1212.6651 (Kenza S. Arraki et al.)

Effects of baryon removal on the structure of dwarf spheroidal galaxies    [PDF]

Kenza S. Arraki, Anatoly Klypin, Surhud More, Sebastian Trujillo-Gomez

[abridged] Dwarf spheroidal galaxies (dSphs) are extremely gas poor, dark matter-dominated galaxies, which make them ideal to test the predictions of the Cold Dark Matter (CDM) model. We argue that the removal of a small baryonic component from the central regions of forming dSphs may substantially reduce their central dark matter density. Thus it may play an important role in alleviating one of the problems of the CDM model related with the structure of relatively massive satellite galaxies of the Milky Way. Traditionally, collisionless cosmological N-body simulations are used when confronting theoretical predictions with observations. However, these simulations assume that the baryon fraction is equal to the cosmic mean, an assumption which can be incorrect for dSphs. We find that the combination of (i) the lower baryon fraction in dSphs compared to the cosmic mean and (ii) the concentration of baryons in the inner part of the Milky Way halo can go a long way towards explaining the observed circular velocity profiles of dSphs. We find that the blowing away of baryons by ram pressure, when the dwarfs fall into larger galaxies, lowers the circular velocity profile of the satellite. In the central ~200-500 pc region of the galaxies the dark matter density is expected to decline by a factor of (1 - f_b)^4 ~ 0.5, where f_b is the cosmological fraction of baryons. In addition, the enhanced baryonic mass in the central regions of the parent galaxy generates tidal forces, which are larger than those experienced by subhaloes in traditional N -body simulations. Increased tidal forces substantially alter circular velocity profiles for satellites that come as close as 50 kpc. We show that these two effects are strong enough to bring the observed structure of dSphs into agreement with the predictions of the subhaloes in CDM simulations, regardless of the details of the baryonic processes.

View original: http://arxiv.org/abs/1212.6651