The cluster RX J1347.5-1145 , the most luminous cluster in the X-ray wavelengths , was imaged with the newly installed Space Telescope Imaging Spectrograph ( STIS ) on-board HST .
Its relatively high redshift ( 0.451 ) and luminosity indicate that this is one of the most massive of all known clusters .
The STIS images unambiguously show several arcs in the cluster .
The largest two arcs ( > 5 \arcsec length ) are symmetrically situated on opposite sides of the cluster , at a distance of \sim 35 arcsec from the central galaxy .
The STIS images also show approximately 100 faint galaxies within the radius of the arcs whose combined luminosity is \sim 4 \times 10 ^ { 11 } L _ { \odot } .
We also present ground-based spectroscopic observations of the northern arc which show one clear emission line at \sim 6730 Å , which is consistent with an identification as [ OII ] 3727 Å , implying a redshift of 0.81 for this arc .
The southern arc shows a faint continuum but no emission features .
The surface mass within the radius of the arcs ( 240 kpc ) , as derived from the gravitational lensing , is \sim 6.3 \times 10 ^ { 14 } M _ { \odot } .
The resultant mass-to-light ratio of \sim 1200 is higher than what is seen in many clusters but smaller than the value recently derived for some ‘ dark ’ X-ray clusters ( Hattori et al .
1997 ) .
The total surface mass derived from the X-ray flux within the radius of the arcs is \sim 2.1 - 6.8 \times 10 ^ { 14 } M _ { \odot } , which implies that the ratio of the gravitational to the X-ray mass is \sim 1 to 3 .
The surface gas mass within this radius is \sim 3.5 \times 10 ^ { 13 } M _ { \odot } , which implies that at least 6 % of the total mass within this region is baryonic .