Deep X-ray and IR study of the distant X-ray cluster XDCP J0044.0-2033 (Il Gioiello) at z=1.6

Galaxy clusters are the largest structures in the Universe held together by gravity. Finding and characterizing these structures at high redshift is fundamental in order to understand how they evolve over time and ultimately provide crucial tests to cosmological models. The high redshift cluster XDCP0044 (z=1.6) was discovered in 2011 in the framework of the XMM Distant Cluster Project (Santos et al. 2011, A&A, 531L, 15). A recent, deep exposure with the NASA's Chandra X-ray Observatory, awarded to the research group led by Paolo Tozzi and including Joana Santos at INAF-Arcetri, showed strong diffuse X-ray emission from this cluster. This is the deepest X-ray observation yet made on a cluster beyond a distance of about 8 billion light years.

The long observation, equivalent to over four days time, allowed the team to gather enough X-ray data from Chandra that, when combined with scientific models, provide an accurate weight of the cluster. The researchers concluded that XDCP0044 is a virialized cluster at redshift z=1.6 with a mass of about 400 trillion times the mass of the Sun, whose Intra Cluster Medium has global properties similar to that of local clusters. It is the first time that a virialized halo is clearly identified at a look back time of 9.6 billion light years.

Such a massive cluster at high redshift, together with few other massive and distant clusters recently discovered, are providing hints for some modification of the standard cosmological model. According to the best current model for how the Universe evolved, there is a low chance of finding clusters as massive as XDCP0044. The new findings are enticing the astronomical community to look for other distant and massive clusters. These results are published in òÀÜChandra Deep Observation of XDCP J0044.0-2033, a Massive Galaxy Cluster at z > 1.5òÀÝ, by P. Tozzi et al. (2015, The Astrophysical Journal, 799, 93) and have been advertised in a Chandra press release.

The cluster is officially known as XDCP J0044.0-2033. The team, however, have nicknamed it "Gioiello", which stands for "jewel" in Italian. This name was chosen because an image of the cluster contains many sparkling colors from the hot, X-ray emitting gas and various star-forming galaxies within the cluster (Fig. 1). Also, Villa Il Gioiello, a 15th century villa near the Observatory of Arcetri, which was the last residence of Galileo Galilei, hosted the first meeting of the group to discuss the Chandra data.

Figure 1 - In this new image of the Gioiello Cluster, X-rays from Chandra are purple, infrared data from ESA's Hershel Space Telescope appear as large red halos around some galaxies, and optical data from the Subaru telescope on Mauna Kea in Hawaii are red, green, and blue. Credit: X-ray: NASA/CXC/INAF/P.Tozzi, et al; Optical: NAOJ/Subaru and ESO/VLT; Infrared: ESA/Herschel/J. Santos, et al.

Another important piece of information comes from the Infrared observation from ESAòÀÙs Herschel Space Observatory. The Herschel telescope looks into the far-infrared observational window, that allows the detection of cold dust in galaxies - an important signature of on-going star formation activity. By measuring the amount of dust in galaxies in the core of the cluster, an area with over one and half million light years across, they found that the cluster centre has a star formation rate of almost two thousand solar masses per year, four times the rate found in its outer regions. The detection of such an unprecedented level of star formation in the cluster centre, typically a harsh environment with many galaxies packed together that hampers star formation, makes this discovery a first for clusters of this mass and distance, and tells us that we are witnessing a very early stage in galaxy cluster evolution. These, potentially fleeting, fireworks will form huge numbers of stars before the process halts and the cluster core becomes like those we see in our local Universe òÀÓ densely packed with òÀÜredòÀÝ and òÀÜdeadòÀÝ elliptical galaxies, and devoid of bright blue stars and of the gas reservoirs needed to create new star. Therefore, this observation gives new clues to the evolution of some of the largest structures in the Universe and constitutes an important a step towards understanding one of the main questions faced by today's astronomers: how and where galaxies form their stars and how this has changed over cosmic time. The results of this study have been published in "The reversal of the SF-density relation in a massive, X-ray selected galaxy cluster at z=1.58: results from Herschel" by J. S. Santos et al. (2015, Monthly Notices of the Royal Astronomical Society: Letters, 447, 65).

The significance of this intense level of star formation is evident when compared to other clusters of a similar mass. The cluster XMMU J2235.3-2557, previously studied by the team (òÀÜDust-obscured star formation in the outskirts of XMMU J2235.3-2557, a massive galaxy cluster at z = 1.4 òÀÝ, by J. S. Santos et al. 2013, MNRAS, 433, 1287), has a very similar mass and lies only a little more than half a billion years later in cosmic history, at 4.8 billion years after the Big Bang, and yet shows no signs of star formation in its core. Finding active star birth in the core of XDCPJ0044.0-2033 is an important step towards establishing at what period in cosmic history the star formation preference changed from the high density environments in cluster cores, to the low density settings at the periphery of clusters, like the star formation we see in our local Universe. To better determine the epoch in which this change occurred astronomers will need to find and study other massive clusters at these large look-back times and search for signs of star birth in their cores.

Figure 2 òÀÓ A close-up of the galaxy cluster XDCPJ0044.0-2033 in the optical and Infrared band. The red glow shows the intense emission from the star-forming member galaxies. Credit: ESA/Herschel/J. Santos et al. 2015; NAOJ/Subaru; ESA/VLT/Hawk-I.

Edited by Anna Gallazzi and Paolo Tozzi, May 2015