Galaxy Clusters around Radio-loud Active Galactic Nuclei at 1.3 lt z lt 3.2 as seen by Spitzer

D. Wylezalek, A. Galametz, D. Stern, J. Vernet, C. De Breuck, N. Seymour, M. Brodwin, P.R.M. Eisenhardt, A.H. Gonzalez, N. Hatch, M.J. Jarvis, A. Rettura, S. A. Stanford, J.A. Stevens

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We report the first results from the Clusters Around Radio-Loud AGN program, a Cycle 7 and 8 Spitzer Space Telescope snapshot program to investigate the environments of a large sample of obscured and unobscured luminous radio-loud active galactic nuclei (AGNs) at 1.2 < z < 3.2. These data, obtained for 387 fields, reach 3.6 and 4.5 μm depths of [3.6]AB = 22.6 and [4.5]AB = 22.9 at the 95% completeness level, which is two to three times fainter than L* in this redshift range. By using the color cut [3.6] – [4.5] > –0.1 (AB), which efficiently selects high-redshift (z > 1.3) galaxies of all types, we identify galaxy cluster member candidates in the fields of the radio-loud AGN. The local density of these Infrared Array Camera (IRAC)-selected sources is compared to the density of similarly selected sources in blank fields. We find that 92% of the radio-loud AGN reside in environments richer than average. The majority (55%) of the radio-loud AGN fields are found to be overdense at a ≥2σ level; 10% are overdense at a ≥5σ level. A clear rise in surface density of IRAC-selected sources toward the position of the radio-loud AGN strongly supports an association of the majority of the IRAC-selected sources with the radio-loud AGN. Our results provide solid statistical evidence that radio-loud AGN are likely beacons for finding high-redshift galaxy (proto-)clusters. We investigate how environment depends on AGN type (unobscured radio-loud quasars versus obscured radio galaxies), radio luminosity and redshift, finding no correlation with either AGN type or radio luminosity. We find a decrease in density with redshift, consistent with galaxy evolution for this uniform, flux-limited survey. These results are consistent with expectations from the orientation-driven AGN unification model, at least for the high radio luminosity regimes considered in this sample
Original languageEnglish
Article number79
Number of pages10
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 1 May 2013


  • galaxies: active, galaxies: clusters: general, galaxies: high-redshift, infrared: galaxies, techniques: photometric


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