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 disc. Using Fornax as an illustrative example, we estimate its Mg(t) and the corresponding net gas flow rate ΔF(t) assuming a global star formation rate ψ(t) = λ*(t)[Mg(t)/M⊙]α consistent with observations of nearby galaxies. We identify the onset of the transition in ΔF(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 current cosmology and its present mass within the half-light radius derived from observations. We examine three different cases of α = 1, 1.5, and 2, and justify the corresponding λ*(t) by comparing the gas mass fraction fg(t) = Mg(t)/Mh(t) at t < tsat with results from simulations of gas accretion by haloes in a reionized universe. We find that the Fornax halo grew to Mh(tsat) ~ 2 × 109M⊙ 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.
Bibliographical noteFunding Information:
We thank Thomas de Boer for providing the data on Fornax?s SFH and for advice on treating uncertainties in the data. We also thank an anonymous referee for constructive criticisms that helped improve our paper. We have benefited from discussions with Fabio Governato, Evan Skillman, and Jerry Wasserburg. We are grateful to Conrad Chan and Alexander Heger for help with the quadraticspline fit. ZY and Y-ZQ acknowledge the hospitality of the Institute of Nuclear and Particle Physics at Shanghai Jiao Tong University where part of thework was done. This work was supported in part by the US DOE (DE-FG02-87ER40328), the NSFC (11320101002), 973-Project 2015CB857000, Shanghai Key Laboratory Grant No. 11DZ2260700, and the CAS/SAFEA International Partnership Program for Creative Research Teams (KJCX2-YW-T23).
- Galaxies: dwarf
- Galaxies: evolution
- Galaxies: formation
- Galaxies: individual: fornax dwarf galaxy
- Local Group