Dynamical properties and star formation history of a low-mass quenched galaxy at Cosmic Noon

Abstract

We present the spectroscopic confirmation and in-depth analysis of AURORA-LQG1, a low-mass quiescent galaxy at $z_{\mathrm{spec}}=2.0834$ with $\log (M_{\star }/M_{\odot })=9.6$ observed with medium-resolution JWST/NIRSpec spectroscopy. The deep medium-resolution spectrum enables the measurement of its stellar velocity dispersion ($\ddot{I}{ƒ}_{\star }=95_{-33}^{+38}\mathrm{km}{\mathrm{s}}^{-1}$), the smallest value recorded among spectroscopically confirmed quiescent galaxies at $z\sim 2$. Coupled with a compact size ($0.41\pm 0.03\mathrm{kpc}$), it yields a dynamical mass estimate of $\log (M_{\mathrm{dyn}}/M_{\odot })=9.75_{-0.38}^{+0.29}$. Its star formation history suggests that half of the stellar mass was in place $\sim 1\mathrm{Gyr}$ before the observed epoch, with quenching occurring $\sim 0.2\mathrm{Gyr}$ prior to $z=2.08$. These results confirm that AURORA-LQG1 is genuinely quenched, rather than in a temporary phase of suppressed star formation rate. AURORA-LQG1 is consistent with the mass fundamental plane at $z\sim 2$, previously constrained only by massive quiescent systems. Compared with more massive counterparts at the same epoch observed with NIRSpec grating spectroscopy, the time since quenching for AURORA-LQG1 is among the shortest observed. The galaxy resides in a possible dense group-scale ($\sim 50$ kpc) environment containing one companion with tentative spectroscopic redshift and five companion candidates, and it is embedded in a known protocluster on Mpc 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. [Abridged]

10 pages, 7 figures, 1 table + Appendix. submitted to A&A