Efficient formation of a massive quiescent galaxy at redshift 4.9

TL;DR

RUBIES-EGS-QG-1 is a massive ($M_*\approx10^{11}\,M_\odot$) quiescent galaxy at $z\approx4.9$ that formed most of its stellar mass in a brief $\Delta t\approx180$ Myr burst and quenched by $z\gtrsim5.5$, as revealed by JWST spectroscopy and SED modeling. The galaxy's rapid assembly and early quenching challenge current galaxy formation models, which predict such objects to be extremely rare at this epoch. Comparisons with large-volume simulations show RUBIES-EGS-QG-1 is a significant outlier (about $2$–$3\sigma$) unless the host halo is exceptionally massive or feedback physics are incomplete, motivating revisions to early-universe star formation and quenching prescriptions. An overdense environment at $z\approx4.9$ supports formation in a substantial halo, while a NOEMA submillimeter non-detection reinforces the quiescent interpretation; collectively, the results constrain the efficiency and timing of early massive galaxy growth.

Abstract

Within the established framework of structure formation, galaxies start as systems of low stellar mass and gradually grow into far more massive galaxies. The existence of massive galaxies in the first billion years of the Universe, suggested by recent observations, appears to challenge this model, as such galaxies would require highly efficient conversion of baryons into stars. An even greater challenge in this epoch is the existence of massive galaxies that have already ceased forming stars. However, robust detections of early massive quiescent galaxies have been challenging due to the coarse wavelength sampling of photometric surveys. Here we report the spectroscopic confirmation with the James Webb Space Telescope of the quiescent galaxy RUBIES-EGS-QG-1 at redshift $z=4.90$, 1.2 billion years after the Big Bang. Deep stellar absorption features in the spectrum reveal that the galaxy's stellar mass of $10^{11}\,M_\odot$, corroborated by the mass implied by its gas kinematics, formed in a short $200\,$Myr burst of star formation, after which star formation activity dropped rapidly and persistently. According to current galaxy formation models, systems with such rapid stellar mass growth and early quenching are too rare to plausibly occur in the small area probed spectroscopically with JWST. Instead, the discovery of RUBIES-EGS-QG-1 implies that early massive quiescent galaxies can be quenched earlier or exhaust gas available for star formation more efficiently than currently assumed.

Figures (9)

• Figure 1: JWST/NIRSpec PRISM spectrum of the massive quiescent galaxy RUBIES-EGS-QG-1 at a redshift of $z=4.8976$. The inset shows the medium-resolution (NIRSpec G395M) spectrum around the wavelength of H$\alpha$. Both spectra were calibrated to the measured photometry using Prospector . The spectrum shows deep Balmer absorption lines, similar to post-starburst galaxies at lower redshifts, and implies a lack of star formation in its recent history. The presence of the emission line doublets [O iii], [N ii], [S ii], and the minimal inferred infilling of the H$\alpha$ absorption line are consistent with AGN activity.
• Figure 2: The history of stellar mass growth in RUBIES-EGS-QG-1 . Top: Star formation history inferred from the modeling to the PRISM spectrum and photometry for the fiducial (free-metallicity) model (purple) and the fixed-solar metallicity model (blue). Dark (light) shaded regions indicate the $1\sigma$ ($2\sigma$) confidence intervals of the posterior distributions. Bottom: The cumulative mass history inferred from the star formation history of the two models. In orange we show the maximum stellar mass formed for a typical halo at the observed number density of massive quiescent galaxies at $z>4$Valentino2023, assuming a universal baryon-to-total matter ratio ($f_{\rm B}$) and different baryon-to-stellar conversion factors ($\epsilon$). This indicates that a short ($\approx 300\,$Myr) burst of star formation with high efficiency of $\epsilon>0.2$ is required to form RUBIES-EGS-QG-1 , corresponding to an efficiency at or greater than the peak of the stellar-halo mass relationWechsler2018.
• Figure 3: Spatial clustering of spectroscopically-confirmed sources at $z\approx4.90$ (circles) around RUBIES-EGS-QG-1 (red star). The submillimeter galaxy, the brightest source among the group of 4 at a projected distance of 16 comoving Mpc, is indicated by a purple triangle. The background image shows the NIRCam F444W mosaic of the EGS field. Compared to the typical projected spatial clustering (within redshift ranges $\Delta z=0.03$) and redshift clustering (within apertures of radius $<3\,\rm Mpc$) for galaxies in the redshift range $4<z<6$ with robust redshifts from JWST spectroscopy, RUBIES-EGS-QG-1 clearly resides in an overdense environment, forming the highest redshift known overdensity hosting a massive quiescent galaxy. We show false-color images (created from NIRCam F150W, F277W and F444W images) of RUBIES-EGS-QG-1 and its 6 nearby neighbors.
• Figure 4: NIRCam F115W and F444W image cutouts of RUBIES-EGS-QG-1 . Orange lines show the location of the NIRSpec microshutters.
• Figure 5: The covariant posteriors for a selected set of parameters in our fiducial fit with Prospector , with contours bounding 68% and 95% of the likelihood and dashed lines capturing the 95% confidence interval on the marginalized posteriors.