Triggering nuclear and galaxy activity in the Bullet cluster

Abstract

The analysis of the cluster environment is a valuable instrument to investigate the origin of AGN and star-forming galaxies gas fuelling and trigger mechanisms. To this purpose, we present a detailed analysis of the point-like X-ray sources in the Bullet cluster field. Thanks to $\sim 600$ ks Chandra observations, we produced a catalogue of 381 X-ray point sources up to a distance of $\sim$1.5 virial radius and with flux limits $\sim 1\times 10^{-16}$ and $\sim 8\times 10^{-16}$ erg cm${}^{-2}$ s${}^{-1}$ in the 0.5-2 keV and 2-10 keV bands, respectively. We found a strong (up to a factor 1.5-2) and significant ($\ge$4$\sigma$) over-density in the full region studied $0.3R_{200}<R<1.5R_{200}$. We identified optical and infrared counterparts for $\sim$84% and $\sim$48% of the X-ray sources, respectively. We obtained new spectroscopic redshifts for 106 X-ray sources. Spectroscopic and photometric redshifts of optical and infrared sources have been also collected, and these sources were used as ancillary samples. We find that the over-density in the region $0.3R_{200}<R<R_{200}$ is likely due to X-ray AGN (mostly obscured) and star-forming galaxies both associated to the cluster, while in the more external region it is likely mostly due to background AGN. The fraction of cluster galaxies hosting an X-ray detected AGN is 1.0$\pm$0.4$\%$, nearly constant with the radius, a fraction similar to that reported in other clusters of galaxies at similar redshift. The fraction of X-ray bright AGN (L${}_{2-10keV}$$>$10${}^{43}$ ergs s${}^{-1}$) in the region $0.3R_{200}<R<R_{200}$ is $0.5_{-0.2}^{+0.6}$$\%$, 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, selected also thanks to their infrared emission, appear similar, thus suggesting that both are triggered by the same mechanism.

28 pages, accepted for publication in the A&A

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