxies is higher than around bluer galaxies at fixed stellar mass. DM haloes exhibit an analogous conformity signal, in which the fraction of haloes formed at earlier times (old haloes) is higher around old haloes than around younger ones at fixed halo mass. A plausible interpretation of galactic conformity can be given as a combination of the halo conformity signal with the galaxy color-halo age relation: at fixed stellar mass, particularly toward the low-mass end, Illustris' galaxy colors correlate with halo age, with the reddest galaxies (often satellites) being preferentially found in the oldest haloes. In fact, the galactic conformity effect can be explained with a simple semi-emprical model, by assigning stellar mass based on halo mass (abundance matching) and by assigning galaxy color based on halo age (age matching). Investigate other interpretations for the galactic conformity, particularly its dependence on the isolation criterion and on the central-satellite information. Regarding comparison to observations, conclude that the adopted selection/isolation criteria, projection effects, and stacking techniques can have significant impact on the measured amplitude of the conformity signal.
1508.05394
Evidence for two spatially separated UV continuum emitting regions in the Cloverleaf broad absorption line quasar
Sluse et al
Testing the standard Shakura-Sunyaev model of accretion is a challenging task because the central region of quasars where accretion takes place is unresolved with telescopes. The analysis of microlensing in gravitationally lensed quasars is one of the few techniques which can test this model, yielding to the measurement of the size and of the temperature profile of the accretion disc. Present spectroscopic observations of the gravitationally lensed broad absorption line quasar H1413+117, which reveal partial microlensing of the continuum emission that appears to originate from two separated regions, a micro lensed region corresponding the compact accretion disc, and a non-microlensed region, more extended and contributing to at least 30% of the total UV-continuum flux. Because this extended continuum is occulted by the broad absorption line clouds, it is not associated to the host galaxy, but rather to light scattered in the neighborhood of the central engine. Measure the amplitude of microlensing of the compact continuum over the rest-frame wavelength range 1000-7000AA. Following a Bayesian scheme, confront the measurements to microlensing simulations of an accreting disc with a temperature varying as T~R^{-1/nu}. Find a most likely source half-light radius of R_1/2 =0.61e16 cm (i.e., 0.002pc) at 0.18 um, and a most likely index of nu=0.4. The standard disc (nu=4/3) model is not ruled out by the data, and found within the 95% confidence interval associated to the measurements. Demonstrate that for H1413+117, the existence of an extended continuum in addition to the disc emission has only a small impact on the inferred disc parameters, and is unlikely to solve the tension between the microlensing source size and standard disc sizes as previously reported in the literature.
1508.05655
An accurate and practical method for inference of weak gravitational lensing from galaxy images
Bernstein, Armstrong, Krawiec, March
Demonstrate recovery of WL shear at parts-per-thousand accuracy using an implementation of the Bayesian Fourier Domain (BFD) method proposed by Bernstein+Armstrong (2014). The BFD formalism is rigourously correct for Nyquist-sampled, background-limited, uncrowded image of background galaxies. BFD does not assign shapes to galaxies, instead compression the pixel data D into a vector of moments M, such that there is an analytic expression for the probability P(M|g) of obtaining the observations with gravitational lensing distortion g along the LoS. Extend the BA14 formalism to include detection and selection of galaxies without inducing biases on the inferred g. Describe a practical algorithm for conducting BFD's integrations over the population of unlined source galaxies. The BFD implementation measures ~10 galaxies per second per core on current hardware, a speed that will be largely independent of the number of images taken of each target. Initial tests of this code on ~500 M simulated lensed galaxy images recover the simulated shear to a fractional accuracy of m=(2.2±0.6)e-3, substantially more accurate than has been demonstrated previously for any generally applicable method. The method is readily extended to use multiple exposures in multiple filters. Deep sky exposures generate a sufficiently accurate approximation to the noiseless, unused galaxy population distribution assumed as input to BFD. Describe the remaining challenges for applying the BFD method to current and future surveys, as well as potential further extensions, such as simultaneous measurement of magnification and shear; multi-band observations; and joint inference of photometric redshifts and lensing tomography.
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