Day 440

Thursday.

1306.0898
The dependence of dark matter profiles on the stellar to halo mass ratio: a prediction for cusps vs cores
Di Cintio, et al

Use 31 simulated galaxies (1e10 to 8e11 Msun haloes, 5e5 to 8e10 Msun stellar mass; from dwarf to L*) to investigate the effects of baryonic feedback on the density profiles of DM haloes.  Several baryonic prescriptions, including stellar feedbacks.  Main results: clear dependence of the inner slope of the DM density profile, alpha (where rho~r^alpha), on the ratio between stellar-to-halo mass ratio (M*/Mhalo).  This relation is independent of the stellar feedback scheme, allowing a prediction for cusp vs core formation.  When M*/Mhalo is low, ~0.01% [that's pretty low], energy from stellar feedback is insufficient to significantly alter the inner DM density and the galaxy retains a cuspy profile.  At higher M*/Mhalo, feedback drives the expansion of the DM and generates cored profiles.  The flattest profiles form where M*/Mhalo ~ 0.5%.  Above this ratio, stars formed in the central regions deepen the gravitational potential enough to oppose this SNe-driven expansion process, resulting in smaller cores and cuspier profiles.  Combining the dependence of alpha on M*/Mhalo with the abundance matching relation between M* and Mhalo provides a prediction for how alpha varies with M*.  Further, using the TF relation allows a prediction for the dependence of the DM inner slope on the observed rotation velocity of galaxies.  The most cored galaxies are expected to have Vrot~50 km/s, with alpha decreasing for more massive disc galaxies: spirals with Vrot~150 km/s have central slopes alpha<-0.8, approaching the NFW profile.  This novel prediction for the dependence of alpha on disc galaxy mass can be tested using current observational data sets, and can be applied to theoretical modeling of mass profiles and populations of disc galaxies.

1306.0901
Mass-sheet degeneracy, power-law models and external convergence: impact on the determination of the Hubble constant from gravitational lensing
Schneider, Sluse

The light travel time differences in SL systems allows an independent determination of the Hubble constant.  This method has been successfully applied to several lens systems.  The formally most precise measurements are, however, in tension with the recent determination of H0 from the Planck satellite for a spatially flat 6-parameters LCDM cosmology.  Reconsider the uncertainties of the method, concerning the mass profile of the lens galaxies, and show that the formal precision relies on the assumption that the mass profile is a global power law. Simple analytical and numerical experiments reveal that mass-sheet like transformation yield significant freedom in choosing the mass profile, even when exquisite Einstein rings are observed.  Furthermore, the characterization of the environment of the lens will not break that degeneracy which is not physically linked to extrinsic convergence.  Present an illustrative example where the multiple imaging properties of a composite (baryons + DM) lens can be extremely well reproduced by a power-law model, but with predictions for the Hubble constant that deviate by about 20%.  Hence conclude that the impact of degeneracies between parameterized models might have been underestimated in current H_0 measurements from lensing, and need to be carefully reconsidered.