RUBIES: Evolved Stellar Populations with Extended Formation Histories at $z \sim 7-8$ in Candidate Massive Galaxies Identified with JWST/NIRSpec

TL;DR

JWST/NIRSpec observations of three candidate massive galaxies at $z\sim6.7-8.4$ reveal unambiguous Balmer breaks, indicating evolved stellar populations formed hundreds of Myr earlier in the universe. Two objects also show broad Balmer lines, pointing to dust-reddened AGN contributing to or dominating the rest-optical continuum, while narrow [O III] lines complicate simple high-mass disk interpretations. Spectral energy distribution modeling with Prospector, under three priors that bracket maximal stellar, mixed, and minimal stellar contributions, yields a broad range of stellar masses ($M_\\\ star \\sim 10^9-10^{11}\,M_\odot$) but consistently implies very early, rapid formation with subsequent quenching. The study highlights the tension between early, massive formation and AGN/stellar contributions to the observed light, and stresses that deeper, redder data are needed to fully understand the nature and evolution of these objects.

Abstract

The identification of red, apparently massive galaxies at $z>7$ in early JWST photometry suggests a strongly accelerated timeline compared to standard models of galaxy growth. A major uncertainty in the interpretation is whether the red colors are caused by evolved stellar populations, dust, or other effects such as emission lines or AGN. Here we show that three of the massive galaxy candidates at $z=6.7-8.4$ have prominent Balmer breaks in JWST/NIRSpec spectroscopy from the RUBIES program. The Balmer breaks demonstrate unambiguously that stellar emission dominates at $λ_{\rm rest} = 0.4\,μ$m, and require formation histories extending hundreds of Myr into the past in galaxies only 600--800 Myr after the Big Bang. Two of the three galaxies also show broad Balmer lines, with H$β$ FWHM $>2500~{\rm km\,s^{-1}}$, suggesting that dust-reddened AGN contribute to, or even dominate, the SEDs of these galaxies at $λ_{\rm rest}\gtrsim 0.6\,μ$m. All three galaxies have relatively narrow [O III] lines, seemingly ruling out a high-mass interpretation if the lines arise in dynamically-relaxed, inclined disks. Yet, the inferred masses also remain highly uncertain. We model the high-quality spectra using Prospector to decompose the continuum into stellar and AGN components, and explore limiting cases in stellar/AGN contribution. This produces a wide range of possible stellar masses, spanning $M_\star \sim 10^9 - 10^{11}\,{\rm M_{\odot}}$. Nevertheless, all fits suggest a very early and rapid formation, most of which follow with a truncation in star formation. Potential origins and evolutionary tracks for these objects are discussed, from the cores of massive galaxies to low-mass galaxies with over-massive black holes. Intriguingly, we find all of these explanations to be incomplete; deeper and redder data are needed to understand the physics of these systems.

Figures (5)

• Figure 1: Sample of this paper. (Left) This work focuses on continuum-detected sources with Balmer breaks, shown as solid diamonds on the color plane. The L23 sample is denoted as plus signs, and all sources in CEERS brighter than 27 AB mag are grey hexagons included for reference. (Right) We show the $2\times2$$^{\prime\prime}$ color images of the three Balmer-break-detected sources, colors from JWST/NIRCam F115W, F277W, and F444W. They are remarkably compact at red wavelengths, with some evidence for spatial structure at blue wavelengths.
• Figure 2: Characteristics of the sample of this paper. (a) Balmer breaks detected at $z_{\rm spec} = 6.7 - 8.4$. The scaled individual spectra which exhibit potential Balmer breaks are plotted in light colors, and the averaged spectrum is shown in black. Over-plotted in red is the averaged best-fit model spectrum (§ \ref{['sec:prosp']}). All breaks are located at the expected wavelength, and show similar shapes, supporting the Balmer break interpretation. The break strength for the average observed spectrum, quantified by the flux ratio in the wavelength windows illustrated in blue and red shades, is 2.0. (b) Emission line fits to the H$\beta$ and [O iii] complex (§ \ref{['sec:lines']}). Unambiguous broadened H$\beta$ lines are found in the G395M spectrum for two sources (RUBIES-EGS-49140 and 55604), indicative of an AGN. A broad component of H$\beta$ is only marginally detected in both the G395M and Prism fits for RUBIES-EGS-966323.
• Figure 3-a: Spectrophotometric modeling for RUBIES-EGS-49140, with models including maximal / medium / minimal stellar contribution shown. The other two objects with detected Balmer breaks are shown in subsequent figures. (Left panels) The photometric and spectroscopic data are shown in gray. The best-fit model spectrum, which includes the marginalized emission lines as annotated, is plotted in light brown. The emission at $\sim 3869$ Å is likely [Ne iii] 3869, although He I 3889 is also possible. The galaxy and the AGN continuum components are over-plotted in blue and red, respectively. The spectral break strength, SBS, predicted by the galaxy model spectrum is indicated to the upper left corner. The spectral regions that are masked due to low S/N or detector gap are shaded in gray. (Right panels) The inferred SFRs are plotted as a function of the age of the universe. The gray and light gray shading indicates 1$\sigma$ and 2$\sigma$ uncertainties, respectively. The post-starburst feature is primarily driven by the Balmer break.
• Figure 3-b: Spectrophotometric modeling for RUBIES-EGS-55604 (identified as 38094 in L23), with format as described in Figure \ref{['fig:sed_a']}.
• Figure 3-c: Spectrophotometric modeling for RUBIES-EGS-966323 (identified as 14924 in L23), with format as described in Figure \ref{['fig:sed_a']}.