The 700 ks Chandra Spiderweb Field: I. Evidence for widespread nuclear activity in the protocluster

P. Tozzi, L. Pentericci, R. Gilli, M. Pannella, F. Fiore, G. Miley, M. Nonino, H. J.A. Röttgering, V. Strazzullo, C. S. Anderson, S. Borgani, A. Calabrò, C. Carilli, H. Dannerbauer, L. Di Mascolo, C. Feruglio, R. Gobat, S. Jin, A. Liu, T. MroczkowskiC. Norman, E. Rasia, P. Rosati, A. Saro

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Aims. We present an analysis of the 700 ks Chandra ACIS-S observation of the field around the radio galaxy J1140-2629 (the Spiderweb Galaxy) at z = 2:156, focusing on the nuclear activity in the associated large-scale environment. Methods. We identified unresolved X-ray sources in the field down to flux limits of 1:3 × 10-16 and 3:9 × 10-16 erg s-1 cm in the soft (0.5 2.0 keV) and hard (2 10 keV) band, respectively. We searched for counterparts in the optical, near-infrared, and submillimeter catalogs available in the literature to identify X-ray sources belonging to the protocluster and derived their X-ray properties. Results. We detect 107 X-ray unresolved sources within 5 arcmin (corresponding to 2.5 Mpc) of J1140-2629, among which 13 have optical counterparts with spectroscopic redshift 2:11 < z < 2:20, and 1 source has a photometric redshift consistent with this range. The X-ray-emitting protocluster members are distributed approximately over a ∼3:2 × 1:3 Mpc2 rectangular region. An X-ray spectral analysis for all the sources within the protocluster shows that their intrinsic spectral slope is consistent with an average hGi ∼ 1:84 ± 0:04. Excluding the Spiderweb Galaxy, the best-fit intrinsic absorption for five protocluster X-ray members is NH > 1023 cm-2, while another six have upper limits of the order of a few times 1022 cm-2. Two sources can only be fitted with very flat G = 1, and are therefore considered Compton-thick candidates. The 0.5 10 keV rest-frame luminosities of the 11 Compton-thin protocluster members corrected for intrinsic absorption are greater than 2 × 1043 erg s-1. These values are typical for the bright end of a Seyfert-like distribution and significantly greater than X-ray luminosities expected from star formation activity. The X-ray luminosity function of the AGN in the volume associated to the Spiderweb protocluster in the range 1043 < LX < 1044:5 erg s-1 is at least ten times higher than that in the field at the same redshift and significantly flatter, implying an increasing excess at the bright end. The X-ray AGN fraction is measured to be 25:5 ± 4:5% of the spectroscopically confirmed members in the stellar mass range log(M*=M) > 10:5. This value corresponds to an enhancement factor of 6:0+9:0-3:0 for the nuclear activity with L0:5-10 keV > 4 × 1043 erg s-1 with respect to the COSMOS field at comparable redshifts and stellar mass range. Conclusions. We conclude that the galaxy population in the Spiderweb protocluster is characterized by enhanced X-ray nuclear activity triggered by environmental effects on megaparsec scales.

Idioma originalInglés
Número de artículoA54
PublicaciónAstronomy and Astrophysics
EstadoPublicada - 1 jun. 2022


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