A massive quiescent galaxy at redshift 4.658

Abstract

The extremely rapid assembly of the earliest galaxies during the first billion years of cosmic history is a major challenge for our understanding of galaxy formation physics. The advent of JWST has exacerbated this issue by confirming the existence of galaxies in significant numbers as early as the first few hundred million years. Perhaps even more surprisingly, in some galaxies, this initial highly efficient star formation rapidly shuts down, or quenches, giving rise to massive quiescent galaxies as little as 1.5 billion years after the Big Bang. However, due to their faintness and red colour, it has proven extremely challenging to learn about these extreme quiescent galaxies, or to confirm whether any exist at earlier times. Here we report the spectroscopic confirmation of a massive quiescent galaxy, GS-9209, at redshift $z=4.658$, just 1.25 billion years after the Big Bang, using JWST NIRSpec. From these data we infer a stellar mass of $M_* = 3.8\pm0.2\times10^{10}\ M_\odot$, which formed over a $\simeq200$ Myr period before this galaxy quenched its star formation activity at $z=6.5^{+0.2}_{-0.5}$, when the Universe was $\simeq800$ million years old. Based on the presence of broad H$α$ in the spectrum and a high narrow-line [NII]/H$α$ ratio, we infer the presence of an accreting supermassive black hole, with a mass of $M_\bullet = 5\pm1\times10^{8}\ M_\odot$. This large black hole mass relative to the stellar mass suggests that active galactic nucleus (AGN) feedback may have been responsible for quenching this galaxy. GS-9209 is also extremely compact, with an effective radius, $r_e=215\pm20$ parsecs. This galaxy is both a likely descendent of the highest-redshift submillimetre galaxies and quasars, and a likely progenitor for the dense, ancient cores of the most massive local galaxies.

Figures (4)

• Figure 1: JWST NIRSpec observations of GS-9209. Data were taken on 16th November 2022, using the G235M and G395M gratings ($R=1000$) with integration times of 3 hours and 2 hours respectively, providing wavelength coverage from $\lambda=1.7-5.1\mu$m. The galaxy is at a redshift of $z=4.6582\pm0.0002$, and exhibits extremely deep Balmer absorption lines. The spectrum strongly resembles that of an A-type star, and is reminiscent of lower-redshift post-starburst galaxies Goto2007Wild2009Wild2020, clearly indicating this galaxy experienced a significant, rapid drop in star-formation rate (SFR) within the past few hundred million years. The spectral region from $\lambda=2.6-4.0\mu$m, containing $H\beta$ and $H\alpha$, is shown at a larger scale in Figure \ref{['fig:spectrum2']}.
• Figure 2: JWST NIRSpec observations of GS-9209: zoom in on H$\boldsymbol{\beta}$ and H$\boldsymbol{\alpha}$. Data are shown in blue, with their associated ($1\sigma$ standard deviation) uncertainties visible at the bottom in purple. The full Bagpipes fitted model is shown in black, with the AGN component shown in red. The narrow H$\alpha$ and [N ii] lines were masked during the Bagpipes fitting process, and subsequently fitted with Gaussian functions, shown in green. Key emission and absorption features are also marked.
• Figure 3: The star-formation rate and stellar mass of GS-9209 as a function of time. Panel a shows the star-formation rate (SFR) as a function of time (the star-formation history). Panel b shows the stellar mass as a function of time. The blue lines show the posterior medians, with the darker and lighter shaded regions showing the 1$\sigma$ and 2$\sigma$ confidence intervals respectively. We find a formation redshift, $z_\mathrm{form}=6.9\pm0.2$ and a quenching redshift, $z_\mathrm{quench} = 6.5^{+0.2}_{-0.5}$. The sample of massive $z\simeq8$ galaxy candidates from JWST CEERS reported by Labbe2022 is also shown in the right panel, demonstrating that these candidates are plausible progenitors for GS-9209. The uncertainties shown on the red points are $1\sigma$ standard deviation values.
• Figure 4: JWST NIRCam imaging of GS-9209. Each cutout image shows an area of $1.5^{\prime\prime}\times1.5^{\prime\prime}$. Panel a is a RGB image, constructed with F430M as red, F210M as green and F182M as blue. Panel b shows the F210M image, with our posterior median PetroFit model shown in Panel c. Panel d shows the residuals between model and data, on the same colour scale as panels b and c.