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arXiv · 1304.0770

Galaxy Clusters around radio-loud AGN at 1.3 < z < 3.2 as seen by Spitzer

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

We report the first results from the Clusters Around Radio-Loud AGN (CARLA) 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 AGN at 1.2 < z < 3.2. These data, obtained for 387 fields, reach 3.6 and 4.5 um 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 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 > 2 {\sigma} level; 10% are overdense at a > 5 {\sigma} level. A clear rise in surface density of IRAC-selected sources towards 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 vs. 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.

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D. Wylezalek, A. Galametz, D. Stern, J. Vernet, C. De Breuck, N. Seymour, M. Brodwin, P. M. Eisenhardt, A. H. Gonzalez, N. Hatch, M. Jarvis, A. Rettura, S. A. Stanford, J. A. Stevens. 2013-04-02. Galaxy Clusters around radio-loud AGN at 1.3 < z < 3.2 as seen by Spitzer. https://doi.org/10.1088/0004-637x/769/1/79

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