James Webb Space Telescope Feed Post

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

NIRCam false-color image (panel a), using F444W/F200W/F090W in the red/green/blue channels. The galaxy displays a compact morphology, with possible signs of a neighbour/merger signatures to the north-west. The rectangles show the nominal position of the NIRSpec/MSA micro-shutters (for clarity, the two shutters overlapping the galaxy are cropped) and the dashed grey circle is the 0.3-arcsec radius aperture we used for the photometry. The 2-d spectrum (panel b) shows a spectral break at 1.25 µm, and tentative evidence for flux blueward of the break. The 3-pixel boxcar extraction (panel c) also shows flux detection blueward of the break. At the same time, we do not detect any strong nebular emission at the wavelengths corresponding to the Lya-drop solution (shaded region to the right) – supporting the hypothesis that this galaxy is at z = 2.34. Even at this fiducial redshift, no strong nebular emission is detected (dashed red vertical lines). The red circles with errorbars are the JWST/NIRCam circular photometry, which is 1.5× brighter than the aperture-corrected spectrum; for comparison, we show the photometry downscaled by a factor of 1.54 as small diamonds. The strong dips in the spectrum at 1.5 and 2.2 µm are likely to be outliers, perhaps due to correlated noise; bootstrapping the individual nods and summing, both these features disappear, which suggests they are not real. 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.