Overdensities of 24um Sources in the Vicinities of High-Redshift Radio Galaxies

TL;DR

This study reveals that high-redshift radio galaxies are frequently located in dense, protocluster environments with significant overdensities of 24um sources, indicating active star formation and galaxy clustering at early cosmic times.

Contribution

It provides the first statistical evidence linking HzRGs to protocluster environments using 24um source overdensity analysis from Spitzer data.

Findings

Overdensity of 24um sources around HzRGs with delta=2.2 +/- 1.2

Over 95% of HzRGs are in higher-than-average density fields

32% of fields are overdense at 3sigma significance, including 9 new protocluster candidates

Abstract

We present a statistical study of the environments of 63 high-redshift radio galaxies (HzRGs) between redshifts 1<z<5.2, using the 24um, waveband of the MIPS instrument aboard the Spitzer Space Telescope. Using a counts-in-cell analysis, a statistically significant source overdensity is found in 1.75arcmin radius circular cells centred on the HzRGs when compared to reference fields. We report an average overdensity of delta (= {N}_{targets} / {N}_{reference}) = 2.2 +/- 1.2 at a flux density cut of f24um=0.3mJy. This result implies that HzRGs are likely to lie in protoclusters of active and star-forming galaxies at high redshift. Over 95% of our targeted HzRGs lie in higher than average density fields. Further, 20 (32%) of our selected fields are found to be overdense to at least a 3sigma significance, of which 9 are newly identified protocluster candidates. We observe a weak correlation between redshift and 24um, source density, and discuss the populations being probed at different redshifts. In our uniformly selected sample, which was designed to cover two orders of magnitude in radio luminosity throughout z=1-4, we find that the 24um, source density does not depend on radio luminosity. We also compare this result with recent work describing IRAC source overdensities around the same HzRGs and find correlations between the results.