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Undisturbed galaxy clusters are characterized by a massive and large elliptical galaxy at their centre, 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 multiwavelength analysis of the core of XMM–Newton Distant Cluster Project (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 active galactic nuclei (AGN) located close to its centre. SINFONI J -, H -, and KMOS YJ -, H -bands spectroscopic data have been analysed, 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 centre 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 centre 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 |$M_{\star }\sim 10^{12} \, \mathrm{M}_{\odot }$|⁠ , hosting a black hole of |$\rm 2 \times 10^8\!-\!10^9 \, \mathrm{M}_{\odot }$|⁠ , in a time-scale of the order of ∼2.5 Gyr. [ABSTRACT FROM AUTHOR]

Context. Recent observational progress has enabled the detection of galaxy clusters and groups out to very high redshifts and for the first time allows detailed studies of galaxy population properties in these densest environments in what was formerly known as the ¡°redshift desert¡± at z > 1.5. Aims. We aim to investigate various galaxy population properties of the massive X-ray luminous galaxy cluster XDCP J0044.0-2033 at z = 1.58, which constitutes the most extreme currently known matter-density peak at this redshift. Methods. We analyzed deep VLT/HAWK-I near-infrared data with an image quality of 0.5" and limiting Vega magnitudes (50% completeness) of 24.2 in J- and 22.8 in the Ks band, complemented by similarly deep Subaru imaging in i and V, Spitzer observations at 4.5 ¥ìm, and new spectroscopic observations with VLT/FORS2. Results. We detect a cluster-associated excess population of about 90 galaxies, most of them located within the inner 30" (250 kpc) of the X-ray centroid, which follows a centrally peaked, compact NFW galaxy surface-density profile with a concentration of c200 ≃ 10. Based on the Spitzer 4.5 ¥ìm imaging data, we measure a total enclosed stellar mass of M.500 ≃ (6.3 ¡¾ 1.6) ¡¿ 1012 M­ and a resulting stellar mass fraction of f.,500 = M.,500/M500 = (3.3 ¡¾ 1.4)%, consistent with local values. The total J- and Ks-band galaxy luminosity functions of the core region yield characteristic magnitudes J* and Ks* consistent with expectations from simple zf = 3 burst models. However, a detailed look at the morphologies and color distributions of the spectroscopically confirmed members reveals that the most massive galaxies are undergoing a very active mass-assembly epoch through merging processes. Consequently, the bright end of the cluster red sequence is not in place, while a red-locus population is present at intermediate magnitudes [Ks*, Ks* + 1.6], which is then sharply truncated at magnitudes fainter than Ks*+ 1.6. The dominant cluster-core population comprises post-quenched galaxies transitioning toward the red sequence at intermediate magnitudes, while additionally a significant blue-cloud population of faint star-forming galaxies is present even in the densest central regions. Based on a color.color selection performed to separate different cluster galaxy types, we find that the blue star-forming population is concentrated in clumpy structures and dominates in particular at and beyond the R500 radius. On the other hand, the fraction of post-starburst galaxies steadily increases toward the center, while the red-locus population and red-sequence transition galaxies seem to reach their peak fractions already at intermediate cluster-centric radii of about r ¡­ 200 kpc. Conclusions. Our observations support the scenario in which the dominant effect of the dense z ≃ 1.6 cluster environment is an accelerated massassembly timescale (¡­1Gyr or shorter) through merging activity that is responsible for driving core galaxies across the mass-quenching threshold of log(M./M­) ≃ 10.4. Beyond this mass limit, star formation is suppressed on timescales of ¡­1Gyr, while the direct environmental quenching process seems to be subdominant and is acting on significantly longer timescales (¡­2.3Gyr). [ABSTRACT FROM AUTHOR]