Day 637

Thursday.

1404.5618
A measurement of the Alcock-Paczynski test using cosmic voids in the SDSS
Sutter, Pisani, Wandelt

AP test on stacked cosmic voids in SDSS DR7 main sample, and DR10 LOWZ and CMASS samples.  Find 1500 voids out to z=0.6.  Assess the impact of peculiar velocities, use mock void catalogs, and found a constant uniform flattening of 14% along the LoS when peculiar velocities are included.  This flattening appears universal for all void sizes at all z and for all tracer densities.  Also use these mocks to identify an optical stacking strategy.  After correcting for systematic effects, find that AP measurement leads to Omega_m~0.15 preferred over Omega_m~1.0 by likelihood ratio of 10.  Find a factor of 4.5 preference of the likelihood ratio of LCDM Omega_m=0.3 model and a null measurement.  Find substantial evidence for AP signal in the cosmic voids.  Measurements with future surveys will provide tighter cosmo parameter constraints.  The void-finding algorithm and catalogs used in this work is available on the web.

1404.5633
Dark matter-baryons separation at the lowest mass scale: the bullet group
Gastaldello et al

SL selected group of 2.4e14 Msun mass has a separation of 124 kpc between X-ray emitting collisional gas and the collision less galaxies and DM, traced by SL.  DM interaction cross section limit of 10 cm^2/g.  Lowest mass object found to date showing DM-baryon separation; detection of bullet-like objects is not rare and confined to mergers of massive objects; possible of a statistical detection of DM-baryons separation with future surveys.

1404.5636
The abundance of Bullet-groups in LCDM
Fernandez-Trincado et al

Estimate the expected distribution of displacements between DM-DM peaks and between DM-gas displacements in DM haloes with masses > 1e13 Msun/h.  Find that 50% of the DM haloes with circular velocities in the range 300 km/s to 700 km/s (groups) show DM-DM displacements equal or larger than 186 kpc/h.  For DM haloes with circular velocities larger than 700 km/s (clusters) this fraction rises to 70%.  Find that 0.1 to 1.0% of the groups should present separations equal or larger than 87kpc/h; for clusters this fraction rises to 7pm3%, consistent with previous studies.  Number density of groups similar to that observed above is~6e-7 Mpc^-3, 3x larger than the estimated value for clusters.  These results open up the possibility for a new statistical test of LCDM by looking for DM-gas displacements in low mass clusters and groups.

1404.5624
Charting the evolution of the ages and metallicities of massive galaxies since z=0.7
Gallazzi, Bell, Zibetti, Brinchmann, Kelson

Perform deep, multi-object rest-frame optical spectroscopy with IMACS/Magellan of ~70 galaxies in the E-CDFS with 0.65<z<0.75, R>22.7 and M*>1e10 Msun.  Constrain the stellar mass, mean stellar age and stellar metallicity of individual galaxies from stellar absorption features.  Characterize for the first time the dependence of stellar metallicity and age on stellar mass at z~0.7 for all galaxies and for quiescent and SF galaxies separately.  These relations for the whole sample have a similar shape as the z=0.1 SDSS analog, but are shifted by ~0.28 dex in age and by -0.13 dex in metallicity, at odds with simple passive evolution.  Find that no additional SF and chemical enrichment are required for z=0.7 quiescent galaxies to evolve into the present-day quiescent population.  However, this must be accompanied by the quenching of a fraction of z=0.7 M*>1e11 Msun SF galaxies with metallicities comparable to those of quiescent galaxies, thus increasing the scatter in age without affecting the metallicity distribution.  However rapid quenching of the entire population of massive SF galaxies at z=0.7 would be inconsistent with the age/metallicity-mass relation for the population as a whole and with the metallicity distribution of SF galaxies only, which are on average 0.12 dex less metal-rich than their local counterparts.  This indicates chemical enrichment until the present in at least a fraction of the z=0.7 massive SF galaxies.