Day 774

Wednesday.

1410.7391
The metallicity of galactic winds
Creasey, Theuns, Bower

Find that winds from more massive galaxies are hotter and more highly enriched, in stark contrast to that which is often assumed in galaxy formation models.  Use these findings in a simple model of galactic enrichment evolution, in which the metallicity of forming galaxies is the result of accretion of nearly pristine gas and outflow of enriched gas along an equilibrium sequence.  Compare these predictions to the observed mass-metallicity relation, and demonstrate how the galaxy's gas fraction is a key controlling parameter.  This explains the observed flattening of the mass-metallicity relation at higher stellar masses.

1410.7400
The colors of satellite galaxies in the Illustris Simulation
Sales, Vogelsberger, Genel, ... Springel, Hernquist

Observationally, the fraction of blue satellite galaxies decreases steeply with host halo mass, and their radial distribution around central galaxies is significantly shallower in massive (M*>1e11 Msun) than in MW-like systems.  Theoretical models, based primarily on SAM techniques, have had a long-standing problem with reproducing these trends, instead predicting too few blue satellites in general but also estimating a radial distribution that is too shallow, regardless of primary mass.  In this Letter, use the Illustrious cosmological simulation to study the properties of satellite galaxies around isolated primaries.  For the first time, find good agreement between theory and observations.  Identify the main source of this success relative to earlier work to be a consequence of the large gas contents of satellite at infall, a factor ~5-10 times larger than in SAMs.  Because of their relatively large gas reservoirs, satellites can continue to form stars long after infall, which a typical timescale for SF to be quenched ~2Gyr in groups but more than ~5Gyr for satellites around MW-like primaries.  The gas contents inferred are consistent with z=0 observations of HI gas in galaxies, although find large discrepancies among reported values in the literature.  A testable prediction of the model is that the gas-to-stellar mass ratio of satellite progenitors should vary only weakly with cosmic time.

1410.7515
Core shapes and orientations of core-Sersic galaxies
Dullo, Graham

24 core-Sersic galaxies using HST images.  Select galaxies with ellipticity and position angle measurements that are robust against HST seeing.  For bulk of the galaxies, there is a good agreement between the ellipticities and position angles at the break radii and the average outer ellipticities and position angles determined over R_e < 2 < R_e, where R_e is the spheroids' effective HLR.  However there are some interesting differences.  Find a median "inner" ellipticity at R_b of e_med=0.13pm0.01, rounder than the median ellipticity of the "outer" regions e_med=0.20pm0.01, which is thought to reflect the influence of the central SMBH at small radii.  In addition find a trend albeit weak (2 sigma), such that galaxies with larger (stellar deficit)-to-(SMBH) mass ratios - thought to be a measure of the number of major dry merger events - tend to have rounder inner and outer isophotes, suggesting a connection between the galaxy shapes and their mergers histories.  Show that this finding is not simply reflecting the well known result that more luminous galaxies are rounder, but it is no doubt related.