We present a snapshot Hubble Space Telescope ( HST ) image of the galaxy cluster A 1201 ( z { = } 0.169 ) , revealing a tangential arc 2 arcsec from the brightest cluster galaxy ( BCG ) .
Keck–ESI spectroscopy confirms that the arc is gravitational in nature and that the source galaxy lies at z { = } 0.451 .
We construct a model of the gravitational potential of the cluster that faithfully reproduces the observed arc morphology .
Despite the relaxed appearance of the cluster in the HST frame , the best fit ellipticity of the total matter distribution is \epsilon _ { total } { \geq } 0.5 , in contrast to the light distribution of the BCG ( \epsilon _ { BCG } { = } 0.23 { \pm } 0.03 ) on 2 ^ { \prime \prime } scales .
Further deep optical observations and pointed X–ray spectro–imaging observations with Chandra are required to determine whether this elongation is due to a single elongated dark matter halo , or a more complex distribution of matter in the cluster core .
We compare the arc with a sample drawn from the published literature , and confirm that it is unique among tangential systems in the small physical scales that it probes ( { \sim } 6 kpc ) .
In anticipation of a more thorough investigation of this cluster across a broad range of physical scales , we use our fiducial lens model to estimate the projected mass and mass–to–light ratio of the cluster within a radius of 6 kpc , obtaining : M { = } ( 5.9 ^ { { + } 0.9 } _ { { - } 0.7 } ) { \times } 10 ^ { 11 } { M _ { \odot } } , M / L _ { V } { = } 9.4 ^ { { + } 2.4 } _ { { - } 2.1 } ( M / L ) _ { \odot } .
Overall our results confirm the importance of HST snapshot surveys for identifying rare lensing constraints on cluster mass distributions .
In combination with follow–up optical and X–ray observations , the arc in A 1201 should help to increase our understanding of the physics of cluster cores .