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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 Å 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 ± 0.002. This redshift, coupled with the observed magnitude of the lensing galaxy (H = 18.45 ± 0.15, in a 2'' aperture), implies the lensing galaxy has a very plausible L* luminosity and a velocity dispersion of 210 ± 20 km s-1 (q0 = 0.5, h = 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 z = 0.42, σ ~ 210 km s-1 galaxy is a viable and probable solution to both the observed morphology and uniquely large (~1 × 1014 h-2 L☉) luminosity of FSC 10214+4724. The true (unlensed) nature of FSC 10214+4724 is likely an ultraluminous IR galaxy of ~3.7 × 1012 h-2 L☉.