Understanding Bursty Star Formation in Dwarf Galaxies, Its Effect on Galactic Dynamics, and Implications for Reionization
Abstract
Dwarf galaxies -- galaxies with stellar masses below 10^9 solar mass are the most abundant galaxies in the universe. Due to their shallow gravitational potential well and small gas reservoirs, dwarfs are easily disrupted by supernovae feedback and can lose a fraction of their cold gas to the intergalactic medium in the form of outflows. Implementation of feedback into hydrodynamical simulations suggests that this phenomenon leads to a stochastic star formation history in these low-mass systems called bursty star formation. Burstiness can cause large variations of many physical quantities such as metallicity, morphology, radial velocity, dark matter density profile, etc. and can reconcile the discrepancies between the predictions from cold dark matter and observations in near-field cosmology.
In order to characterize burstiness, we use two different star formation rate indicators that are sensitive to the recent and current star formation changes (Hα and UV1500 luminosities). We apply exponentially rising/falling burst models to determine the timescales and amplitudes of the bursts for each mass bin and compare the results to the simulations. We find that in lower mass galaxies, burstiness becomes stronger and shorter.
Furthermore, we discuss the importance of dwarf galaxies to the reionization of the universe and describe our measurement of ξ_ion as one of the key components in determining the ionizing emissivity of dwarf galaxies. For that, we use of a sample of lensed dwarf galaxies at a redshift of 1< z <3, pushing the limits to higher redshifts and lower luminosities than others have studied before. We do not find any strong dependence between log(ξ_ion) and M_UV or UV spectral slope (β) and report a value of log(ξ_ion)= 25.5-25.6 for 21< M_UV<18.
Lastly, we discuss the effect of burstiness on the dynamics and morphology of stars and gas inside dwarf galaxies. Our findings show that there is evidence of both size fluctuation and gas velocity variations being correlated with the UV1500-inferred star formation indicators especially at masses below 10^9 solar mass.