On the nature of starbursts

Starbursts are a fascinating phenomenon that can significantly impact the host galaxy and the surrounding intergalactic medium. Understanding the nature of a starburst requires a detailed analysis of the resolved stellar populations and the recent star formation history (SFH) of the galaxy. Using a CMD fitting technique and stellar evolution models, we derive the SFHs of twenty nearby dwarf starburst galaxies from Hubble Space Telescope V and I band images. The star formation rates (SFRs) from this diverse sample of dwarf galaxies span three orders of magnitude but all show elevated levels of star formation (SF) in their recent past when viewed in the context of the host galaxy's past SFH. Fifteen of the twenty galaxies show currently bursting SF and five galaxies show "fossil" bursts. From our reconstructed SFHs, it is evident that the elevated SFRs of a burst are sustained for hundreds of Myr. The SF migrates around the host galaxies in many cases as derived from the temporally and spatially resolved stellar populations and is a cumulation of SF not only in star clusters but also in field regions of low surface brightness in the galaxies. Contrary to the shorter time of 3-10 Myr often cited, the starburst durations we measure range from 450--600 Myr in fifteen of the dwarf galaxies and up to 1.3 Gyr in four galaxies; comparable to or longer than the dynamical timescales for each system. The same feedback loop from massive stars that may quench flickering SF does not disrupt the overall burst event in this sample of galaxies. In the fifteen galaxies that show ongoing bursts, the final durations may be longer than we report here. One galaxy shows a burst that has been ongoing for only 20 Myr; we are likely seeing the beginning of a burst event in this system. Using the duration of the starbursts, we calculate that the bursts deposited 1053.9--10 57.2 erg of energy into the interstellar medium through stellar winds and supernovae and produced 3.2%--26% of the host galaxy's mass. We also explore two other metrics for identifying starbursts: iv the gas consumption timescale and the strength of Halpha emission produced by the burst. Interestingly, four galaxies classified as starbursts in our most recent time bin of 4-10 Myr show non-starburst levels of Halpha emission from the last ∼5 Myr indicating that, while the bursts are long-lasting events, the SFR can change on timescales of only a few Myr.