A STRONGLY LENSED MASSIVE ULTRACOMPACT QUIESCENT GALAXY AT z similar to 2.4 IN THE COSMOS/UltraVISTA FIELD

International audienceWe 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(s) near-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 Spersic profile of the galaxy. The best-fit model is an ultracompact galaxy (R-e = 0.64(-0.18)(+ 0.08) kpc, lensing-corrected), with a Sersic profile that is intermediate between a disk and a bulge profile (n = 2.2(-0.9)(+2.3)), albeit with considerable uncertainties on the Sersic 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-circle dot) = 10.8(-0.1)(+0.1), lensing-corrected) with an age of 1.0(-0.4)(+1.0) Gyr, moderate dust extinction (A(v) = 0.8(-0.6)(+0.5)), and a low specific star formation rate (log(SSFR)\textless-11.0 yr(-1)). 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