Abstract
We discuss the X-ray and optical properties of the massive galaxy cluster MACSJ1206.2-0847 (z = 0.4385), discovered in the Massive Cluster Survey (MACS). Our Chandra observation of the system yields a total X-ray luminosity of 2.4 × 1045ergs-1 (0.1-2.4 keV) and a global gas temperature of 11.6 +/- 0.7keV, very high values typical of MACS clusters. In both optical and X-ray images, MACSJ1206.2-0847 appears close to relaxed in projection, with a pronounced X-ray peak at the location of the brightest cluster galaxy (BCG); we interpret this feature as the remnant of a cold core. A spectacular giant gravitational arc, 15 arcsec in length, bright (V ~ 21) and unusually red (R - K = 4.3), is seen 20 arcsec west of the BCG; we measure a redshift of z = 1.036 for the lensed galaxy. From our Hubble Space Telescope image of the cluster, we identify the giant arc and its counter image as a sevenfold imaged system. An excess of X-ray emission in the direction of the arc coincides with a mild galaxy overdensity and could be the remnant of a minor merger with a group of galaxies. We derive estimates of the total cluster mass as well as of the mass of the cluster core using X-ray, dynamical and gravitational-lensing techniques. For the mass enclosed by the giant arc (r < 119 kpc), our strong-lensing analysis based on Hubble Space Telescope imaging yields a very high value of 1.1 × 1014Msolar, inconsistent with the much lower X-ray estimate of 0.5 × 1014Msolar. Similarly, the virial estimate of 4 × 1015Msolar for the total cluster mass, derived from multi-object spectroscopy with Canada-France-Hawaii Telescope (CFHT) and the VLT of 38 cluster members, is significantly higher than the corresponding X-ray estimate of 1.7 × 1015Msolar. We take the discrepancy between X-ray and other mass estimates to be indicative of pronounced substructure along the line of sight during an ongoing merger event, an interpretation that is supported by the system's very high velocity dispersion of 1580 km s-1. Based on observations collected with the VLT-UT3 Melipal Telescope (ESO). E-mail: ebeling@ifa.hawaii.edu
Highlights
The concentration of both dark and baryonic matter in the cores of clusters of galaxies has many profound implications for our understanding of cluster growth and cosmology
We present a comprehensive multi-wavelength study of a complex gravitational arc and its host cluster MACS J1206.2−0847, an X-ray selected system at intermediate redshift found by the Massive Cluster Survey (MACS) (Ebeling, Edge & Henry 2001; Ebeling et al 2007)
MACS J1206.2−0847 was observed on 2002 December 18 with the Advanced CCD Imaging Spectrometer (ACIS)-I detector aboard the Chandra X-ray Observatory for a nominal duration of 23.5 ks, as part of a Chandra Large Programme awarded to the MACS team
Summary
The concentration of both dark and baryonic matter in the cores of clusters of galaxies has many profound implications for our understanding of cluster growth and cosmology. The selection of massive clusters through X-ray emission has proved very successful at providing cosmological constraints (Henry 2000; Borgani et al 2001; Allen et al 2003; Pierpaoli et al 2003; Allen et al 2008; Mantz et al 2008), and follow-up observations of X-ray luminous clusters have revealed many spectacular cases of gravitational lensing (Gioia & Luppino 1994; Smith et al 2001; Dahle et al 2002; Covone et al 2006). We present a comprehensive multi-wavelength study of a complex gravitational arc and its host cluster MACS J1206.2−0847, an X-ray selected system at intermediate redshift found by the Massive Cluster Survey (MACS) (Ebeling, Edge & Henry 2001; Ebeling et al 2007).
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