Fig. 1.— as a function of the Ha [N ii] EW (a measure of the normalized MB star formation activity in a galaxy; i.e., burst indicator) for all known spiral and irregular galaxies with B ! 15 and in the 11HUGS sample. We FbF 1 20 take starbursts to be those galaxies with Ha EW 1 100 A. The dashed line indicates the completeness limit of the survey at 11 Mpc. Galaxies without EW measurements are plotted in gray. An open question in galaxy evolution research is whether the star formation histories (SFHs) 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. These parameters determine the efficacy of starbursts as critical sinks of fuel and impulsive sources of energy, and are essential in our understanding of the role of starbursts in such varied phenomena as the galaxy mass-metallicity relationship (Tremonti et al. 2004; H. Lee et al. 2006), the presence of metals in the intergalactic medium (e.g., Martin 1999; Garnett 2002), and the existence of (and possible relationship between) earlyand late-type dwarf galaxies (e.g., Dekel & Silk 1986; Skillman & Bender 1995; Gil de Paz & Madore 2005). Our approach here is to directly tally the number of bursting dwarfs in a deep, 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 colors from the literature, integrated Ha equivB V alent widths (EWs) from 11HUGS (the 11 Mpc Ha UV Galaxy Survey), and stellar evolutionary synthesis models in order to place constraints on the average starburst duty cycle. Our 11HUGS GALEX Legacy program is providing narrowband Ha and UV imaging for an essentially volume-limited, statistical sample of∼400 star-forming galaxies within 11 Mpc of the Milky Way. While the UV component of the survey is ongoing, the Ha component has been completed (Lee et al. 2006; R. Kennicutt et al. 2007, in preparation). The 11HUGS Ha data set yields measurements of the star formation rates (SFRs) and birthrates (current SFR normalized by past average SFR) over the past few million years and forms the basis for this analysis. To carry out our starburst-counting experiment, we require an objective method to distinguish between bursting and nonbursting low-mass galaxies. We use the integrated Ha emission line EW—a measure of the normalized star formation activity in a galaxy—as a burst indicator and examine its distribution with two tracers of the galaxy mass, the blue luminosity (Fig. 1) and the maximum circular rotational velocity of the disk. Such EWmagnitude diagrams are cousins of the now widely used galaxy color-magnitude diagrams and have great value in revealing the nature of star formation along the Hubble sequence. We briefly comment on the features of the galaxy distribution in Figure 1, before reporting our results on dwarf galaxy starbursts. Galaxies in the local universe outline clear sequences in the -EW and -EW planes that exhibit two characteristic tranM V B max sitions. One occurs at and km s , where M ∼ 19 V ∼ 120 B max the more luminous, massive galaxies tend to have Sa–Sb bulgedominated morphologies and EWs that turn off toward lower values. The EW distribution of galaxies in this regime has a dispersion that is 50% larger than that of the intermediate-mass systems below the transition. The systematic drop of the EW