Exploring the galaxy cluster-group transition regime at high redshifts:
  Physical properties of two newly detected z > 1 systems
  Context: Multi-wavelength surveys for clusters of galaxies are opening a
window on the elusive high-redshift (z>1) cluster population. Well controlled
statistical samples of distant clusters will enable us to answer questions
about their cosmological context, early assembly phases and the thermodynamical
evolution of the intracluster medium. Aims: We report on the detection of two
z>1 systems, XMMU J0302.2-0001 and XMMU J1532.2-0836, as part of the XMM-Newton
Distant Cluster Project (XDCP) sample. We investigate the nature of the
sources, measure their spectroscopic redshift and determine their basic
physical parameters. Methods: The results of the present paper are based on the
analysis of XMM-Newton archival data, optical/near-infrared imaging and deep
optical follow-up spectroscopy of the clusters. Results: We confirm the X-ray
source XMMU J0302.2-0001 as a gravitationally bound, bona fide cluster of
galaxies at spectroscopic redshift z=1.185. We estimate its M500 mass to
(1.6+/-0.3) times 10^{14} Msun from its measured X-ray luminosity. This ranks
the cluster among intermediate mass system. In the case of XMMU J1532.2-0836 we
find the X-ray detection to be coincident with a dynamically bound system of
galaxies at z=1.358. Optical spectroscopy reveals the presence of a central
active galactic nucleus, which can be a dominant source of the detected X-ray
emission from this system. We provide upper limits of X-ray parameters for the
system and discuss cluster identification challenges in the high-redshift
low-mass cluster regime. A third, intermediate redshift (z=0.647) cluster, XMMU
J0302.1-0000, is serendipitously detected in the same field as XMMU
J0302.2-0001. We provide its analysis as well.
