Estimating the evolution of gas in the Fornax dwarf spheroidal galaxy from its star formation history: an illustrative example
Abstract
We propose that detailed data on the star formation history of a dwarf spheroidal galaxy (dSph) may be used to estimate the evolution of the total mass Mg(t) for cold gas in its star-forming disk. Using Fornax as an illustrative example, we estimate its Mg(t) and the corresponding net gas flow rate Delta F(t) assuming a global star formation rate psi(t) = lambda*(t)[Mg(t)/Msun]alpha consistent with observations of nearby galaxies. We identify the onset of the transition in Delta F(t) from a net inflow to a net outflow as the time tsat at which the Fornax halo became a Milky Way satellite and estimate the evolution of its total mass Mh(t) at t < tsat using the median halo growth history in the LambdaCDM cosmology and its present mass within the half-light radius derived from observations. We examine three different cases of alpha = 1, 1.5, and 2, and justify the corresponding lambda*(t) by comparing the gas mass fraction fg(t) = Mg(t)/Mh(t) at t < tsat with results from simulations of gas accretion by halos in a reionized universe. We find that the Fornax halo grew to Mh(tsat) ~ 2x109 Msun at tsat ~ 5 or 8 Gyr, in broad agreement with previous studies using data on its stellar kinematics and its orbital motion. We describe qualitatively the evolution of Fornax as a satellite and discuss potential extension of our approach to other dSphs.