Discovery of a massive X-ray luminous galaxy cluster at z = 1.579

We report on the discovery of a very distant galaxy cluster serendipitously detected in the archive of the XMM-Newton mission, within the scope of the XMM-Newton Distant Cluster Project (XDCP). XMMUJ0044.0-2033 was detected at a high significance level (5σ) as a compact, but significantly extended source in the X-ray data, with a soft-band flux f(r < 40″) = (1.5 ± 0.3) × 10^-14 erg s^-1 cm². Optical/NIR follow-up observations confirmed the presence of an overdensity of red galaxies matching the X-ray emission. The cluster was spectroscopically confirmed to be at z = 1.579 using ground-based VLT/FORS2 spectroscopy. The analysis of the I - H colour - magnitude diagram shows a sequence of red galaxies with a colour range [3.7 < I - H < 4.6] within 1' from the cluster X-ray emission peak. However, the three spectroscopic members (all with complex morphology) have significantly bluer colours relative to the observed red-sequence. In addition, two of the three cluster members have [OII] emission, indicative of on-going star formation. Using the spectroscopic redshift we estimated the X-ray bolometric luminosity, L_bol,40″ ~ 5.8 × 10⁴⁴ erg s^-1, implying a massive galaxy cluster. This places XMMU J0044.0-2033 at the forefront of massive distant clusters, closing the gap between lower redshift systems and recently discovered proto- and low-mass clusters at z > 1.6.

This work is based on observations obtained with XMM-Newton, an ESA science mission with instruments and contributions directly funded by ESA Member States and the USA (NASA); and on observations carried out using the New Technology Telescope and the Very Large Telescope at the La Silla Paranal Observatory, under Program IDs 079.A-0634(B), 081.A-0312(A) and 084.A-0844(A).