On the Prevalence of Starbursts in Dwarf Galaxies

Abstract

An outstanding question in galaxy evolution research is whether the star formation histories of low mass systems are dominated by global starbursts or modes that are more quiescent and continuous. In this thesis, we quantify the prevalence of global starbursts in dwarf galaxies at the present epoch, and attempt to infer their characteristic durations, frequencies and amplitudes in the past. Our approach is to directly tally the number of bursting dwarfs in a complete local sample, and to compute the fraction of star formation that is concentrated in these systems. The resulting starburst number and mass fractions are then combined with B-V colors from the literature, the H-alpha EWs presented here, and stellar evolutionary synthesis models in order to place constraints on the average starburst duty cycle. The primary dataset used has been put together by the 11 Mpc H-alpha UV Galaxy Survey, who have collected data on an approximately volume-limited, statistical sample of star-forming galaxies within 11 Mpc of the Milky Way.Our main observational results, along with the accumulation of star formation studies of dwarf galaxies over the past three decades, paint a consistent picture where systems that are currently experiencing a massive global burst are just the 6% +/- 3% tip of a low-mass galaxy iceberg. Moreover, bursts are responsible for 22% +/- 10% of the total star formation in the overall dwarf galaxy population, so the majority of stars in low-mass systems do not appear to be formed in this mode today.Over their lifetimes, however, a greater fraction of the stellar mass of a dwarf may be formed in the burst mode. Synthesis modeling suggests that bursts cycles appear to be necessary in order to simultaneously explain the present-day observed blue B-V colors and modest H-alpha EWs of TYPICAL, CURRENTLY NON-BURSTING dwarf irregulars, unless non-standard assumptions concerning the IMF and the escape fractions of Lyman continuum photons are made. The starburst cycle that we converge upon involves burst durations of 50-100 Myrs, cycle frequencies of 1 to 3 per Gyr, and elevated burst SFRs that are a factor of 6-10 higher than the rate in the quiescent state. Galaxies characterized by such a SFH would spend ~10% of their lives in the burst state, and form ~50% of their stellar mass during this time.