James Webb Space Telescope Feed Post

JADES: deep spectroscopy of a low-mass galaxy at redshift 2.3 quenched by environment

The environment surrounding our low-mass quiescent galaxy (red diamond), centred on the light-weighted centre of the structure (halfway between the most massive galaxy and the most massive pair). The inset shows our target and the most massive galaxy. Candidate group members are highlighted by the yellow circles; these are selected from robust photometric redshifts or spectroscopic redshifts. The smaller, white diamonds are a selection of fainter and close (< 10 arcsec) photometrically selected Balmer break galaxies, showing possible evidence for additional quenched satellites. The grey dotted curves are the contours of 3- and 4-s significance for the photometric overdensity. We report the discovery of a quiescent galaxy at z=2.34 with a stellar mass of only M?=9.5+1.8-1.2×108M?, based on deep JWST/NIRSpec spectroscopy. This is the least massive quiescent galaxy found so far at high redshift. We use a Bayesian approach to model the spectrum and photometry, and find the target to have been quiescent for 0.6 Gyr with a mass-weighted average stellar age of 0.8-1.7 Gyr (dominated by systematics). The galaxy displays an inverse colour gradient with radius, consistent with environment-driven quenching. Based on a combination of spectroscopic and robust (medium- and broad-band) photometric redshifts, we identify a galaxy overdensity near the location of the target (5-s above the background level at this redshift). We stress that had we been specifically targetting galaxies within overdensities, the main target would not have been selected on photometry alone; therefore, environment studies based on photometric redshifts are biased against low-mass quiescent galaxies. The overdensity contains three spectroscopically confirmed, massive, old galaxies (M?=8-17×1010M?). The presence of these evolved systems points to accelerated galaxy evolution in overdensities at redshifts z > 2, in agreement with previous works. In projection, our target lies only 35 pkpc away from the most massive galaxy in this overdensity (spectroscopic redshift z = 2.349) which is located close to overdensity's centre. This suggests the low-mass galaxy was quenched by environment, making it possibly the earliest evidence for environment-driven quenching to date.