Day 756

Thursday.

1409.8298
The Zurich Environmental Study (ZENS) of galaxies in groups along the cosmic web.  V.  Properties and frequency of merging satellites and centrals in different environments
Pipino, et al

Investigate the dependence of the merger fraction and merging galaxy properties on environment in a sample of ~1300 group galaxies with M>1e9.2 Msun and 0.05< z< 0.0585.  At any galaxy mass scale of our study, the strongest environmental effect on the merger fraction is a decrease by a factor of ~2-3 in groups with halo masses >1e13.5 Msun, indicating a suppression of merger activity in massive groups.  The observed dependence on halo mass is almost independent of galaxy mass and merger stage.  At galaxy masses >1e10.2 Msun, most mergers are 'dry' accretions of quenched satellites onto quenched centrals, leading to a strong increase of the merger fraction with decreasing group-centric distance at these control sample of non-merging galaxies of equal mass and rank.  However, the increase in both size and SFR leads to similar surface SF densities in the merging and control-sample satellite populations.

1410.0007
Fluctuation spectroscopy: a new probe of old stellar populations
van Dokkum, Conroy

A new method to determine the relative contributions of different types of stars to the integrated light of nearby early-type galaxies.  As is well known, the surface brightness of these galaxies shows pixel-to-pixel fluctuations due to Poisson variations in the number of giant stars.  Differential spectroscopy of pixels as a function of fluctuation strength ("fluctuation spectroscopy") effectively measures the spectral variation of stars as a function of their luminosity, information that is otherwise difficult to obtain for individual stars outside of the LG. Apply this technique to the elliptical galaxy NCG4472, using HST/ACS imaging in six narrowband ramp filters tuned to spectral features in the range 0.8-1.0 micron.  Pixels with +- 5% broad-band variations show differential color variations of 0.1%-1.5% in the narrow-band filters.  These variations are primarily due to the systematic increase in TiO absorption strength with increasing luminosity on the upper giant branch.  The data are very well reproduced by the same Conroy&vanDokkum (2012) stellar populate synthesis model that is the best fit to they integrated light, with residuals in the range 0.03%-0.09%.  Models with ages or metallicities that are significantly different from the integrated-light values do not yield good fits.  Can also rule out several modifications to the underlying model, including the presence of a significant (>3% of the light) population of late M giants.  The current observations constitute a powerful test of the expected luminosities and temperatures of metal-rich giants in massive early-type galaxies.  Studies of pixels with much larger (negative) fluctuations will provide unique information on main sequence stars and the stellar IMF.

1410.0010
An adaptive particle-mesh gravity solver for ENZO
Passy, Bryan

Implement an APM algorithm to solve the Poisson equation for grid-based hydro codes with nested grids.  The algorithm is implemented and extensively tested within the astrophysical code Enzo against the multi grid solver available by default.  Find that while both algorithms show similar accuracy for smooth mass distributions, the adaptive particle-mesh algorithm is more accurate for the case of point masses, and is generally less noisy.  Also demonstrate that the two-body problem can be solved accurately in a configuration with nested grids.  In addition, discuss the effect of sub cycling, and demonstrate that evolving all the levels with the same time step yields even greater precision