The Spitzer High Redshift Radio Galaxy Survey

TL;DR

This study uses Spitzer imaging to analyze high-redshift radio galaxies, revealing their stellar masses, dust emission, and possible merger activity, and explores the relationship between radio and infrared properties across cosmic time.

Contribution

It provides the first comprehensive infrared survey of high-redshift radio galaxies, measuring stellar masses, dust emission, and jet orientation effects, and suggests a link between mergers and radio activity.

Findings

Median stellar mass ~2x10^11 M_sun at 1<z<3

Evidence for decreased stellar mass at z>3

Correlation between hot dust luminosity and radio core dominance

Abstract

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