Day 935

Thursday.


1507.07694
Digital sky surveys from the ground: status and perspectives
Shanks

Review the status of Digital Sky Surveys.  The focus will be on extragalactic surveys with an area of more than 100 sq.deg.  The SDSS is the archetype of such imaging surveys and it is its great success that has prompted great activity in this field.  The latest surveys explore wider, fainter and higher resolution and also a longer wavelength range than SDSS.  Many of these surveys overlap particularly in the Southern Hemisphere where there are Pan-STARRS, DES and the ESO VST surveys, and the aim here is to compare their properties.  Since there is no dedicated article on the VST ATLAS in this symposium, review the properties of this particular survey.   This easily fits onto the other main focus which is the compare overlapping Southern Surveys and see how they best fit wit the available NIR imaging data.  Conclude that the Southern Hemisphere will soon overtake the North in terms of multi wavelength imaging.  However, note that the South has more limited opportunities for spectroscopic follow-up and this weakness will persist during the LSST era.  Some new perspectives are offered on this and other aspects of survey astronomy.


1507.07937
Lens galaxies in the Illustris simulation: power-law models and the bias of the Hubble constant from time-delays
Xu, Sluse, Schneider, Springel, Vogelsberger, Nelson, Hernquist

The combination of dynamical and strong GL studies of massive galaxies shows that their total density profile in the central region (i.e., up to a few half-light rides) can be described by a power law, rho(r)~r^-gamma.  Therefore, such a power-law model is employed for a large number of SL applications, including the so-called time-delay technique used to infer the Hubble constant H0.  However, since the radial scale at which strong lensing features are formed (i.e., the Einstein radius) corresponds to the transition from the dominance of baryonic matter to dark matter, there is no known reason why galaxies should follow a power law in density.  The assumption of a power law artificially breaks the mass-sheet degeneracy, a well-known invariance transformation in gravitational lensing which affects the product of Hubble constant and time delay and can therefore cause a bias in the determination of H0 from the time-delay technique.  In this paper, use the Illustris hydro sims to estimate the amplitude of this bias, and to understand how it is related to observational properties of galaxies.  Investigating a large sample of Illustris galaxies that have velocity dispersion sigma_SIE>160 km/s at redshifts below z=1, find that the bias on H0 introduced by the power-law assumption can reach 20-50%, with a scatter of 10-30 % (rms).  However, find that by selecting galaxies with an inferred power-law model slope close to isothermal, it is possible to reduce the bias on H0 to <5%, and the scatter to <10%.  This could potentially be used to form less biased statistical samples for H0 measurements in the upcoming large survey era.


1507.08031
Clustering and bias measurements of SDSS voids
Clampitt, Jain, Sánchez

Using a void catalog from the SDSS survey, present the first measurements of void clustering and the corresponding void bias.  Over the range 30-200 Mpc/h the void auto-correlation is detected at 5-sigma significance for voids of radius 15-20 Mpc/h.  Also measure the void-galaxy cross-correlation at higher S/N and compare the inferred void bias with the autocorrelation results.  Void bias is constant with scale for voids of a given size, but its value falls from 5.6±1.0 to below zero as the void radius increases from 15-30 Mpc/h.  The comparison of the measurements with carefully matched galaxy mock catalogs, with no free parameters related to the voids, shows that model predictions can be reliably made for void correlations.  Study the dependence of void bias on tracer density and void size with a few to future applications.  In combination with the previous lensing measurements of void mass profiles, these clustering measurements provide another step towards using voids as cosmo tracers.