A spectacular giant arc in the massive cluster lens MACSJ1206.2-0847

TL;DR

This paper studies the massive galaxy cluster MACSJ1206.2-0847, revealing a giant gravitational arc, analyzing its properties through X-ray, optical, and lensing data, and highlighting discrepancies in mass estimates indicative of ongoing merger activity.

Contribution

The study combines multi-wavelength observations and multiple mass estimation techniques to analyze the complex structure and dynamics of MACSJ1206.2-0847, revealing evidence of a merger.

Findings

Discovery of a 15" giant arc at z=1.036

Mass estimates from lensing and X-ray are inconsistent

Evidence of substructure and ongoing merger activity

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 x 10^45 erg/s (0.1-2.4 keV) and a global gas temperature of (11.6 +/- 0.7) keV, very high values typical of MACS clusters. In both optical and X-ray images MACSJ1206 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" in length, bright (V~21) and unusually red (R-K=4.3), is seen 20" west of the BCG; we measure a redshift of z=1.036 for the lensed galaxy. From our HST image of the cluster we identify the giant arc and its counter image as a seven-fold 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 HST imaging yields a very high value of 1.1 x 10^14 M_sun, inconsistent with the much lower X-ray estimate of 0.5 x 10^14 M_sun. Similarly, the virial estimate of 4 x 10^15 M_sun for the total cluster mass, derived from multi-object spectroscopy of 38 cluster members, is significantly higher than the corresponding X-ray estimate of 1.7 x 10^15 M_sun. We take the discrepant mass estimates to be indicative of 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.