Day 628

Tuesday.

1404.3209
The Smith Cloud and its dark matter halo: survival of a galactic disc passage
Nichols etal

The current velocity of the Smith Cloud indicates that it has undergone at least one passage of the Galactic disc.  Using hydrodynamics simulations, examine the present day structure of the Smith Cloud.  Find that the DM supported cloud is able to reproduce the observed present day neutral hydrogen mass, column density distribution and morphology.  In this case, the DM halo becomes elongated, owing to the tidal interaction with the Galactic disc.  Clouds in models neglecting DM confinement are destroyed upon disc passage, unless the initial cloud mass is well in excess of what is observed today.  Then determine integrated flux upper limits o the gamma-ray emission around such a hypothesized DM core in the Smith Cloud.  No statistically significant core or extended gamma-ray emission are detected down to a 95% CL upper limit of 1.4e-10 ph /cm2/s in the 1-300 GeV energy range.  For the derived distance of 12.4 kpc, the Fermi upper limits set the first tentative constrains on the DM cross sections annihilating into tau+tau- and b bbar for a high-velocity cloud.

1404.3212
The Argo simulation: I. quenching of massive galaxies at high redshift as a result of cosmological starvation
Feldmann, Mayer

Observations show a prevalence of high-z galaxies with large stellar masses and predominantly passive stellar populations.  A variety of processes have been suggested that could reduce the SF in such galaxies to observed levels, including quasar mode feedback, virial shock heating, or galactic winds driven by stellar feedback.  However, the main quenching mechanisms have yet to be identified.  Study the origin of SF quenching using Argo, a cosmological zoom-in simulation that follows the evolution of a massive galaxy at z>=2.  This simulation adopts the same sub-grid recipes of the Eris simulations, which have been shown to form realistic disk galaxies, and, in one version, adopts also a mass and spatial resolution identical to Eris.  The resulting galaxy has properties consistent with those of observed, massive (M*~1e11 Msun) galaxies at z~2 and with abundance matching predictions.  The models do not include AGN feedback, indicating that SMBHs likely play a subordinate role in determining masses and sizes of massive galaxies at high z.  The sSFR of the simulated galaxy matches the observed M*-sSFR relation at early times.  This period of smooth stellar mass growth comes to a sudden halt at z=3.5 when the sSFR drops by almost an order of magnitude within a few hundred Myr.  The suppression is initiated by a leveling off and a subsequent reduction of the cool gas accretion ratio onto the galaxy, and not by feedback processes.  This "cosmological starvation" occurs as the parent DM halo switches from a fast collapsing mode to a slow accretion mode.  Additional mechanisms, such as perhaps radio mode feedback from an AGN, are needed to quench any residual SF of the galaxy and to maintain a low sSFR until the present time.