Triggering nuclear and galaxy activity in the Bullet cluster^⋆

¹ Agenzia Spaziale Italiana-Unita’ di Ricerca Scientifica, Via del Politecnico, 00133 Roma, Italy
e-mail: simonetta.puccetti@asi.it
² INAF Osservatorio Astronomico di Trieste, Via G. Tiepolo 11, Trieste, Italy
³ INAF Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy
⁴ Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
⁵ Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, Giessenbachstr., 85741 Garching, Germany
⁶ OmegaLambdaTec GmbH, Lichtenbergstr. 8, 85748 Garching, Germany
⁷ Department for Astrophysics, University of Vienna, Türkenschanzstr. 17, 1180 Vienna, Austria

Received: 8 June 2018
Accepted: 28 November 2019

Abstract

The analysis of a cluster environment is a valuable instrument to investigate the origin of gas fuelling and trigger mechanisms in active galactic nuclei (AGN) and star-forming galaxies. To this purpose, we present a detailed analysis of the point-like X-ray sources in the Bullet cluster field. Based on ∼600 ks Chandra observations, we produced a catalogue of 381 X-ray point sources up to a distance of ∼1.5 virial radius and with flux limits ∼1 × 10⁻¹⁶ and ∼8 × 10⁻¹⁶ erg cm⁻² s⁻¹ in the 0.5–2 keV and 2–10 keV bands, respectively. We find a strong (up to a factor 1.5–2) and significant (≥4σ) over-density in the full region studied 0.3R₂₀₀ <  R <  1.5R₂₀₀. We identified optical (R band) and infrared (Spitzer IRAC) counterparts for ∼84% and ∼48% of the X-ray sources, respectively. We obtained new spectroscopic redshifts for 106 X-ray sources and collected from the literature additional 13 spectroscopic and 8 photometric redshifts of X-ray sources. Twenty-nine X-ray sources turned out to be cluster members. Spectroscopic and photometric redshifts of optical and infrared sources were also collected, and these sources were used as ancillary samples. We used these multi-wavelength data to characterise the nature of the Bullet cluster X-ray point sources. We find that the over-density in the region 0.3R₂₀₀ <  R <  R₂₀₀ is likely due to X-ray AGN (mostly obscured) and star-forming galaxies associated with the cluster, while in the more external region this over-density is likely to be mostly due to background AGN. The fraction of cluster galaxies hosting an X-ray detected AGN is 1.0 ± 0.4%, which is nearly constant with the radius; this fraction is similar to that reported in other clusters of galaxies at similar redshift. The fraction of X-ray bright AGN (L_{2 − 10 keV} >  10⁴³ ergs s⁻¹) in the region 0.3R₂₀₀ <  R <  R₂₀₀ is 0.5_−0.2^+0.6%, which is higher than that in other clusters at similar redshift and more similar to the AGN fraction in the field. Finally, the spatial distributions of AGN and star-forming galaxies, which we selected for their infrared emission, appear similar, thus suggesting that both are triggered by the same mechanism.