The Galaxy Luminosity Function at z ≃ 1 in the HUDF: Probing the Dwarf Population

We present a catalog of spectrophotometric redshifts for 1308 galaxies from the Grism ACS Program for Extragalactic Science (GRAPES) observations with the Hubble Space Telescope. These low-resolution spectra between 6000 Å and 9500 Å are supplemented with U, J, H, and Ks data from various facilities, resulting in redshifts computed with ~40 spectral bins per galaxy. For 75 galaxies in the range 0.5 < z < 1.5 with spectroscopic redshifts, the standard deviation in the fractional error in 1 + z is 0.037. With this catalog, we compute the B-band luminosity function in this redshift range from 72 galaxies. Due to the depth of the GRAPES survey, we are able to accurately constrain the faint-end slope by going to MB ≃ -18 mag at z = 1.0 ± 0.2, nearly 2 mag fainter than previous studies. The faint-end slope is α = -1.32 ± 0.07. When compared to numerous published values at various redshifts, we find strong evidence for a steepening of the faint-end slope with redshift, which is expected in the hierarchical formation scenario of galaxies.