The MOSDEF Survey: The Dependence of H$\alpha$-to-UV SFR Ratios on SFR and Size at $z\sim2$

TL;DR

This study compares dust-corrected Hα and UV star formation rates in z~2 galaxies, revealing that Hα-based SFRs tend to be higher in galaxy centers, suggesting patchy dust distribution and inside-out growth.

Contribution

It provides a detailed aperture-matched analysis of Hα and UV SFRs, highlighting the impact of dust distribution and galaxy size on SFR measurements at high redshift.

Findings

Hα-to-UV SFR ratios increase with galaxy SFR.

Hα SFRs are higher in galaxy centers, indicating inside-out growth.

Dust distribution is patchier in high-SFR galaxies.

Abstract

We perform an aperture-matched analysis of dust-corrected H$\alpha$ and UV SFRs using 303 star-forming galaxies with spectroscopic redshifts $1.36<z_\text{spec}<2.66$ from the MOSFIRE Deep Evolution Field (MOSDEF) survey. By combining H$\alpha$ and H$\beta$ emission line measurements with multi-waveband resolved CANDELS/3D-HST imaging, we directly compare dust-corrected H$\alpha$ and UV SFRs, inferred assuming a fixed attenuation curve shape and constant SFHs, within the spectroscopic aperture. Previous studies have found that H$\alpha$ and UV SFRs inferred with these assumptions generally agree for typical star-forming galaxies, but become increasingly discrepant for galaxies with higher SFRs ($\gtrsim$100 M$_\odot$ yr$^{-1}$), with H$\alpha$-to-UV SFR ratios being larger for these galaxies. Our analysis shows that this trend persists even after carefully accounting for the apertures over which H$\alpha$ and UV-based SFRs (and the nebular and stellar continuum reddening) are derived. Furthermore, our results imply that H$\alpha$ SFRs may be higher in the centers of large galaxies (i.e., where there is coverage by the spectroscopic aperture) compared to their outskirts, which could be indicative of inside-out galaxy growth. Overall, we suggest that the persistent difference between nebular and stellar continuum reddening and high H$\alpha$-to-UV SFR ratios at the centers of large galaxies may be indicative of a patchier distribution of dust in galaxies with high SFRs.