2012ApJ...761..142M

We report the discovery of a massive ultracompact quiescent galaxy that has been strongly lensed into multiple images by a foreground galaxy at z = 0.960. This system was serendipitously discovered as a set of extremely K_s-bright high-redshift galaxies with red J - K_s colors using new data from the UltraVISTA YJHK_snear-infrared survey. The system was also previously identified as an optically faint lens/source system using the COSMOS Advanced Camera for Surveys (ACS) imaging by Faure et al. Photometric redshifts for the three brightest images of the source galaxy determined from 27-band photometry place the source at z = 2.4±0.1. We provide an updated lens model for the system that is a good fit to the positions and morphologies of the galaxies in the ACS image. The lens model implies that the magnification of the three brightest images is a factor of 4-5. We use the lens model, combined with the K_s-band image, to constrain the size and Sérsic profile of the galaxy. The best-fit model is an ultracompact galaxy (R_e= 0.64^+0.08_- 0.18_ kpc, lensing-corrected), with a Sérsic profile that is intermediate between a disk and a bulge profile (n = 2.2^+2.3_- 0.9_), albeit with considerable uncertainties on the Sérsic profile. We present aperture photometry for the source galaxy images that have been corrected for flux contamination from the central lens. The best-fit stellar population model is a massive galaxy (log(M_star/M_☉) = 10.8^+0.1_- 0.1_, lensing-corrected) with an age of 1.0^+1.0_- 0.4_Gyr, moderate dust extinction (A_v= 0.8^+0.5 _- 0.6_), and a low specific star formation rate (log(SSFR) ←11.0/yr). This is typical of massive "red-and-dead" galaxies at this redshift and confirms that this source is the first bona fide strongly lensed massive ultracompact quiescent galaxy to be discovered. We conclude with a discussion of the prospects of finding a larger sample of these galaxies.