Day 511

Saturday.

1309.3565
Galaxy cluster baryon fractions revisited
Gonzalez, Sivanandam, Zabludoff, Zaritsky

Measure the baryons contained in both the stellar and hot gas components for twelve galaxy clusters and groups at z~0.1 with M=1-5e14 Msun.  This paper improves upon the previous work through the addition of XMM data, enabling measurements of the total mass and masses of each major baryonic component, --- ICM, intracluster stars, and stars in galaxies --- for each system.  Recover a relation for the stellar mass versus halo mass consistent with previous result. Confirm that the partitioning of baryons between the stellar and hot gas components is a strong function of M500; the fractions of total mass in stars and X-ray gas within r500 scale as M^-0.45 and M500^0.26, respectively.  Also confirm that the combination of the BCG and intracluster stars is an increasingly important contributor to the stellar baryon budget in lower halo masses.  Find a weak, but statistically significant, dependence of the total baryon fraction upon halo mass, scaling as M500^0.16 [baryon fraction not constant?!].  For M500>2e14 Msun, the total baryon fractions within r500 are on average 18% (7%) below the Universal value from the WMAP7 (Planck) analysis.  For Planck, the difference between the Universal value and cluster baryon fractions is less than the systematic uncertainties associated with M500.  The total baryon fractions exhibit significant scatter, particularly at M500 < 2e14 Msun where they range from 60-90% (65-100%) of the Universal value for WMAP7 (Planck).  The ratio of the stellar-to-gas mass within r500, a measure of SF efficiency, strongly decreases with M500.  The fact that this relation remains tight at low mass implies that the larger scatter in the total baryon fractions at these masses arises from either true scatter in the total baryon content or observational scatter in M500, rather than late-time physical processes such as redistribution of gas beyond r500 [why?].

1309.3567
HST/WFC3 Near-infrared spectroscopy of quenched galaxies at z~1.5 from the WISP survey: stellar population properties
Bedregal et al

Combine HST NIR spectroscopy with HST UVIS, IR and Spitzer IRAC 3.6um photometry to assemble a sample of massive (log(M*/Msun)~11) and quenched galaxies at z~1.5.  Sample of 41 galaxies; largest for NIR spectroscopy for quenched sources at these redshifts.  z~1.5 quenched galaxies in the high-mass range have a wide range of stellar population properties, compared to z~0.  Find their SEDs are well fitted with exponentially decreasing SFHs, and short SF time -scales (tau<100 Myr).  Quenched galaxies also show a wide distribution in ages, between 1-4 Gyr.  In the (u-r)^0 vs. mass space, quenched galaxies have a large spread in rest-frame color at a given mass.  Most quenched galaxies populate the z~1.5 red-sequence (RS), but an important fraction of them (32%) have substantially bluer colors.  Although with a large spread, find that the quenched galaxies on the RS have older median ages (3.1 Gyr) than the quenched galaxies off the RS (1.5 Gyr).  Show that a rejuvenated SED cannot reproduce the observed stacked spectra of (the bluer) quenched galaxies off the RS.  Derive the upper limit on the fraction of massive galaxies on the RS at z~1.5 to be <43% [so a lot of them are in the green valley? (only ~32% are bluer than RS?)].  Speculate that the young quenched galaxies off the RS are in a transition phase between vigorous SF at z>2 and the z~1.5 RS.  According to their estimated ages, the time required for quenched galaxies off the RS to join their counterparts on the z~1.5 RS is of the order of ~1 Gyr.