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

We present a catalog of spectro-photometric 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 A and 9500 A are supplemented with U, J, H, and Ks from various facilities, resulting in redshifts computed with ~40 spectral bins per galaxy. For 81 galaxies between 0.5<z<1.5 with spectroscopic redshifts, the standard deviation in the fractional error in (1+z) is 0.046. With this catalog, we compute the B-band luminosity function in this redshift range from 72 galaxies. Owing to the depth of the GRAPES survey, we are able to accurately constrain the faint-end slope by going to M_B~-18 mag at 0.8<z<1.2, nearly two magnitudes fainter than previous studies. The faint-end slope is alpha=-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.