Day 740

Tuesday.

1409.1919
Neutral hydrogen in galaxy haloes at at the peak of the cosmic star formation history
Faucher-Giguere, Hopkins, Keres, Muratov, Quataert, Murray

Gas inflows and outflows regulate SF in galaxies.  Probing these processes is one of the central motivations for spectroscopic measurements of the circum-galactic medium.  Use high-resolution cosmological zoom-in simulations from the FIRE project to make predictions for the covering fractions of neutral hydrogen around galaxies at z=2-4. These simulations resolve the interstellar medium of galaxies and explicitly implement a comprehensive set of stellar feedback mechanisms.  Simulation sample consists of 16 main halos covering the mass range Mh~2e9-8e12 Msun at z=2, including 12 haloes in the mass range Mh~1e11-1e12 Msun corresponding to LBGs.  Process simulations with a ray tracing method to compute the ionization state of the gas.  Galactic winds increase the HI covering fractions in galaxy haloes by direct ejection of cool gas from galaxies and through interactions with gas inflowing from the IGM.  Simulations predict HI covering fraction for LLSs consistent with measurements around z~2-2.5 LBGs; these covering fractions are a factor ~2 higher than previous calculations without galactic winds.  The fractions of HI absorbers arising in inflows and in outflows are on average ~50% but exhibit significant time variability.  For the most massive halos, find a factor ~3 deficit in the LLS covering fraction relative to what is measured around quasars at z~2, suggesting that the presence of a quasar may affect the properties of halo gas on ~100 kpc scales.  The predicted covering fractions peak at Mh~1e11-12 Msun, near the peak of the SF efficiency in DM haloes.  In the simulations, SF and galactic outflows are highly time dependent; HI covering fractions are also time variable but less so because they represent averages over large areas.

1409.1924
The re-distribution of matter in the cores of galaxy clusters
Laporte, White

Present cosmo N-body sims of the assembly of BCGs in rich clusters.  At z=2 populate DM sub haloes with self-gravitating stellar systems whose abundance and structure match observed high-z galaxies.  By z=0, mergers have built much larger galaxies at cluster centre.  Their DM density profiles are shallower than in corresponding DM-only simulations, but their total mass density profiles (stars + DM) are quite similar.  Differences are found only at radii where the effects of central BHs may be significant.  DM density slopes shallower than gamma=1.0 occur for r/r200<0.015, close to the half-light radii of the BCGs.  Experiments up port earlier suggestions that NFW-like profiles are an attractor for the hierarchical growth of structure in collision less systems - total mass density profiles asymptote to the solution found in DM-only simulations over the radial range where mergers produce significant mixing between stars and DM.  Simulated DM fractions are substantially higher in BCGs than in field ellpticals, reaching 80% within the HLR.  Also estimate the SMBH mergers should create BCG cores as large as rc~3kpc.  The good agreement of all these properties with recent observational studies of BCG structure suggests that dissipation processes have not played a dominant role in the assembly of the observed systems.