Double Trouble: Two spectroscopically confirmed low-mass quiescent galaxies at z>5 in overdensities

Abstract

We present the discovery of two low-mass, high-redshift, quiescent galaxies, GS-z5-Q1 and COS-z5-Q1, using JWST NIRSpec spectroscopy alongside NIRCam and MIRI photometry. Observed at a redshift of z=5.39 and z=5.11 respectively, and with stellar masses of $\rm 10^{9.6}M_\odot$ and $\rm 10^{9.5}M_\odot$, GS-z5-Q1 and COS-z5-Q1 are two of the most distant quiescent galaxies spectroscopically confirmed to-date, and are by far the least massive ($\sim10\times$ lower mass). Full spectrophotometric modelling reveals that COS-z5-Q1 appears to have quenched more than 300Myr prior to observation ($z\sim 7$) and has a formation redshift of around z$\sim$11, whilst GS-z5-Q1 formed and quenched in a single burst around 150Myr prior to observation ($z\sim6$). GS-z5-Q1 is found to lie near the centre of a known high-z overdensity in GOODS-S, as would be expected by galaxy formation models, while COS-z5-Q1 lies towards the outskirts of an overdense region. This highlights the role that environment could play in accelerating galaxy evolutionary processes and could possibly be linked to the galaxies' quiescent nature. By modelling their stellar populations, we show that these types of low-mass quiescent galaxies could potentially be descendants of the higher-z "mini-quenched" galaxies. The discovery of these two low-mass $z>5$ quiescent galaxies illuminates a previously undiscovered galaxy population and motivates dedicated follow-up surveys to investigate the overall population.

Figures (6)

• Figure 1: Left: $UVJ$ colour diagram. GS-z5-Q1 and COS-z5-Q1 are indicated with full red circles, along with the sources in Weibel2024qgalLooser2024deGraaff2025 as labelled. The background gray-scaled sources are QG candidates at $z>2$ from Baker2025d, where the mass cut has been relaxed ($M_\star>10^{9.3}\,M_\odot$). Right: The same galaxies are shown in the stellar mass versus redshift plane. Quiescent galaxies with JWST medium/high-resolution spectra compiled in Ito2025b are also shown in squares. It highlights how low mass GS-z5-Q1 and COS-z5-Q1 are compared to the deGraaff2025 galaxy at $z=4.9$, i.e. almost 1.5 dex less massive, despite having similar colours and redshifts.
• Figure 2: Upper: Observed 1D NIRSpec prism spectra (grey), photometry (yellow points) and best fit model spectra and photometry (red) for GS-z5-Q1 (left) and COS-z5-Q1 (right). The MIRI points are shown in the right extension to the figure and correspond to F560W (just GS-z5-Q1) and F770W (both). Lower middle: $\chi$ values for the spectral fit with the NIRCam filters overplotted (left) and an RGB image of the galaxy (right). Notable emission lines are overplotted. Lower: Star-formation histories for the two galaxies (orange line) showing SFR against lookback time (redshift). The formation time ($\rm t_{50}$, the time 50% of the mass is formed) and quenching time ($\rm t_{90}$, the time 90% of the mass is formed) and overplotted as blue and red lines respectively.
• Figure 3: Sérsic modeling of the surface brightness in NIRCam/F277W of COS-z5-Q1 (top) and GS-z5-Q1 (bottom). In each row, the first panel shows the image in pixel units ($3"$ side, $0\farcs04$ pixel size), the best-fit model, and the residuals.
• Figure 4: Distribution of galaxies around the GS-z5-Q1 (left) and COS-z5-Q1 (right). These QGs are located in the centre of the field and marked as stars. The coloured circles and smaller black circles correspond to galaxies at $5.34<z<5.44$ (for GS-z5-Q1) and $5.06<z<5.16$ (for COS-z5-Q1) according to their spectroscopic and photometric redshift, respectively. The contours show the overdensity measured from photometric and spectroscopic galaxies. The number in these contours indicates the significance of the overdensity. For GS-z5-Q1, the location of the overdensity centre reported in Helton2024 is shown as a green square.
• Figure 5: Comparison between GS-z5-Q1, COS-z5-Q1 and three sources from the literature. These are a low-mass "mini-quenched" galaxy from Looser2024, a massive quiescent galaxy at $z=4.9$ from deGraaff2025 and the current highest-redshift massive quiescent galaxy spectrum from Weibel2024qgal. The spectra have been rebinned and renormalised to deal better with noise and improve clarity. It shows that GS-z5-Q1 and COS-z5-Q1 are consistent with the spectral shapes of the Weibel2024qgal and deGraaff2025 spectra, but are completely different to the classic Looser2024 "mini-quenched" galaxy spectrum. GS-z5-Q1 and COS-z5-Q1 have red UV slopes and more prominent Balmer breaks whereas the Looser2024 galaxy has a much steeper UV slope and significantly weaker Balmer break.