COOL-LAMPS I. An Extraordinarily Bright Lensed Galaxy at Redshift 5.04
Abstract
We report the discovery of COOL J1241+2219, a strongly-lensed galaxy at redshift z=5.0430.002 with observed magnitude zAB=20.47, lensed by a moderate-mass galaxy cluster at z=1.0010.001. COOL J1241+2219 is the brightest lensed galaxy currently known at optical and near-infrared wavelengths at z 5; it is 5 times brighter than the prior record-holder lensed galaxy, and several magnitudes brighter than the brightest unlensed galaxies known at these redshifts. It was discovered as part of COOL-LAMPS, a collaboration initiated to find strongly lensed systems in recent public optical imaging data. We characterise the lensed galaxy, as well as the central galaxy of the lensing cluster using ground-based grizJH imaging and optical spectroscopy. We report model-based magnitudes, and derive stellar masses, dust content, metallicity and star-formation rates via stellar-population synthesis modeling. Our lens mass modeling, based on ground-based imaging, implies a median source magnification of 30, which puts the stellar mass and star formation rate (in the youngest age bin, closest to the epoch of observation) at logM* = 10.11+0.21-0.26 and SFR = 27+13-9 M/yr, respectively. We constrain a star formation history for COOL J1241+2219 consistent with constant star formation across 1 Gyr of cosmic time, and that places this galaxy on the high-mass end of the star-forming main sequence. COOL J1241+2219 is 2-4 times more luminous than a galaxy with the characteristic UV luminosity at these redshifts. The UV continuum slope β= -2.20.2 places this galaxy on the blue side of the observed distribution of galaxies at z=5, although the lack of Lyα emission indicates dust sufficient to suppress this emission.