Multiple AGN activity during the BCG assembly of XDCPJ0044.0-2033 at z ∼ 1.6

Abstract

Undisturbed galaxy clusters are characterized by a massive and large elliptical galaxy at their center, i.e. the Brightest Cluster Galaxy (BCG). How these central galaxies form is still debated. According to most models, a typical epoch for their assembly is z∼1-2. We have performed a detailed multi-wavelength analysis of the core of XDCP J0044.0-2033 (XDCP0044), one of the most massive and densest galaxy clusters currently known at redshift z∼1.6, whose central galaxy population shows high star formation compared to lower-z clusters and an X-ray AGN located close to its center. SINFONI J-, H- and KMOS YJ-, H- bands spectroscopic data have been analyzed, together with deep archival HST photometric data in F105W, F140W, and F160W bands, Chandra X-ray, radio JVLA data at 1-2 GHz, and ALMA band-6 observations. In the very central region of the cluster (∼70 kpc × 70 kpc), two systems of interacting galaxies have been identified and studied (Complex A and B), with a total of seven confirmed cluster members. These galaxies show perturbed morphologies and three of them show signs of AGN activity. In particular, two type-1 AGN with typical broad lines have been found at the center of each complex (both of them X-ray obscured and highly accreting with $\rm \lambda _{Edd}\sim 0.4-0.6$), while a type-2 AGN has been discovered in Complex A. The AGN at the center of Complex B is also detected in X-ray while the other two are spatially related to radio emission. The three AGN provide one of the closest AGN triple at z>1 revealed so far with a minimum (maximum) projected distance of 10 (40) kpc. The observation of high star formation, merger signatures and nuclear activity in the core of XDCP0044 suggests that all these processes are key ingredients in shaping the nascent BCG. According to our data, XDCP0044 could form a typical massive galaxy of $\rm M_{\star }\sim 10^{12} M_{\odot }$, hosting a Black Hole (BH) of $\rm 2 \times 10^8 - 10^9 M_{\odot }$, in a time scale of the order of ∼2.5 Gyrs.

Mass assembly and active galactic nucleus activity at z ≳ 1.5 in the dense environment of XDCP J0044.0–2033

“…This can be seen as a lower limit, considering that we find nine sources identified in HST images. This region is denser than the core region of XDCP0044 (Travascio et al 2020), which shows a surface number density of one order of magnitude lower, and is also even denser than the core region of the Spiderweb protocluster at z ∼ 2.156 (Roettgering et al 1994, Pentericci et al 1997, Miley et al 2006, which shows a surface number density of ∼ 7 × 10 −4 kpc −2 (Kuiper et al 2011). Furthermore, the six confirmed cluster members form two different complexes, Complex M and Complex N, which are located at a distance of d ∼ 13 kpc and have a relative velocity of v ∼ 300 − 400 km/s.…”

“…In our analysis, we use the images obtained by combining the archival drizzled (DRZ) frames after performing the astrometry and aligning them. Due to the low signal-to-noise ratio (S/N), the F814W band has not been included (see also Travascio et al 2020). The depth of the three images at 5σ is [28.53, 28.33, 28.18], respectively, for F105W, F140W, and F160W.…”

“…Two additional unresolved X-ray sources may turn out to be additional cluster members, but redshift measurements are not yet available. Furthermore, radio Jansky Very Large Array (JVLA) data reveal the presence of at least four sources emitting in the radio band and extended radio emission associated to Complex A (see Travascio et al 2020). In particular, the core region (∼ 70 kpc × 70 kpc) of XDCP0044, studied by Travascio et al (2020), is the densest among those observed at z > 1.5 and shows the presence of two luminous, obscured, and highly accreting AGNs and an optically obscured Type-2 AGN, with signatures of an ongoing merger.…”

“…Furthermore, radio Jansky Very Large Array (JVLA) data reveal the presence of at least four sources emitting in the radio band and extended radio emission associated to Complex A (see Travascio et al 2020). In particular, the core region (∼ 70 kpc × 70 kpc) of XDCP0044, studied by Travascio et al (2020), is the densest among those observed at z > 1.5 and shows the presence of two luminous, obscured, and highly accreting AGNs and an optically obscured Type-2 AGN, with signatures of an ongoing merger. The presence of multiple AGN activity and high SF in the core of a high-z cluster suggest that these processes have a key role in shaping the nascent BCGs observed at the center of local clusters.…”

“…On the basis of these results, we decided to investigate SF and nuclear activity in another region located ∼ 157 kpc from the core of XDCP0044 that appears to be extremely dense and active as shown by the presence of an unresolved X-ray source at its center, intense SF at the level of SFR=825±120 M ⊙ /yr (source ID 95 in Santos et al 2014), and the presence of a compact radio source with luminosity L 1.5GHz = 2.6 ± 0.3 × 10 24 W/Hz (source r3 in Travascio et al 2020). To perform our study, we used highresolution HST photometric data combined with K-band multiobject spectrometry provided by the ESO Very Large Telescope (VLT) (KMOS IFU) and X-ray Chandra data.…”