The evolution of dwarf galaxy satellites with different dark matter density profiles in the ErisMod simulations. I. The early infalls

We present the first simulations of tidal stirring of dwarf galaxies in the Local Group carried out in a cosmological context. We use the ErisDARK simulation of a MW-sized galaxy to identify some of the most massive subhalos ($M_{vir} > 10^8 M_{\odot}$) that fall into the main host before $z=2$. Subhalos are replaced before infall with high-resolution models of dwarf galaxies comprising a faint stellar disk embedded in a dark matter halo. The set of models contains cuspy halos as well as halos with "cored" profiles (with asymptotic inner slope $\gamma = 0.6$). The simulations are then run to $z=0$ with as many as 54 million particles and resolution as small as $\sim 4$ pc using the N-Body code ChaNGa. The stellar components of all satellites are significantly affected by tidal stirring, losing stellar mass and undergoing a morphological transformation towards a pressure supported spheroidal system. However, while some remnants with cuspy halos maintain significant rotational flattening and disk-like features, all the shallow halo models achieve $v/\sigma < 0.5$ and round shapes typical of dSph satellites of the MW and M31. Mass loss is also enhanced in the latter, and remnants can reach luminosities and velocity dispersions as low as those of Ultra Faint Dwarfs (UFDs). We argue that cuspy progenitors must be the exception rather than the rule among satellites of the MW since all the MW and M31 satellites in the luminosity range of our remnants are dSphs, a result matched only in the simulation with "cored" models

Tidal stirring of satellites with shallow density profiles prevents them from being too big to fail

The "too big to fail" problem is revisited by studying the tidal evolution of populations of dwarf satellites with different density profiles. The high resolution cosmological $\rm \Lambda CDM$ "ErisMod" set of simulations is used. These simulations can model both the stellar and dark matter components of the satellites, and their evolution under the action of the tides of a MW-sized host halo at a force resolution better than 10 pc. The stronger tidal mass loss and re-shaping of the mass distribution induced in satellites with $\gamma=0.6$ dark matter density distributions, as those resulting from the effect of feedback in hydrodynamical simulations of dwarf galaxy formation, is sufficient to bring the circular velocity profiles in agreement with the kinematics of MW's dSphs. In contrast, in simulations in which the satellites retain cusps at $z=0$ there are several "massive failures" with circular velocities in excess of the observational constraints. Various sources of deviations in the conventionally adopted relation between the circular velocity at the half light radius and the one dimensional line-of-sight velocity dispersions are found. Such deviations are caused by the response of circular velocity profiles to tidal effects, which also varies depending on the initially assumed inner density profile, and by the complexity of the stellar kinematics, which include residual rotation and anisotropy. In addition tidal effects naturally induce large deviations in the stellar mass-halo mass relation for halo masses below $\rm 10^9 ~ M_{\odot}$, preventing any reliable application of the abundance matching technique to dwarf galaxy satellites.Comment: submitted to Ap

Microlensing as a possible probe of event-horizon structure in quasars

In quasars which are lensed by galaxies, the point-like images sometimes show sharp and uncorrelated brightness variations (microlensing). These brightness changes are associated with the innermost region of the quasar passing through a complicated pattern of caustics produced by the stars in the lensing galaxy. In this paper, we study whether the universal properties of optical caustics could enable extraction of shape information about the central engine of quasars. We present a toy model with a crescent-shaped source crossing a fold caustic. The silhouette of a black hole over an accretion disc tends to produce roughly crescent sources. When a crescent-shaped source crosses a fold caustic, the resulting light curve is noticeably different from the case of a circular luminosity profile or Gaussian source. With good enough monitoring data, the crescent parameters, apart from one degeneracy, can be recovered