Search for Hα Emitters at z 7.8: A Constraint on the Hα-based Star Formation Rate Density

Abstract

We search for Hα emitters at z ~ 7.8 in four gravitationally lensed fields observed in the Hubble Frontier Fields program. We use the Lyman break method to select galaxies at the target redshift and perform photometry in the Spitzer/IRAC 5.8 μm band to detect Hα emission from the candidate galaxies. We find no significant detections of counterparts in the IRAC 5.8 μm band, and this gives a constraint on the Hα luminosity function (LF) at z ~ 7.8. We compare the constraint with previous studies based on rest-frame UV and far-infrared observations using the correlation between the Hα luminosity and the star formation rate. Additionally, we convert the constraint on the Hα LF into an upper limit for the star formation rate density (SFRD) at this epoch assuming the shape of the LF. We examine two types of parameterization of the LF and obtain an upper limit for the SFRD of ${\mathrm{log}}_{10}({\rho }_{\mathrm{SFR}}\ [{M}_{\odot }\,{\mathrm{yr}}^{-1}\ {\mathrm{Mpc}}^{-3}])\lesssim -1.1$ at z ~ 7.8. With this constraint on the SFRD, we present an independent probe into the total star formation activity including dust-obscured and unobscured star formation at the Epoch of Reionization.