FIRST RESULTS FROM THE FAINT INFRARED GRISM SURVEY (FIGS): FIRST SIMULTANEOUS DETECTION OF Ly alpha EMISSION AND LYMAN BREAK FROM A GALAXY AT z=7.51

摘要

Galaxies at high redshifts are a valuable tool for studying cosmic dawn, therefore it is crucial to reliably identify these galaxies. Here, we present an unambiguous and first simultaneous detection of both the Ly alpha emission and the Lyman break from a z = 7.512 +/- 0.004 galaxy, observed in the Faint Infrared Grism Survey (FIGS). These spectra, taken with the G102 grism on the Hubble Space Telescope (HST), show a significant emission line detection (6 sigma) in two observational position angles (PAs), with Lya line flux of 1.06 +/- 0.19 x 10(-17) erg s(-1) cm(-2). The line flux is nearly a factor of four higher than that in the archival MOSFIRE spectroscopic observations. This is consistent with other recent observations, implying that ground-based near-infrared spectroscopy underestimates the total emission line fluxes, and if confirmed, can have strong implications for reionization studies that are based on ground-based Ly alpha measurements. A 4 sigma detection of the NV line in one PA also suggests a weak active galactic nucleus (AGN), and if confirmed, would make this source the highest-redshift AGN yet found. These observations from HST thus clearly demonstrate the sensitivity of the FIGS survey, and the capability of grism spectroscopy for studying the epoch of reionization.