The 700 ks Chandra Spiderweb Field

II. Evidence for inverse-Compton and thermal diffuse emission in the Spiderweb galaxy

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi, 50122 Firenze, Italy
e-mail: paolo.tozzi@inaf.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio, Via Piero Gobetti 93/3, 40129 Bologna, Italy
³ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁴ Astronomy Unit, Department of Physics, University of Trieste, Via Tiepolo 11, 34131 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, 34143 Trieste, Italy
⁶ IFPU – Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
⁷ INFN – Sezione di Trieste, via Valerio 2, 34127 Trieste, Italy
⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monteporzio (RM), Italy
⁹ National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
¹⁰ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands
¹¹ European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
¹² Dipartimento di Fisica e Scienze della Terra, Università degli Studi di Ferrara, Via Saragat 1, 44122 Ferrara, Italy
¹³ Jansky Fellow of the National Radio Astronomy Observatory, PO Box 0, Socorro, NM 87801, USA
¹⁴ Instituto de Astrofísica de Canarias (IAC), 38205 La Laguna Tenerife, Spain
¹⁵ Universidad de La Laguna, Dpto. Astrofísica, 38206 La Laguna Tenerife, Spain
¹⁶ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
¹⁷ Cosmic Dawn Center, Rådmandsgade 62, 2200 København N, Denmark
¹⁸ DTU-Space, Technical University of Denmark, Elektrovej 327, 2800 Kgs. Lyngby, Denmark
¹⁹ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21210, USA
²⁰ Johns Hopkins University, 3400 N. Charles Street, Baltimore, MD 21218, USA

Received: 23 June 2022
Accepted: 31 August 2022

Abstract

Aims. We present the X-ray imaging and spectral analysis of the diffuse emission around the radio galaxy J1140-2629 (the Spiderweb galaxy) at z = 2.16 and of its nuclear emission, based on a deep (700 ks) Chandra observation.

Methods. We obtained a robust characterization of the unresolved nuclear emission, and carefully computed the contamination in the surrounding regions due to the wings of the instrument point spread function. Then, we quantified the extended emission within a radius of 12 arcsec. We used the Jansky Very Large Array radio image to identify the regions overlapping the jets, and performed X-ray spectral analysis separately in the jet regions and in the complementary area.

Results. We find that the Spiderweb galaxy hosts a mildly absorbed quasar, showing a modest yet significant spectral and flux variability on a timescale of ∼1 year (observed frame). We find that the emission in the jet regions is well described by a power law with a spectral index of Γ ∼ 2 − 2.5, and it is consistent with inverse-Compton upscattering of the cosmic microwave background photons by the relativistic electrons. We also find a roughly symmetric, diffuse emission within a radius of ∼100 kpc centered on the Spiderweb galaxy. This emission, which is not associated with the jets, is significantly softer and consistent with thermal bremsstrahlung from a hot intracluster medium (ICM) with a temperature of kT = 2.0_−0.4^+0.7 keV, and a metallicity of Z <  1.6 Z_⊙ at 1σ c.l. The average electron density within 100 kpc is n_e = (1.51  ±  0.24 ± 0.14) × 10⁻² cm⁻³, corresponding to an upper limit for the total ICM mass of ≤(1.76  ±  0.30  ±  0.17) × 10¹² M_⊙ (where error bars are 1σ statistical and systematic, respectively). The rest-frame luminosity L_{0.5 − 10 keV} = (2.0 ± 0.5) × 10⁴⁴ erg s⁻¹ is about a factor of 2 higher than the extrapolated L − T relation for massive clusters, but still consistent within the scatter. If we apply hydrostatic equilibrium to the ICM, we measure a total gravitational mass M(<100 kpc) = (1.5_−0.3^+0.5) × 10¹³ M_⊙ and, extrapolating at larger radii, we estimate a total mass M₅₀₀ = (3.2_−0.6^+1.1) × 10¹³ M_⊙ within a radius of r₅₀₀ = (220 ± 30) kpc.

Conclusions. We conclude that the Spiderweb protocluster shows significant diffuse emission within a radius of 12 arcsec, whose major contribution is provided by inverse-Compton scattering associated with the radio jets. Outside the jet regions, we also identified thermal emission within a radius of ∼100 kpc, revealing the presence of hot, diffuse baryons that may represent the embryonic virialized halo of the forming cluster.