Spectrophotometric Redshifts in the Faint Infrared Grism Survey

A galaxy's redshift is vital for understanding its place in the growth and evolution of the universe. Determination of a galaxy’s redshift generally requires measuring prominent features in the galaxy’s spectrum with known rest-frame wavelengths, such as known strong emission or absorption lines (eg, H$\alpha$) or characteristic breaks. Spectroscopic redshifts maintain high accuracy, but are unable to probe fainter sources. Redshifts for these sources can also be calculated with photometric catalogs, but with a generally less accurate result. Using a technique pioneered by Ryan et al. 2007, one can combine spectra and photometry to yield an accurate spectrophotometric redshift (SPZ) while still measuring faint sources. By taking mid-resolution spectra from the HST Faint Infrared Grism Survey, SPZs can be found for measurements potentially down to 27th magnitude (the typical brightness of a dwarf galaxy at redshift $\sim$1.5), more completely filling the faint-end and high-redshift portions of the luminosity function than before. The improved redshift and distance measurements allowed for the identification of a structure at z=0.83 in one of the FIGS fields.