HST Hα grism spectroscopy of ROLES: A flatter low-mass slope for the z ~ 1 SSFR-mass relation

We present measurements of star formation rates (SFRs) for dwarf galaxies (M* ~ 108.5M⊙) at z ~ 1 using near-infrared slitless spectroscopy from the Hubble Space Telescope (HST) by targeting and measuring the luminosity of the Ha emission line. Our sample is derived from the Redshift One LDSS3 Emission Line Survey (ROLES), which used [OII]λ3727 as a tracer of star formation to target very low stellar masses down to very low SFRs (~0.1M⊙ yr-1) at this epoch. Dust corrections are estimated using SED fits and we find, by comparison with other studies using Balmer decrement dust corrections, that we require a smaller ratio between the gas phase and stellar extinction than the nominal Calzetti relation, in agreement with recent findings by other studies. By stacking the Wide-Field Camera (WFC)3 spectra at the redshifts obtained from ground-based [O II] detections, we are able to push the WFC3 spectra to much lower SFRs and obtain the most complete spectroscopic measurement of the low-mass end of the SSFR-mass relation to date. We measure a flatter low-mass power-law slope (-0.47 ± 0.04) than found by other (shallower) Hα-selected samples (≈-1), although still somewhat steeper than that predicted by the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulation (-0.14 ± 0.05), hinting at possible missing physics not modelled by EAGLE or remaining incompleteness for our Hα data.