THE FAINT END OF THE CLUSTER-GALAXY LUMINOSITY FUNCTION AT HIGH REDSHIFT

We measure the faint-end slope of the galaxy luminosity function (LF) for cluster galaxies at 1 < z < 1.5 using Spitzer IRAC data. We investigate whether this slope, [alpha], differs from that of the field LF at these redshifts, and with the cluster LF at low redshifts. The latter is of particular interest as low-luminosity galaxies are expected to undergo significant evolution. We use seven high-redshift spectroscopically confirmed galaxy clusters drawn from the IRAC Shallow Cluster Survey to measure the cluster-galaxy LF down to depths of M* +3 (3.6 [mu]m) and M* + 2.5 (4.5 [mu]m). The summed LF at our median cluster redshift (z = 1.35) is well fit by a Schechter distribution with [alpha] sub(3.6)[mu]m = -0.97 + or - 0.14 and [alpha] sub(4.5)[mu]m = -0.91 + or - 0.28, consistent with a flat faint-end slope and is in agreement with measurements of the field LF in similar bands at these redshifts. A comparison to [alpha] in low-redshift clusters finds no statistically significant evidence of evolution. Combined with past studies which show that M* is passively evolving out to z ~ 1.3, this means that the shape of the cluster LF is largely in place by z ~ 1.3. This suggests that the processes that govern the buildup of the mass of low-mass cluster galaxies have no net effect on the faint-end slope of the cluster LF at z [lap] 1.3.