A STRONGLY LENSED MASSIVE ULTRACOMPACT QUIESCENT GALAXY ATz∼ 2.4 IN THE COSMOS/UltraVISTA FIELD

Abstract

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 Ks-bright high-redshift galaxies with red J − Ks colors using new data from the UltraVISTA YJHKs 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 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(Mstar/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) <−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.