Day 1007

Friday.


1511.03659
He II Proximity effect and the lifetime of quasars
Khrykin, Hennawi, McQuinn, Worseck

The lifetime of quasars is fundamental for understanding the growth of SMBHs, and is an important ingredient in models of the reionization of the IGM.  However, despite various attempts to determine quasar lifetimes, current estimates from a variety of methods are uncertain by orders of magnitude.  This work combines cosmo hydro sims and 1d radiative transfer to investigate the structure and evolution of the He II Lyalpha proximity zones around quasars at z~3-4.  Show that the time evolution in the proximity zone can be described by a simple analytical model for the approach of the He II fraction x_HeII(t) to ionization equilibrium, and use this picture to illustrate how the transmission profile depends on the quasar lifetime, quasar UV luminosity, and the ionization state of he in the ambient IGM (i.e. the average He II fraction, or equivalently the metagalactic He II ionizing background).  A significant degeneracy exists between the lifetime and the average He II fraction, however the latter can be determined from measurements of the He II Lya optical depth far from quasars, allowing the lifetime to be measured.  Advocate stacking existing He II quasar spectra at z~3, and show that the shape of this average proximity zone profile is sensitive to lifetimes as long as ~30 Myr.  At higher redshift z~4 where the He II fraction is poorly constrained, degeneracies will make it challenging to determine these parameters independently.  The analytical model for He II proximity zones should also provide a useful description of the properties of H I proximity ones around quasars at z~6-7.


1511.03662
The mass-sheet degeneracy and time-delay cosmography: analysis of the strong lens RXJ1131-1231
Birder, Amara, Rrefregier

Present extended modeling of the SL RXJ1131-1231 with archival data in two HST bands in combination with existing LoS contribution and velocity dispersion estimates.  Focus on the accuracy and reliability of the source reconstruction scale and lens model assumptions and its implication on time-delay cosmography.  Map out the mass-sheet degeneracy and especially the degeneracy pointed out by Schneider and Sluse (2013) using the source reconstruction scale.  In a second step, fold in velocity dispersion and external convergence measurements.  Then infer angular diameter distance relations for the time-delays without cosmo priors.  For a flat LCDM cosmology, these constraints lead to constraints of the Hubble constant H0 as a function of the matter density Omega_m in the form of H0=H0*/[1+0.5(Omega_m=Omega_m*]±5% with H0*=71.7±3.6 km/s/Mpc being the value for H0 at Omega_m*=0.3.  This is a significant improvement in the uncertainty of the lens modeling and is consistent with recent CMB measurements.  Describe the full cosmological information of the lens system data in an analytic form such that this information can be combined with other cosmo probes.


1511.03734
The SPLASH survey: quiescent galaxies are more strongly clustered but are not necessarily located in high-density environments
Lin, Capak, et al

Study the environments of galaxies via galaxy density and clustering up to z~2.5.  The clustering strength of quiescent galaxies exceeds that of SF galaxies, implying that quiescent galaxies are preferentially located in more massive haloes.  When using local density measurement, find a clear positive quiescent fraction -- density relation at z<1, consistent with earlier results.  However, the quiescent fraction -- density relation reverses its trend at intermediate redshifts (1<z<1.5) with marginal significance (<1.8 sigma), and is found to be scale dependent (1.6 sigma).  The lower fraction of quiescent galaxies seen in large-scale dense environments, if confirmed to be true, may be associated with the fact that the SF can be more easily sustained via cold stream accretion in 'large-scale' high-density regions, preventing galaxies from permanent quenching.  Finally at z>1.5, the quiescent fraction depends little on the local density, even though clustering shows that quiescent galaxies are in more massive halos.  Argue that at high z the typical halo size falls below 1e13 Msun, where intrinsically the local density measurements are so varied that they do not trace the halo mass.  The results thus suggest that in the high-z Universe, halo mass may be the key in quenching the SF in galaxies, rather than the conventionally measured galaxy density.