Abstract
We review numerical works carried out over the last decade on the role of environmental mechanisms in shaping nature of the faintest galaxies known, dwarf spheroidals (dSphs). The combination of tidally induced morphological transformation, termed tidal stirring, with mass loss due to tidal and ram-pressure stripping aided by heating due to the cosmic ionizing background can turn late-type dwarfs resembling present-day dIrrs into classic dSphs. The time of infall into the primary halo is shown to be a key parameter. Dwarfs accreting at when the cosmic ultraviolet ionizing flux was much higher than today, and was thus able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark-matter-dominated objects with truncated star-formation histories, such as the Draco dSph. The low star-formation efficiency expected in such low-metallicity objects prior to their infall was crucial for keeping their disks gas dominated until stripping took over.Therefore gas stripping along with inefficient star-formation provides a new feedback mechanism, alternative to photoevaporation or supernovae feedback, playing a crucial role in dwarf galaxy formation and evolution. We also discuss how the ultra-faint dSphs belong to a different population of lower-mass dwarf satellites that were mostly shaped by reionization rather than by environmental mechanisms (“reionization fossils”). Finally, we scrutinize the various caveats in the current understanding of environmental effects as well as other recent ideas on the origin of Local Group dSphs.
Highlights
Dwarf spheroidals in the Local Group are the faintest galaxies known
Dwarfs accreting at z > 1, when the cosmic ultraviolet ionizing flux was much higher than today, and was able to keep the gas in the dwarfs warm and diffuse, were rapidly stripped of their baryons via ram pressure and tidal forces, producing very dark-matter-dominated objects with truncated star-formation histories, such as the Draco dwarf spheroidals (dSphs)
We discuss how the ultra-faint dSphs belong to a different population of lower-mass dwarf satellites that were mostly shaped by reionization rather than by environmental mechanisms (“reionization fossils”)
Summary
Dwarf spheroidals (dSphs) in the Local Group are the faintest galaxies known. Including the ultrafaint dwarf spheroidals discovered in the last few years, they span luminosities in the range −3 < MB < −14. We discuss how the combination of tidal effects, ram pressure stripping and the cosmic ultraviolet background radiation at high redshift may provide an explanation to both the similarities and differences among the many known Local Group dwarf spheroidal galaxies. Based on the current measures of their internal stellar velocity dispersions and on theoretical predictions on tidal mass loss of subhalos, we will argue that classic dwarf spheroidals were assembled in fairly massive halos, with masses in the range 108–1010M. For these dwarfs, reionization played some role in depleting their baryonic content enhancing the effect of gas removal mechanisms such as tidal stripping and tidal stirring as well as ram pressure stripping. The theoretical models that we present here are based on the results of N-Body+hydrodynamical simulations of the detailed interaction of individual dwarf galaxies with a primary host, as well as of high-resolution cosmological simulations of Milky-Way-sized galaxies.In the interaction simulations, the model galaxies and the choice of the orbits are consistent with the predictions of ΛCDM models [18–20, hereafter MA06]
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