THE<i>SPITZER</i>HIGH-REDSHIFT RADIO GALAXY SURVEY

Abstract

We present results from a comprehensive imaging survey of 70 radio galaxies\nat redshifts 1&lt;z&lt;5.2 using all three cameras onboard the Spitzer Space\nTelescope. The resulting spectral energy distributions unambiguously show a\nstellar population in 46 sources and hot dust emission associated with the\nactive nucleus in 59. Using a new restframe S_3um/S_1.6um versus S_um/S_3um\ncriterion, we identify 42 sources where the restframe 1.6um emission from the\nstellar population can be measured. For these radio galaxies, the median\nstellar mass is high, 2x10^11 M_sun, and remarkably constant within the range\n1&lt;z&lt;3. At z&gt;3, there is tentative evidence for a factor of two decrease in\nstellar mass. This suggests that radio galaxies have assembled the bulk of\ntheir stellar mass by z~3, but confirmation by more detailed decomposition of\nstellar and AGN emission is needed.\n The restframe 500 MHz radio luminosities are only marginally correlated with\nstellar mass but are strongly correlated with the restframe 5um hot dust\nluminosity. This suggests that the radio galaxies have a large range of\nEddington ratios. We also present new Very Large Array 4.86 and 8.46 GHz\nimaging of 14 radio galaxies and find that radio core dominance --- an\nindicator of jet orientation --- is strongly correlated with hot dust\nluminosity. While all of our targets were selected as narrow-lined, type 2\nAGNs, this result can be understood in the context of orientation-dependent\nmodels if there is a continuous distribution of orientations from obscured type\n2 to unobscured type 1 AGNs rather than a clear dichotomy. Finally, four radio\ngalaxies have nearby (&lt;6") companions whose mid-IR colors are suggestive of\ntheir being AGNs. This may indicate an association between radio galaxy\nactivity and major mergers.\n