The 700 ks Chandra Spiderweb Field I: evidence for widespread nuclear activity in the Protocluster

Abstract

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/cm 2 in the soft (0.5-2.0 keV) and hard (2-10 keV) band, respectively. We searched for counterparts in the optical, near-infrared (NIR), 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 Mpc 2 rectangular region. An Xray spectral analysis for all the sources within the protocluster shows that their intrinsic spectral slope is consistent with an average Γ ∼ 1.84 ± 0.04. Excluding the Spiderweb Galaxy, the best-fit intrinsic absorption for five protocluster X-ray members is N H > 10 23 cm −2 , while another six have upper limits of the order of a few times 10 22 cm −2 . Two sources can only be fitted with very flat Γ ≤ 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 × 10 43 erg/s. 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 10 43 < L X < 10 44.5 erg/s 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 L 0.5−10keV > 4 × 10 43 erg/s 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.

The Spiderweb proto-cluster is being magnetized by its central radio jet

“…McCarthy et al 1990;Chambers et al 1990;van Ojik 1995;Villar-Martín et al 2003;Falkendal et al 2021), so assuming T Lyα ∼ 10 4 K, the Lyα-emitting gas will then have a pressure P Lyα = n e kT = 5.5 × 10 −11 dyne cm −2 . The most plausible confining agent is a hot X-ray-emitting cluster gas, which has now been detected by Tozzi et al (2022). They derive P hot ∼ 8.2 × 10 −11 dyne cm −2 (Section 3.1.1), which is within 50% of the P Lyα value estimated above -consistent with a state of pressure equilibrium within the substantial uncertainties, and a viable candidate for the Faraday-active Fig.…”

“…The star formation rate is also an order of magnitude greater than present-day levels (Madau & Dickinson 2014), galaxy clusters are still in the process of forming (e.g. Muldrew et al 2015;Tozzi et al 2022), and magnetic fields may not yet fully permeate the intergalactic gas (Donnert et al 2018), which can affect its viscosity, pressure support, and thermal conductivity. Therefore, to better understand how super-massive black holes have shaped the cosmic ecology, it is desirable to observe radio galaxies in nascent galaxy clusters at high redshift, using techniques that illuminate the locus of interaction between the jet and ambient gas, and probe the physics occurring therein.…”

“…High resolution, broadband, full-polarisation radio observations dis not previously exist, but represent a singular probe of magnetised gas physics in and around the radio jet, and may thereby shed new light on jet-gas interactions in this archetypal protocluster system. We therefore undertook such observations using the Jansky Very Large Array (VLA), which possess sub-arcsecond resolution and span 340 MHz to 36 GHz, as well as a 700 ksec Chandra exposure, as part of a definitive new study of the Spiderweb system (Carilli et al 2022;Tozzi et al 2022). In this paper, we present an analysis of polarization and Faraday rotation in the Spiderweb system, in order to better understand the properties of the radio jet, the surrounding proto-cluster medium, and interactions between the two, at an epoch where such interactions may peak in their cosmological importance.…”

“…Dissecting the faint, extended emission in three components, has been possible thanks to the 700 ks Chandra Large Program observation with ACIS-S granted in Cycle 20 (PI P. Tozzi). The details of the ICM, Inverse Compton and AGN X-ray component separation are presented in Tozzi et al (2022), while the X-ray emission associated to the radio jets is discussed in detail in Carilli et al (2022).…”

“…Unfortunately, the faintness of this emission, coupled with the uncertainty inherent in the AGN subtraction, means that a detailed radial brightness profile could not be measured for the gas. To first order however, the ICM appears to be roughly isotropic, with a characteristic radius of ∼ 100 kpc, a volume averaged thermal electron density of n e = (1.51 ± 0.24 ± 0.14) × 10 −2 cm −3 (statistical and systematic uncertainties are quoted, respectively; see Tozzi et al 2022), yielding M ICM = (1.76 ± 0.30 ± 0.17) × 10 12 M , and kT = 3.4 +4 −1.5 keV, meaning P hot ∼ 8.2 × 10 −11 dyne cm −2 . These values are used in calculations appearing in Section 5.…”