Constraints on Fluctuating Star Formation Rates for Intermediate-mass Galaxies with Hα and UV Luminosities

We study the recent star formation histories (SFHs) of 575 intermediate-mass galaxies (IMGs, 10(9) M/M(?)10(10)) in COSMOS at 0.3 < z < 0.4 by comparing their H alpha and UV luminosities. These two measurements trace star formation rates (SFRs) on different timescales and together reveal fluctuations in recent activity. We compute LH alpha from Magellan IMACS spectroscopy, while LUV is derived from rest-frame 2800.1 photometry. Dust corrections are applied to each band independently. We compare the deviation of L-H alpha and L-UV from their respective star-forming sequences (i.e., delta log L-Ha and delta log L-UV), and after accounting for observational uncertainties we find a small intrinsic scatter between the two quantities (sigma(delta) ? 0.03 dex). This crucial observational constraint precludes strong fluctuations in the recent SFHs of IMGs: simple linear SFH models indicate that a population of IMGs would be limited to only factors of ?2 change in SFR over 200 Myr and ?30% on shorter timescales of 20 Myr. No single characteristic SFH for IMGs, such as an exponentially rising/falling burst, can reproduce the individual and joint distribution of delta log L-Ha and delta log L-UV. Instead, an ensemble of SFHs is preferred. Finally, we find that IMG SFHs predicted by recent hydrodynamic simulations, in which feedback drives rapid and strong SFR fluctuations, are inconsistent with our observations.