Dynamical properties and star formation history of a low-mass quenched galaxy at cosmic noon

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Abstract
We present the spectroscopic confirmation and in-depth analysis of AURORA-LQG1, a low-mass quiescent galaxy at z spec = 2.0834 with log( M ★ / M ⊙ ) = 9.6, observed with medium-resolution JWST/NIRSpec spectroscopy. Its stellar mass places approximately ten times lower than the knee of the stellar mass function for quiescent galaxies at z ∼ 2. The deep medium-resolution spectrum enables the measurement of its stellar velocity dispersion ( σ ★ = 95 −33 +38 km s −1 ), the smallest value recorded among spectroscopically confirmed quiescent galaxies at z ∼ 2. Coupled with a compact size (0.41 ± 0.03 kpc in the rest-frame optical), the stellar velocity dispersion yields a dynamical mass estimate of log( M dyn M ⊙ = 9.75 −0.38 +0.29 , consistent with the stellar mass, confirming the true low-mass nature of this galaxy and placing a first constraint on its initial mass function. Joint spectro-photometric spectral energy distribution fitting reveals a star formation history in which half the stellar mass was in place ∼1 Gyr before the observed epoch, with quenching occurring ∼0.2 Gyr prior to z = 2.08. These results confirm that AURORA-LQG1 is genuinely quenched, rather than in a temporary phase of suppressed star formation. AURORA-LQG1 is consistent with the mass fundamental plane at z ∼ 2, which was previously constrained only by massive quiescent systems with M ★ ≥ 10 11 M ⊙ at cosmic noon. Compared with more massive counterparts at the same epoch observed with similar NIRSpec grating spectroscopy, the time since quenching for AURORA-LQG1 is among the shortest observed. The star formation history of AURORA-LQG1 is broadly consistent with predictions for quiescent galaxies of similar mass and redshift in the IllustrisTNG and SHARK models. The galaxy resides in a possible dense group-scale (∼50 kpc) environment containing one companion with tentative spectroscopic redshift and five low-mass companion candidates with similar redshifts, and it is embedded in a large known protocluster on megaparsec scales. A potential environmental influence on its evolution could explain the outside-in quenching suggested by the positive gradient of size with wavelength. This study demonstrates that deep JWST/NIRSpec spectroscopy enables low-mass quiescent galaxies at cosmic noon to be characterized with a level of detail long reserved for massive systems, offering valuable new insights into how quenching operates in these underexplored, low-mass systems.
Funding Information
  • Danmarks Frie Forskningsfond (3120-00043B)
  • Japan Society for the Promotion of Science (JP23K13141)
  • Japan Society for the Promotion of Science (JP25K07361)
  • Villum Fonden (VIL54489)

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