Day 771

Sunday.

1410.6161
Galactic tides and the shape and orientation of dwarf galaxy satellites
Barber, Starkenburg, Navarro, McConnachie

Use cosmo N-body sims from Aquarius to study the tidal effects of a dark matter halo on the shape and orientation of its substructure.  Although tides are often assumed to enhance sphericity and to stretch sub haloes tangentially, these effects are short lived: as in earlier work, find that sub haloes affected by tides become substantially more spherical and show a strong radial alignment toward the center of the host halo.  These results, combined with SAM of galaxy formation, may be used to assess the effect of Galactic tides on the observed population of dSph satellites of the MW and Andromeda galaxies.  If the relatively low DM content of luminous dSphs such as Fornax and Leo I is due to tidal stripping, then their gravitational potential must be substantially more spherical than that of more heavily DM dominated systems such as Draco or Carina.  The model also predicts a tidally-induced statistical excess of satellites whose major axis aligns with the direction to the central galaxy.  Find evidence of this in the M31 satellite population, which suggest that tides may have played an important role in its evolution.

1410.6169
Early flattening of dark matter cusps in dwarf spheroidal galaxies
Nipoti, BInney

Simulations of the clustering of CDM yield DM haloes that have central density cusps, but observations of totally DM dominated dSph imply that they do not have cusp central density profiles.  Use analytic calculations and numerical modeling to argue that whenever stars form, central density cups are likely to be erased.  Gas that accumulates in the potential well of an initially cuspy DM halo settles into a disc.  Eventually the surface density of the gas exceeds the threshold for fragmentation into self-gravitating clouds.  The clouds are massive enough to transfer energy to the DM particles via dynamical friction on a short time-scale.  The halo's central cusp is heated to form a core with central logarithmic density slope gamma=0 before stellar feedback makes its impact.  Since star formation is an inefficient process, the clouds are disrupted by feedback when only a small fraction of their mass has been converted to stars, and the DM dominates the final mass distribution.