Day 643

Wednesday.

1404.7130
XMM-Newton and Chandra cross calibration using HIFLUGCS galaxy clusters
Schellenberger, Reiprich, Lovisari, Nevalainen, David

Cosmo constraints from clusters rely on accurate gravitational mass estimates, which strongly depend on cluster gas temperature measurements.  Therefore, systematic calibration difference may result in biased, instrument-dependent cosmological constraints.  This is of special interest in the light of the tension between Planck results of the primary temperature anisotropies of the CMB and SZ plus X-ray cluster counts analyses.  Quantify in detail the systematics and uncertainties of the cross-calibraton of the effective area between five X-ray instruments, EPICS-MOS1/MOS2/PN onboard XMM-Newtaon and ACIS-I/S onboard Chandra, and the influence on temperature measurements.  Furthermore, assess the impact of the cross calibration uncertainties on cosmology.  Using the HIFLUGCS sample, consisting of the 64 X-ray brightest galaxy clusters, constrain the ICM temperatures through spectral fitting int the same, mostly isothermal, regions and compare them.  This work is an extension to a previous one using X-ray clusters by the IACHEC.  Performing spectral fitting in the full energy band, find that best-fit temperatures determined with XMM-Newton/EPIC are significantly lower than Chandra/ACIS temperatures.  Demonstrate that effects like multi temperature structure and different relative sensitivities of the instruments at certain energy bands cannot explain the observed differences.  Conclude that using XMM-Newton/EPIC, instead of Chandra/ACIS to derive full energy band temperature profiles for cluster mass determination results in an 8% shift towards lower OmegaM values and <1% shift towards higher sigma8 values in a cosmological analysis of a complete sample of galaxy clusters.  Such a shift is insufficient to significantly alleviate the tension between Planck CMB anisotropies and SZ plus XMM-Newton cosmological constraints.

1404.7144
The star formation histories of local group dwarf galaxies I.  Hubble space telescope / wide field planetary camera 2 observations
Weisz, … Dalcanton, et al

Present uniformly measured SFHs of 50 LG dwarf galaxies based on color-magnitude diagram (CMD) analysis from archival Hubble imaging.  SFHs can be recovered from CMDs that do not reach the oldest MS turn-off (MSTO), but the oldest MSTO is critical for precisely constraining the earliest epochs of SF.  Find: (1) the average lifetime SFHs of dSphs can be approximated by an exponentially declining SFH with tau~5 Gyr; (2) the lower luminosity dSphs are less likely to have extended SFHs than more luminous dSphs; (3) the average SFHs of dIrrs, transition dwarfs (dTrans), and dwarf ellipticals (dEs) can be approximated by the combination of an exponentially declining SFH (tau~3-4 Gyr) for loopback ages >10-12 Gyr ago and a constant SFH thereafter; (4) the observed fraction of stellar mass formed prior to z=2 ranges considerably (80% for galaxies with M<1e5 Msun to 30% for galaxies with M>1e7 Msun) and is largely explained by environment; (5) the distinction between "ultra-faint" and "classical" dSphs is arbitrary; (6) LG dIrrs formed a significantly higher fraction of stellar mass prior to z=2 than the SDSS galaxies from Leiter 2012 and the SFHs from the abundance matching models of Behroozi+2013.  This may indicate higher than expected SF efficiencies at early times in low mass galaxies.  Finally, provide all the SFHs in tabulated electronic format.