Simultaneous ram pressure and tidal stripping; how dwarf spheroidals lost their gas

Abstract

We perform high-resolution N-Body+SPH simulations of gas-rich dwarf galaxy satellites orbiting within a Milky Way-sized halo and study for the first time the combined effects of tides and ram pressure. The structure of the galaxy models and the orbital configurations are chosen in accordance to those expected in a LCDM Universe.While tidal stirring of disky dwarfs produces objects whose stellar structure and kinematics resembles that of dwarf spheroidals after a few orbits, ram pressure stripping is needed to entirely remove their gas component. Gravitational tides can aid ram pressure stripping by diminishing the overall potential of the dwarf, but tides also induce bar formation which funnels gas inwards making subsequent stripping more difficult. This inflow is particularly effective when the gas can cool radiatively. Assuming a low density of the hot Galactic corona consistent with observational constraints, dwarfs with V_{peak} < 30 km/s can be completely stripped of their gas content on orbits with pericenters of 50 kpc or less. Instead, dwarfs with more massive dark haloes and V_{peak} > 30 km/s lose most or all of their gas content only if a heating source keeps the gas extended, partially counteracting the bar-driven inflow. We show that the ionizing radiation from the cosmic UV background at z > 2 can provide the required heating. In these objects most of the gas is removed or becomes ionized at the first pericenter passage,explaining the early truncation of the star formation observed in Draco and Ursa Minor. The stripped gas breaks up into individual clouds pressure confined by the outer gaseous medium that have masses, sizes and densities comparable to the HI clouds recently discovered around M31.(abridged)