Spectroscopic and morphological evidence that iras fsc 10214+4724 is a gravitational lens

We have obtained a 0"7 FWHM H image of IRAS FSC 10214+4724. This image suggests FSC 10214+4724 is an arc formed by a gravitational lens located 1"25 to the north. We have also obtained a coarse spectroscopic continuum of the lensing galaxy. The location and strength of a break in the continuum suggests the 4000 A break of an early type galaxy at a redshift of 0.42 ± 0.02. A galaxy 23” south is found to have a very similar redshift of 0.429 H 0.002.This redshift, coupled with the observed magnitude of the lensing galaxy (好=18.45 ± 0.15, in a 2” aperture), implies the lensing galaxy has a very plausible luminosity and a velocity dispersion of 210 ± 20 km s^_1(^0 = 0.5,六=0.5). Such a lens would produce a faint counter-arc image 1"6 ± 0"2 north of the main arc. We observe such a counter-arc in our H image, indicating that FSC 10214+4724 is indeed gravitationally lensed. We find that lensing by a 2 = 0.42, a 〜210 km s^_1galaxy is a viable and probable solution to both the observed morphology and uniquely large (〜1 X 10¹⁴六—²_Lj) luminosity of FSC 10214+4724. The true (unlensed) nature of FSC 10214+4724 is likely an ultraluminous IR galaxy of 〜3.7 X 10¹²六^_2_Lj.