Constraints on Fluctuating Star Formation Rates for Intermediate-mass Galaxies with H$\alpha$ and UV Luminosities

Abstract

We study the recent star formation histories (SFHs) of 575 intermediate-mass galaxies (IMGs, $10^9\le M/M_{\odot }\le 10^{10}$) 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 $L_{\mathrm{H}\alpha }$ from Magellan IMACS spectroscopy while $L_{\mathrm{UV}}$ is derived from rest-frame 2800 $\text{A˚}$ photometry. Dust corrections are applied to each band independently. We compare the deviation of $L_{\mathrm{H}\alpha }$ and $L_{\mathrm{UV}}$ from their respective star forming sequences (i.e., $\Delta \log L_{\mathrm{H}\alpha }$ and $\Delta \log L_{\mathrm{UV}}$) and after accounting for observational uncertainties we find a small intrinsic scatter between the two quantities (${\sigma }_{\delta }\lesssim 0.03$ 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 $\lesssim 2$ change in SFR over $200$ Myr and $\lesssim 30\%$ 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 $\Delta \log L_{\mathrm{H}\alpha }$ and $\Delta \log L_{\mathrm{UV}}$. 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.

15 pages, 9 figures, Accepted to ApJ

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