  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.

Astrophysics