High Resolution HST Imaging Survey of Local Star-Forming Galaxies I: Spatially-Resolved Obscured Star Formation with H$\alpha$ and Paschen-$\beta$ Recombination Lines

Abstract

We present a sample of 24 local star-forming galaxies observed with broad- and narrow-band photometry from the Hubble Space Telescope, that are part of the GOALS survey of local luminous and ultra-luminous infrared galaxies. With narrow-band filters around the emission lines H$\alpha$ (and [NII]) and Pa$\beta$, we obtain robust estimates of the dust attenuation affecting the gas in each galaxy, probing higher attenuation than can be traced by the optical Balmer decrement H$\alpha$/H$\beta$ alone by a factor of $>1$ mag. We also infer the dust attenuation towards the stars via a spatially-resolved SED-fitting procedure that uses all available HST imaging filters. We use various indicators to obtain the star formation rate (SFR) per spatial bin, and find that Pa$\beta$ traces star-forming regions where the H$\alpha$ and the optical stellar continuum are heavily obscured. The dust-corrected Pa$\beta$ SFR recovers the 24$\mu$m-inferred SFR with a ratio $-0.14\pm0.32$ dex and the SFR inferred from the $8\mathrm{-}1000\,\mu\mathrm{m}$ infrared luminosity at $-0.04\pm0.23$ dex. Both in a spatially-resolved and integrated sense, rest-frame near infrared recombination lines can paint a more comprehensive picture of star formation across cosmic time, particularly with upcoming JWST observations of Paschen-series line emission in galaxies as early as the epoch of reionization.