Day 481

Monday.

1308.0328
Dwarf galaxies with optical signatures of active massive black holes
Reines, Greene, Geha

Present a sample of 151 dwarf galaxies (M*=1e8.5-9.5) that exhibit optical spectroscopic signatures of accreting massive HBs, increasing the number of known active galaxies in this stellar mass range by more than an order of magnitude.  Use SDSS DR8 and NASA-Sloan Atlas stellar masses; systematically search for active BHs in ~25k emission-line galaxies with stellar masses comparable to the Magellanic Clouds and z < 0.055.  Using the narrow-line [OIII]/H-beta versus [NII]/H-alpha diagnostic diagram, find photoionization signatures of BH accretion in 136 galaxies, a small fraction of which also exhibit broad H-alpha emission.  For these broad-line AGN candidates, estimate BH masses using standard virial techniques and find a range of 1e5<M_BH<1e6 Msun and a median of M_BH~2e5 Msun.  Also detect broad H-alpha in 15 galaxies that have narrow-line ratios consistent with SF galaxies.  Follow-up observations are required to determine if these are true type 1 AGN or if the broad H-alpha is from stellar processes.  The median absolute magnitude of the host galaxies in the active sample is Mg = -18.1 mag, which is ~1-2 magnitudes fainter than previous samples of AGN hosts with low-mass BHs.  This work constrains the smallest galaxies that can form a massive BH, with implications for BH feedback in low-mass galaxies and the origin of the first SMBH seeds.

1308.0333
Cosmological simulations of black hole growth: AGN luminosities and downsizing
Hirschmann, Dolag, Saro, Borgani, Burkert

Statistical analysis of BH growth and evolution of AGN with cosmo hydro-sims down to z=0.  Radiative cooling, SF, metal enrichment, BH growth and associated feedback processes from both SNe II/Ia and AGN included.  Two sims: one larger, one with better mass resolution.  In good agreement with previous BH property studies; can reproduce the evolution of the bolometric AGN luminosity function for both the low-and the high-luminosity end up to z=2.5.  The smaller but higher resolution run can match the observational data of the low bolometric luminosity end up to z=4-5.  Also perform a direct comparison with the observed soft and hard X-ray LFs of AGN, including an empirical correction for a torus-level obscuration, and find a similarly good agreement.  These results show that simulations can self-consistently predict the observed downsizing trend in the AGN number density evolution, i.e., the number densities of luminous AGN peak at higher redshifts that hose of faint AGN.  Implications of the downsizing behaviour on active BHs, their masses and Eddington-ratios are discussed.  Overall, the downsizing behaviour in the AGN number density can be attributed to a combination of the gas density evolution in the resolved vicinity of a (massive) BH (which is depleted with evolving time mainly as a consequence of the radio-mode feedback) and to the decreasing mean relative velocities between the (low mass) BHs and the surrounding gas with decreasing redshift.

1308.0334
Reconciling 56Ni production in Type Ia supernovae with double degenerate scenarios
Piro, Thompson, Kochanek

Binary WD coalescence driven by GW or collisions in triple systems [! why triple?] are potential progenitors of SNe Ia.  Combine distribution of 56Ni inferred from observation with the results of both sub-Chandrasekhar detonation models and direct collision calculations to estimate what mass WDs should be exploding in each scenario to reproduce the observations.  These WD mass distributions are then compared with the observed Galactic WD mass distribution and MC simulations of WD-WD binary populations.  For collisions, find that the average mass of the individual components of the WD-WD binary must be peaked at ~0.75 Msun, significantly higher than the average WD mass in binaries or in the field of ~0.55-0.60 Msun. Thus, if collisions indeed produce a large fraction of the SNe Ia, then a mechanism must exist that favors large mass WDs.  In particular, collisions between WDs of average mass must be highly suppressed.  For sub-Chandrasekhar detonations, find that the average mass of the exploding WDs must be peaked at ~1.1 Msun, consistent with the average sum of the masses in WD-WD binaries [!].  This interesting similarity should be tested by future calculations of the 56Ni yield from double degenerate mergers.  These models may also explain why SNe Ia are on average dimmer in early-type hosts: in old environments binaries evolve too quickly to have mergers between two high mass WDs at current times.  As future simulations explore the 56Ni yield over a wider range of parameters, the general framework discussed here will be an important tool for continuing to assess double degenerate scenarios.