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We have used 4000 break and H A indices in combination with SFR/M * derived from emission line flux measurements to constrain the recent star formation histories of galaxies with stellar masses in the range 10 8 -10 10 M . The fraction of the total SFR density in galaxies with ongoing bursts is a strong function of stellar mass, declining from 0.85 at a stellar mass of 10 8 M to 0.25 for galaxies with M * 10 10 M . Low-mass galaxies are not all young. The distribution of half-mass formation times for galaxies with stellar masses less than 10 9 M is broad, spanning the range 1-10 Gyr. The peak-to-trough variation in star formation rate among the bursting population ranges lies in the range 10-25. In low-mass galaxies, the average duration of the bursts is comparable to the dynamical time of the galaxy. Galaxy structure is correlated with estimated burst mass fraction, but in different ways in low-and high-mass galaxies. High-mass galaxies with large burst mass fractions are more centrally concentrated, indicating that bulge formation is at work. In low-mass galaxies, stellar surface densities * decrease as a function of F burst . These results are in good agreement with the observational predictions of Teyssier et al. and lend further credence to the idea that the cuspy halo problem can be solved by energy input from multiple starbursts over the lifetime of the galaxy. We note that there is no compelling evidence for initial mass function variations in the population of star-forming galaxies in the local Universe.
Guinevere Kauffmann (Mon,) studied this question.