HST Grism Observations of Paschen-Line Star-Formation and Dust Attenuation: A Precursor to the JWST Era

We use Paβ (1282 nm) observations from the Hubble Space Telescope (HST) G141 grism to study the star-formation and dust attenuation properties of a sample of 29 low-redshift (z < 0.287) galaxies in the CANDELS Lyα Emission at Reionization (CLEAR) survey. We first compare the nebular attenuation from Paβ/Hα with the stellar attenuation inferred from the spectral energy distribution, finding that the galaxies in our sample are consistent with an average ratio of the continuum attenuation to the nebular gas of 0.44, but with a large amount of excess scatter beyond the observational uncertainties. Much of this scatter is linked to a large variation between the nebular dust attenuation as measured by (space-based) Paβ to (ground-based) Hα to that from (ground-based) Hα/Hβ. This implies there are important differences between attenuation measured from grism-based / wide-aperture Paβ fluxes and the ground-based / slit-measured Balmer decrement. We compare star-formation rates (SFRs) from Paβ to those from dust-corrected UV, and perform a survival analysis to infer a census of Paβ emission implied by both detections and non-detections. We find evidence that galaxies with lower stellar mass have more scatter in their ratio of Paβ to attenuation-corrected UV SFRs. When considering our Paβ detection limits, this observation supports the idea that lower mass galaxies experience “burstier” star-formation histories. Together, these results show that Paβ is a valuable tracer of a galaxy’s SFR, probing different timescales of star-formation and potentially revealing star-formation that is otherwise missed by UV and optical tracers. The capabilities of the James Webb Space Telescope (JWST) NIRSpec spectrograph and NIRCam grism will give both Paβ and Paα around the peak of cosmic star formation at z~2.