Day 779

Wednesday.

1411.0667
The formation of massive, compact galaxies at z=2 in the Illustrious Simulation
Wellons, ... Ma, ... Vogelsberger, Kriek, van Dokkum, ... Springel, ... Hernquiest, et al

Massive, quiescent galaxies at high z have been found to be considerably more compact than galaxies of similar mass in the local universe.  How these compact galaxies formed has yet to be determined, though several progenitor populations have been proposed.  Investigate the formation process and quantify the assembly histories of such galaxies in Illustris, a suite of hydrodynamical cosmo sims encompassing a sufficiently large volume to include rare objects, while simultaneously resolving the internal structure of galaxies.  Select massive 1e11 Msun and compact (stellar half-mass radius < 2 kpc) galaxies from the simulation at z=2.  Within the Illustrius suite, find that these quantities are not perfectly converged, but are reasonably reliable for this purpose.  The resulting population is composed primarily of quiescent galaxies, but also find several SF compact galaxies.  The simulated compact galaxies are similar to observed galaxies in SF activity, appearance, and number density.  Follow their evolution at high z in the simulation and find that there are multiple pathways to form these compact galaxies, dominated by two mechanisms: (i) intense, centrally concentrated starbursts generally triggered by gas-rich major mergers between z~2-4, reducing the galaxies' half-mass radii by a factor of a few to below 2 kpc, and (ii) assembly at very early times when the universe was much denser; the galaxies formed compact and remained so until z~2.  These compact galaxies represent a tail in the overall galaxy distribution rather than a special class, with their small sizes simply one end of a smooth distribution which is a natural consequence of all the various physical processes galaxies may undergo a high z.

1411.0050
A Suzaku search for dark matter emission lines in the X-ray brightest galaxy clusters
Urban et al

Present the results of a search for unidentified emission lines in deep Suzaku X-ray spectra for the central regions of the 4 X-ray brightest galaxy clusters: Perseus, Coma, Virgo and Ophiuchus.  Employ an optimized energy range for the analysis (3.2-5.3 keV) that is relatively free of instrumental features, and a baseline plasma emission model that incorporates the abundances of elements with the strongest expected emission lines at these energies (X,Ar,Ca) as free parameters.  For the Perseus Cluster core, employing this baseline model, find evidence for an additional emission feature at an energy 3.51 keV with flux of ~2.87e-7 ph/s/cm2/arcmin2.  At slightly larger radii, detect an emission line at 3.59keV with a flux of ~4.8e-8.  The energies and fluxes of these features are broadly consistent with previous claims, although the radial variation of the line strength appears in tension with standard DM model predictions.  Assuming a decaying DM origin for the Perseus emission features allows prediction of the energies and line fluxes for the other clusters in the sample.  Critically, do not detect an emission feature at the predicted energy and line flux in the rest.  The formal 99.5% upper limit on the strengths of an emission line in each cluster are below the decaying DM model predictions, scaling from the Perseus Cluster center, apparently ruling the model out.  In light of these results, search for other explanations for the ~3.55 keV emission feature in Perseus.  Results suggest that systematic effects associated with modeling the complex spectra for the Perseus Cluster core, details of the assumed ionization balance, and errors in the predicted emissivities of the spectral lines may in part be responsible for the ~3.5 keV feature.