Cosmology

Abstract:

A dark-matter-only Horizon Project simulation is used to investigate the environment and redshift dependences of accretion on to both haloes and subhaloes. These objects grow in the simulation via mergers and via accretion of diffuse non-halo material, and we measure the combined signal from these two modes of accretion. It is found that the halo accretion rate varies less strongly with redshift than predicted by the Extended Press-Schechter formalism and is dominated by minor merger and diffuse accretion events at z= 0, for all haloes. These latter growth mechanisms may be able to drive the radio-mode feedback hypothesised for recent galaxy-formation models, and have both the correct accretion rate and the form of cosmological evolution. The low-redshift subhalo accretors in the simulation form a mass-selected subsample safely above the mass resolution limit that reside in the outer regions of their host, with ∼70 per cent beyond their host's virial radius, where they are probably not being significantly stripped of mass. These subhaloes accrete, on average, at higher rates than haloes at low redshift and we argue that this is due to their enhanced clustering at small scales. At cluster scales, the mass accretion rate on to haloes and subhaloes at low redshift is found to be only weakly dependent on environment, and we confirm that at z∼ 2 haloes accrete independently of their environment at all scales, as reported by other authors. By comparing our results with an observational study of black hole growth, we support previous suggestions that at z > 1, dark matter haloes and their associated central black holes grew coevally, but show that by the present-day, dark matter haloes could be accreting at fractional rates that are up to a factor of 3 - 4 higher than their associated black holes. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We present the results of a gravitational lensing analysis of the bright z s = 2.957 submillimeter galaxy (SMG) HERMES found in the Herschel/SPIRE science demonstration phase data from the Herschel Multi-tiered Extragalactic Survey (HerMES) project. The high-resolution imaging available in optical and near-IR channels, along with CO emission obtained with the Plateau de Bure Interferometer, allows us to precisely estimate the intrinsic source extension and hence estimate the total lensing magnification to be μ = 10.9 ± 0.7. We measure the half-light radius R eff of the source in the rest-frame near-UV and V bands that characterize the unobscured light coming from stars and find R eff, * = [2.0 ± 0.1] kpc, in good agreement with recent studies on the SMG population. This lens model is also used to estimate the size of the gas distribution (Reff, gas = [1.1 ± 0.5] kpc) by mapping back in the source plane the CO (J = 5 → 4) transition line emission. The lens modeling yields a relatively large Einstein radius R Ein = 4.″10 ± 0″.02, corresponding to a deflector velocity dispersion of [483 ± 16] km s -1. This shows that HERMES is lensed by a galaxy group-size dark matter halo at redshift z l ∼ 0.6. The projected dark matter contribution largely dominates the mass budget within the Einstein radius with f dm(< R Ein) ∼ 80%. This fraction reduces to f dm(< R eff, G1 ≃ 4.5 kpc) ∼ 47% within the effective radius of the main deflecting galaxy of stellar mass M *, G1 = [8.5 ± 1.6] × 1011 M ⊙. At this smaller scale the dark matter fraction is consistent with results already found for massive lensing ellipticals at z ∼ 0.2 from the Sloan Lens ACS Survey. © 2011. The American Astronomical Society. All rights reserved.