Star Formation Histories Of Dwarfs Research Articles

How do Dwarf Galaxies Acquire Their Mass & When do They Form Their Stars?

We apply a simple, one-equation, galaxy formation model on top of the halos and subhalos of a high-resolution dark matter cosmological simulation to study how dwarf galaxies acquire their mass and, for better mass resolution, on over 10^5 halo merger trees, to predict when they form their stars. With the first approach, we show that the large majority of galaxies within group- and cluster-mass halos have acquired the bulk of their stellar mass through gas accretion and not via galaxy mergers. We deduce that most dwarf ellipticals are not built up by galaxy mergers. With the second approach, we constrain the star formation histories of dwarfs by requiring that star formation must occur within halos of a minimum circular velocity set by the evolution of the temperature of the IGM, starting before the epoch of reionization. We qualitatively reproduce the downsizing trend of greater ages at greater masses and predict an upsizing trend of greater ages as one proceeds to masses lower than m_crit. We find that the fraction of galaxies with very young stellar populations (more than half the mass formed within the last 1.5 Gyr) is a function of present-day mass in stars and cold gas, which peaks at 0.5% at m_crit=10^6-8 M_Sun, corresponding to blue compact dwarfs such as I Zw 18. We predict that the baryonic mass function of galaxies should not show a maximum at masses above 10^5.5, M_Sun, and we speculate on the nature of the lowest mass galaxies.

The Cosmological Structures and the Orbits of Satellite Galaxies

Most of the orbits of the Milky Way (MW) satellite galaxies and some of the orbits of the MW globular cluster system lie near Great Circles in the Sky. The existence of satellites in the same orbital plane could be understood if they were part of the same accretion event. However, this argument cannot explain, for example, the different star formation histories of dwarfs belonging to the same Great Circle. In this paper we will show, using hydrodynamical cosmological simulations, that the origin of Great Circles can be found in the filaments of cosmological structures, since satellites accreted along the same, or close, filaments will orbit the host roughly on the same plane.