Day 776

Friday.

1410.8141
Forty-seven Milky Way-sized, extremely diffuse galaxies in the Coma Cluster
van Dokkum, ... Geha, Conroy, et al

Discovery of 47 low surface brightness objects in deep images of 3x3 deg field centered on the Coma cluster, obtained with the Dragonfly Telephoto Array.  The objects have central surface brightness mu(g,0) ranging from 24-26 mag/arcsec2 and effective radii r_e=3-10", as measured from CFHT images.  From their spatial distribution, infer that most or all of the objects are galaxies in the Coma cluster.  This relatively large distance is surprising as it implies that the galaxies are very large: with r_e=1.5-4.6 kpc their sizes are similar to those of L* galaxies even though their median stellar mass is only ~6e7 Msun.  The galaxies are relatively red and round, with <g-i> =0.8 and <b/a>=0.74.  One of the 47 galaxies is fortuitously covered by a HST ACS observation.  The ACS imaging shows a large spheroidal object with a central surface brightness mu(g,0)=25.8 mag/as2, a Sersic index n=0.6, and an effective radius of 7", corresponding to a 3.4 kpc at the distance of Coma.  The galaxy is unresolved, as expected for a Coma cluster object.  As known, such "ultra-diffuse galaxies" have not been predicted in any modern galaxy formation model.  Speculate that UDGs may have lost their gas supply at early times, possibly resulting in very high DM fractions.

1410.8161
Star formation quenching in simulated group and cluster galaxies: when, how, and why?
Bahe, McCarthy

SF is observed to be suppressed in group and cluster galaxies compared to the field.  Analyze ~2k galaxies in cosmo hydro sims.  Time of quenching varies from ~2Gyr before accretion (first crossing of r200,c) to >4 Gyr after, depending on satellite and host mass.  Once begun, quenching is rapid (>~500 Myr) in low-mass galaxies (M*<1e10 Msun), but significantly more protracted for more massive satellites.  The simulation predict a substantial role of outflows driven by ram pressure - but not tidal forces - in removing the SF ISM from satellite galaxies, especially dwarfs (M*~1e9 Msun) where they account for nearly 2/3 of ISM loss in both groups and clusters.  Immediately before quenching is complete, this fraction rises to ~80% even for MW analogues (M*~1e10.5 Msun) in groups (M_host~1d13.5 Msun).  Show that (i) ISM stripping was significantly more effective at early times than at z=0; (ii) approximately half the gas is stripped from `galactic fountains` and half directly from the SF disk; (iii) galaxies undergoing stripping experience ram pressure up to ~100 times the average at a given group/cluster-centric raids, because they are preferentially located in overdense ICM regions.  Remarkably, stripping causes at most half the loss of the extended gas haloes surrounding the simulated satellites.  These results contrast sharply with the current picture of strangulation - removal of the ISM through SF after stripping of the hot halo - being the dominant mechanism quenching group and cluster satellites.

1410.8452
Born-corrections to weak lensing of the cosmic microwave background temperature and polarisation anisotropies
Hagstotz, Schäfer, Merkel

Affects especially the polarization spectra, leading to relative changes of the order of 1% of the E-mode spectrum and up to 10% on all scales to the B-mode spectrum.  In contrast, there is only little change of spectra involving the CMB temperature.  The corrections excite one more degree of freedom resulting in a deflection component which can not be described as a gradient of the lensing potential as it is related to image rotation in lens-lens coupling.  Estimate the magnitude of this effect on the CMB-spectra and find it to be negligible.