When relics were made: vigorous stellar rotation and low dark matter content in the massive ultra-compact galaxy GS-9209 at z=4.66

TL;DR

This study presents JWST/NIRSpec IFU observations of GS-9209, a massive, ultra-compact quiescent galaxy at z=4.66, to test how such systems assembled and quenched. Through PSF-deconvolved photometry, MGE-based dynamical modelling, and full-spectrum fitting (ppXF), the authors derive a rotation-supported structure with a low dark matter fraction inside 2R_eff, and a Milky-Way-like IMF. The fiducial JAM analysis yields f_DM(<2R_eff) ≈ 14.5%, i ≈ 57.8°, λ_{2R_eff} ≈ 0.85 and λ_{R_eff} ≈ 0.72, indicating a fast rotator that preserves a disc-like component after quenching. These results imply gentle quenching and potential links to local relic galaxies, while highlighting tensions with some simulations that underproduce such compact high-z MQGs; they also demonstrate the power of spatially resolved kinematics to constrain early galaxy evolution.

Abstract

JWST uncovered a large number of massive quiescent galaxies (MQGs) at $z>3$, which theoretical models struggle to reproduce. Explaining the number density of such objects requires extremely high conversion efficiency of baryons into stars in early dark matter halos. Using stellar kinematics, we can investigate the processes shaping the mass assembly histories of MQGs. We present high-resolution JWST/NIRSpec integral field spectroscopy of GS-9209, a massive, compact quiescent galaxy at $z=4.66$ ($\log (M_{\ast}/M_{\odot})=10.52 \pm 0.06$, $R_{eff}=220 \pm 20$ pc). Full spectral fitting of the spatially resolved stellar continuum reveals a clear rotational pattern, yielding a spin parameter of $λ_{2R_{eff}}=0.85 \pm 0.10$. This study suggests that at least a fraction of the earliest quiescent galaxies were fast rotators and that quenching was a dynamically gentle process, preserving the stellar disc even in highly compact objects. Using Jeans anisotropic modelling and assuming a NFW profile, we measure a dark matter fraction of $f_{DM} (<2R_{eff}) = 14.5^{+6.0}_{-4.2} \% $. Our findings use stellar kinematics to confirm the massive nature of early quiescent galaxies, previously inferred from stellar population modelling. We suggest that GS-9209 has a similar structure to low-redshift `relic' galaxies. However, unlike relic galaxies which have bottom-heavy initial mass functions (IMF), the dynamically inferred mass-to-light ratio of GS-9209 is consistent with a Milky-Way like IMF. The kinematical properties of GS-9209 are different from those of $z<1$ early-type galaxies and more similar to those of recently quenched post-starburst galaxies at $z>2$.

Figures (16)

• Figure 1: Top panel: Composite image of GS-9209 obtained by the superposition of F090W, F200W, and F444W NIRCam photometry images. The dotted line is the effective field of view of our NIRSpec observations. A possible satellite is indicated. Bottom panel: Zoomed-in portion of the same image, where we highlight the $0.35 \arcsec \times 0.35 \arcsec$ aperture used for measuring the integrated stellar velocity dispersion $\sigma_{\ast}^{\prime}$.
• Figure 2: Spectrum of the galaxy GS-9209 in blue (within a $0.35 \arcsec \times 0.35 \arcsec$ square aperture) smoothed and cleaned for outliers and non-valid values. It shows a strong Balmer break of $\approx$ 2 - 2.25, indicating that old stellar populations (spectral class A and later) dominate. Some particular absorption and emission lines are highlighted. We also show the spectrum fitted with the ppxf algorithm (with the red colour) and the (weak) emission lines from the gas templates in magenta. Residuals are shown in green.
• Figure 3: The original image in the NIRCam F200W band (a $40 \times 40$ pixels zoom-in around the brightest pixel; the NIRCam photometry pixel scale is $0.03 \arcsec$ per pixel) is shown in the left panel. Results of the PySersic fitting procedure of the original image are displayed in the middle panel. The panel on the right hand side illustrates the residuals from the fit i.e. $\rm res = \rm original - \rm model$. On each of these images, we display the FWHM size ($0.065 \arcsec$) of the PSF of the NIRCam instrument in the F200W band.
• Figure 4: Maps of the observed (i.e. not de-projected) first (left panel) and second (right panel) moments of the stellar kinematics for the massive quiescent galaxy GS-9209 i.e. the stellar line-of-sight velocity ($V_{\ast}$) and the stellar velocity dispersion ($\sigma_{\ast}$). The left panel indicates a rotational pattern of the motions of the stars in this galaxy. The right panel shows a dispersion dominated central region. The spatially resolved kinematics maps extend to about $4 \ \rm spaxels=0.2\arcsec$, which means approximately $6 \ R_{\rm eff}\approx 1.3 \ \rm kpc$. We furthermore show the outlines separating the Voronoi bins from each other and the positions of the centres of each Voronoi bin. The spaxels contained within each Voronoi bin are all colour coded according to either $V_{\ast}$ (left panel) or $\sigma_{\ast}$ (right panel). The plots also display the PSF FWHM of the NIRSpec IFU instrument at $\lambda_{\rm obs} =2 \ \mu m$.
• Figure 5: Outputs of the fiducial dynamical model. In this model, we consider a Gaussian prior on the mass of the central BH (as shown on the bottom right corner plot panel; see the details in \ref{['sec:dynamical_modelling']}) and we furthermore assume a classic NFW density profile of the dark matter halo. We obtain a Dark Matter fraction of about $14.5^{+6.0}_{-4.2}$% within two effective radii. In this plot, we denote $\log \left(M_{\ast}/L\right) \equiv \log \left[ \left(M_{\ast}/L\right)/\left(M_{\odot}/L_{\odot}\right) \right]$. The fiducial dynamical model has $\chi_{r}^{2}=1.4$. The central black dashed line corresponds to the major axis of the galaxy which we will use in the calculation of $\left(V/\sigma\right)_{\ast}$ in Section \ref{['sec:V_sigma']}.