Day 686

Thursday.

1406.6361
The Lyman-$\alpha$ forest in optically-thin hydrodynamical simulations
Lukic, Stark, Nugent, White, Meiksin, Almgren

Study the statistics of the Lya forest in flat LCDM cosmology with N-body+Eulerian hydrodynamics code Nyx.  Produce a suite of simulations, covering the observationally relevant redshift range 2<z<4.  Find that a grid resolution of 20 kpc/h is required to produce 1% convergence of Lya flux statistics, up to k=10 h/Mpc.  In addition to establishing resolution requirements, study the effects of missing modes in these simulations, and find that box sizes of L>40 Mpc/h are needed to suppress numerical errors to a sub-percent level.  This optically-thin simulations with the ionizing background prescription of Haardt&Madau(2012) reproduce an IGM equation of state with T0~1e4K and gamma~1.55 at z=2, with a mean transmitted flux close to the observed values.  When using the ionizing background prescription of Faucher-Giguere+(2009), the mean flux is 10-15% below observed values at z=2, and a factor of 2 too small at z=4.  Show the effects of the common practice of rescaling optical depths to the observed mean flux and how it affects convergence rates.  Also investigate the common practice of 'splicing' results from a number of different simulations to estimate the 1d flux PS and show it is accurate at the 10% level.  Finally, find that collisional heating of the gas from DM particles is negligible in modern cosmo sims.

1406.6362
The chosen few: the low mass halos that host faint galaxies
Sawala, Frenk, Fattahi, Navarro, Theuns, Bower, .. et al

Since reionization prevents SF in most haloes below 3e9 Msun, dwarf galaxies only populate a fraction of existing DM haloes.  Use hydro cosmo sims on the Local Group to study the discriminating factors for galaxy formation in the early Universe and connect them to the present-day properties of galaxies and haloes.  A combination of selection effects related to reionization, and the subsequent evolution of haloes in different environments, introduces strong biases between the population of haloes that host dwarf galaxies, and the total halo population.  Haloes that host galaxies formed earlier and are more concentrated.  In addition, haloes more affected by tidal stripping are more likely to host a galaxy for a given mass or maximum circular velocity, vmax, today.  Consequently, satellite haloes are populated more frequently than field halos, and satellite haloes of 1e8-9 Msun or vmax of 12-20 km/s, similar to the LG dwarf spheroidals, have experienced a greater than average reduction in both mass and vmax after infall.  They are on closer, more radial orbits with higher infall velocities and earlier infall times.  Together, these effects make dwarf galaxies highly biased tracers of the underlying DM distribution.