A strongly lensed massive ultracompact quiescent galaxy at z ∼ 2.4 in the COSMOS/UltraVISTA field

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 - Kscolors using new data from the UltraVISTA YJHKsnear-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 Ks-band image, to constrain the size and Sérsic profile of the galaxy. The best-fit model is an ultracompact galaxy (Re= 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 (Av= 0.8+0.5- 0.6), and a low specific star formation rate (log(SSFR)