We discuss the X-ray and optical properties of the massive galaxy cluster MACS J1206.2 - 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 \times 10 ^ { 45 } erg s ^ { -1 } ( 0.1–2.4 keV ) and a global gas temperature of 11.6 \pm 0.7 keV , very high values typical of MACS clusters .
In both optical and X-ray images MACS J1206.2 - 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″in length , bright ( V \sim 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 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 Hubble Space Telescope imaging yields a very high value of 1.1 \times 10 ^ { 14 } M _ { \odot } , inconsistent with the much lower X-ray estimate of 0.5 \times 10 ^ { 14 } M _ { \odot } .
Similarly , the virial estimate of 4 \times 10 ^ { 15 } M _ { \odot } for the total cluster mass , derived from multi-object spectroscopy with CFHT and the VLT of 38 cluster members , is significantly higher than the corresponding X-ray estimate of 1.7 \times 10 ^ { 15 } M _ { \odot } .
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 } .