HST/WFC3 grism observations of $z\sim1$ clusters: Evidence for rapid outside-in environmental quenching from spatially resolved H$\alpha$ maps

We present and publicly release (https://www.gclasshst.com) the first spatially resolved H$\alpha$ maps of star-forming cluster galaxies at $z\sim1$, made possible with the Wide Field Camera 3 (WFC3) G141 grism on the Hubble Space Telescope (HST). Using a similar but updated method to 3D-HST in the field environment, we stack the H$\alpha$ maps in bins of stellar mass, measure the half-light radius of the H$\alpha$ distribution and compare it to the stellar continuum. The ratio of the H$\alpha$ to stellar continuum half-light radius, $R[\mathrm{H}\alpha/\mathrm{C}]=\frac{R_{\mathrm{eff, H}\alpha}}{R_{\mathrm{eff, Cont}}}$, is smaller in the clusters by $(6\pm9)\%$, but statistically consistent within $1\sigma$ uncertainties. A negligible difference in $R[\mathrm{H}\alpha/\mathrm{C}]$ with environment is surprising, given the higher quenched fractions in the clusters relative to the field. We postulate that the combination of high quenched fractions and no change in $R[\mathrm{H}\alpha/\mathrm{C}]$ with environment can be reconciled if environmental quenching proceeds rapidly. We investigate this hypothesis by performing similar analysis on the spectroscopically-confirmed recently quenched cluster galaxies. 87% have H$\alpha$ detections, with star formation rates $8\pm1$ times lower than star-forming cluster galaxies of similar stellar mass. Importantly, these galaxies have a $R[\mathrm{H}\alpha/\mathrm{C}]$ that is $(81\pm8)\%$ smaller than coeval star-forming field galaxies at fixed stellar mass. This suggests the environmental quenching process occurred outside-in. We conclude that disk truncation due to ram-pressure stripping is occurring in cluster galaxies at $z\sim1$, but more rapidly and/or efficiently than in $z\lesssim0.5$ clusters, such that the effects on $R[\mathrm{H}\alpha/\mathrm{C}]$ become observable just after the cluster galaxy has recently quenched.