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Register a new userThe Bursty Star Formation Histories of Low-mass Galaxies at $0.4<z<1$ Revealed by Star Formation Rates Measured from H$beta$ and FUV
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We investigate the burstiness of star formation histories (SFHs) of galaxies at $0.4<z<1$ by using the ratio of star formation rates (SFRs) measured from H$beta$ and FUV (1500 AA) (H$beta$--to--FUV ratio). Our sample contains 164 galaxies down to stellar mass (M*) of $10^{8.5} M_odot$ in the CANDELS GOODS-N region, where Team Keck Redshift Survey DEIMOS spectroscopy and HST/WFC3 F275W images from CANDELS and Hubble Deep UV Legacy Survey are available. When the {it ratio} of H$beta$- and FUV-derived SFRs is measured, dust extinction correction is negligible (except for very dusty galaxies) with the Calzetti attenuation curve. The H$beta$--to--FUV ratio of our sample increases with M* and SFR. The median ratio is $sim$0.7 at M*$sim10^{8.5} M_odot$ (or SFR$sim 0.5 M_odot/yr$) and increases to $sim$1 at M*$sim10^{10} M_odot$ (or SFR $sim 10 M_odot/yr$). At M*$<10^{9.5} M_odot$, our median H$beta$--to--FUV ratio is lower than that of local galaxies at the same M*, implying a redshift evolution. Bursty SFH on a timescale of a few tens of megayears on galactic scales provides a plausible explanation of our results, and the importance of the burstiness increases as M* decreases. Due to sample selection effects, our H$beta$--to--FUV ratio may be an upper limit of the true value of a complete sample, which strengthens our conclusions. Other models, e.g., non-universal initial mass function or stochastic star formation on star cluster scales, are unable to plausibly explain our results.
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We use Pa$beta$ (1282~nm) observations from the Hubble Space Telescope ($textit{HST}$) G141 grism to study the star formation and dust attenuation properties of a sample of 32 low redshift ($z < 0.287$) galaxies in the CLEAR survey. Many of the galaxies in the sample have significantly higher Pa$beta$ emission than expected from the star formation rates (SFRs) measured from their (attenuation-corrected) UV continuum or H$alpha$ emission, suggesting that Pa$beta$ is revealing star formation that is otherwise hidden within gas that is optically thick to UV-continuum and Balmer line emission. Galaxies with lower stellar mass tend to have more scatter in their ratio of Pa$beta$ to attenuation-corrected UV SFRs. When considering our Pa$beta$ detection limits, this observation is consistent with burstier star formation histories in lower mass galaxies. We also find a large amount of scatter between the nebular dust attenuation measured by Pa$beta$/H$alpha$ and H$alpha$/H$beta$, implying that the Balmer decrement underestimates the attenuation in galaxies across a broad range of stellar mass, morphology, and observed Balmer decrement. Comparing the nebular attenuation from Pa$beta$/H$alpha$ with the stellar attenuation inferred from the spectral energy distribution, our galaxies are consistent with an average stellar to nebular ratio of 0.44, but with a large amount of excess scatter beyond the observational uncertainties. Together, these results show that Pa$beta$ is a valuable tracer of a galaxys star formation rate, often revealing star formation that is otherwise missed by UV and optical tracers.
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