Spectroscopic evidence that the extreme properties of IRAS F10214+4724 are due to gravitational lensing

The extreme bolometric luminosity of IRAS F10214+4724, and in particular thehuge mass in molecular gas inferred from the CO line fluxes have led tosuggestions that this is a giant galaxy in the process of formation. Anarc-like structure and the closeness of a second object suggest, however, thatgravitational lensing may be responsible for its anomalously high luminosityand mass. In this paper, we use an optical spectrum taken in conditions of0.7-arcsec seeing to provide further evidence that F10214+4724 is agravitationally lensed system. We measure tentative redshifts of 0.896 and0.899 for galaxies projected $\approx 1$ and $\approx 3$ arcsec from IRASF10214+4724 respectively. Identifying the former as the lensing galaxy weobtain a mass:light ratio consistent with those derived for other lenses, andfind that its luminosity is consistent with the velocity dispersion deducedfrom the radius of the Einstein ring. If lensed, our models suggestmagnification by a factor $\sim 10$, and hence an intrinsic bolometricluminosity for F10214+4724 similar to local ULIRGs.Comment: 5 pages, uuencoded compressed PostScript, 3 figures included. Accepted for publication in MNRAS pink pages; submitted 1 May 199