Gravitationally Lensed Images in Abell 370

Several blue arclike images have been found in the galaxy cluster A370, some up to 1' from the cluster center. We show that all the images can be explained by gravitational lensing of background galaxies, provided there is a significant amount of dark matter in the cluster. We use the observations to constrain the distribution of the dark matter. We find a mass-to-light ratio M/L_B_ ~ 200 and a projected half-density core radius of the dark matter <~ 26" or <~ 110 kpc. The isodensity contours of the dark matter need to be elongated in the north-south direction with an axis ratio greater than about 4:3. The ratio is probably closer to 2:1. The longest arc has a measured redshift of 0.724, as opposed to the cluster redshift of 0.374. For the smaller arcs we require source redshifts z_s_ >~ 1.2, with z_s_ >~ 1.5 giving the best fit. We suggest that the arcs are elongated and magnified images of a very numerous population of high-redshift blue galaxies identified by Tyson. Deep images of other galaxy clusters may reveal many more distorted images. An analysis similar to the one presented here may then lead to unique information on the morphology of the dark matter in these clusters.