Discovery of the most massive cluster of galaxies known in the distant Universe
25 August 2008XMM-Newton has discovered a rare, very massive cluster of galaxies at a distance of about 7700 million light years (or z~1). The object, designated 2XMM J083026+524133, was discovered during a systematic analysis of the 2XMM X-ray source catalogue. In a paper, to appear in Astronomy & Astrophysics, Georg Lamer and colleagues present the discovery and analysis of this exceptional cluster of galaxies.
A team of astronomers at the Astrophysikalisches Institut Potsdam, led by Georg Lamer, searched the catalogue for distant and bright clusters of galaxies. These large collections of gravitationally bound galaxies have a tenuous but very hot intra-cluster gas component with a temperature of up to 100 million Kelvin, causing thermal emission at X-ray wavelengths.
Lamer et al. selected all extended sources in the 2XMM catalogue and correlated these with data from the Sloan Digital Sky Survey (SDSS) to search for optical counterparts. About a third of the sky covered in the 2XMM catalogue is also covered by the SDSS. Within this region the majority of the selected X-ray sources were found to have an optical counterpart in the SDSS data. However, for a small number of X-ray sources no visible optical counterpart was detected. These sources were considered candidates for distant clusters of galaxies at redshifts above ~0.8. The large distance would render the optical light too faint to have been detected by the SDSS.
The brightest of the selected X-ray sources without an SDSS counterpart was 2XMM J083026+524133. The source was observed for an accumulated total of nearly 24 hours by XMM-Newton, gathering sufficient X-ray photon counts for a good X-ray spectrum to be obtained. Lamer et al. determined the best fit spectrum using a plasma model for a cluster of galaxies. The fitted spectrum implies a redshift of 0.99 ± 0.03 and a temperature of 8.2 ± 0.9 keV or ~95 million K for the intra-cluster gas.
This makes 2XMM J083026+524133 the hottest, most X-ray bright cluster of galaxies at redshifts z ≥ 1, with a bolometric luminosity Lbol of 1.8 × 1045 erg s-1. Using a mass model and the measured temperature, Lamer et al. also derive a mass of about 5.6 × 1014 Msun for this cluster of galaxies, equal to ~1000 times the mass of our Milky Way galaxy.
Follow-up observations were performed in May 2008 with the world's largest telescope, the Large Binocular Telescope (LBT) on Mount Graham in southeast Arizona, USA. Two broad filters were used to make deep observations of the field around 2XMM J083026+524133 in the R band (Sloan r' ~550-690 nm) and Z band (Sloan z ~850-1000 nm (upper cut-off defined by the detector and not by the filter)).
These optical and near-infrared images provided the sought after counterpart and support the identification of 2XMM J083026.2+524133 as a cluster of galaxies (Figure 1) because:
- there is an increased density of red galaxies within the X-ray contours of the extended source from the 2XMM catalogue
- the magnitudes and colours of these galaxies are consistent with a redshift of ~1
Clusters of galaxies at a redshift of ~1 or higher are routinely identified with XMM-Newton data and in dedicated surveys with other observatories. However, 2XMM J083026.2+524133 is a factor of ~100 brighter in X-rays than the majority of the other known clusters of galaxies at these redshifts, making it an important find.
The evolution of the number of high-mass, high-luminosity clusters of galaxies, similar to 2XMM J083026.2+524133, over the age of the Universe, strongly depends on cosmological parameters. Large X-ray surveys such as the 2XMM catalogue can provide the necessary observational constraints on these parameters through the observed number of these massive clusters at large distances. The counts can be compared with the number of massive clusters of galaxies in the local Universe to characterise their number evolution with time. This in turn has implications for the validity of cosmological models.
G. Lamer, M. Hoeft, J. Kohnert, A. Schwope, and J. Storm, "2XMM J083026+524133: The most X-ray luminous cluster at redshift 1," Astronomy and Astrophysics, 487, 2, L33-L36, 2008, DOI: 10.1051/0004-6361:200810255