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

¹ Cosmic Dawn Center (DAWN), Copenhagen, Denmark
² DTU Space, Technical University of Denmark, Elektrovej 327, DK2800 Kgs. Lyngby, Denmark
³ DARK, Niels Bohr Institute, University of Copenhagen, Jagtvej 155A, DK-2200 Copenhagen, Denmark
⁴ Niels Bohr Institute, University of Copenhagen, Jagtvej 128, 2200 Copenhagen N, Denmark
⁵ Department of Astronomical Science, The Graduate University for Advanced Studies, SOKENDAI, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁶ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁷ International Centre for Radio Astronomy Research (ICRAR), M468, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia
⁸ ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia
⁹ Subaru Telescope, National Astronomical Observatory of Japan, National Institutes of Natural Sciences (NINS), 650 North A’ohoku Place, Hilo, HI 96720, USA
¹⁰ Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA
¹¹ Department of Physics & Astronomy, University of California, Riverside, 900 University Avenue, Riverside, CA 92521, USA
¹² Department of Physics and Astronomy, University of Kentucky, 505 Rose Street, Lexington, KY 40506, USA
¹³ Department of Physics & Astronomy, University of California, Los Angeles, 430 Portola Plaza, Los Angeles, CA 90095, USA

^★ Corresponding author.

Received: 26 December 2025
Accepted: 1 April 2026

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_★ ≥ 10¹¹ 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.