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

**Authors:** Bingjie Wang, Joel Leja, Anna de Graaff, Gabriel B. Brammer, Andrea Weibel, Pieter van Dokkum, Josephine F. W. Baggen, Katherine A. Suess, Jenny E. Greene, Rachel Bezanson, Nikko J. Cleri, Michaela Hirschmann, Ivo Labbe, Jorryt Matthee, Ian McConachie, Rohan P. Naidu, Erica Nelson, Pascal A. Oesch, David J. Setton, Christina C. Williams  
**arXiv:** [2405.01473](https://arxiv.org/abs/2405.01473)  
**Date:** 2024-05-02  
**Summary page:** https://www.summarizepaper.com/en/paper/2405.01473v1/

## Key points

- A recent study has identified red massive galaxies at $z>7$ in early JWST photometry
- Uncertainty remains regarding the interpretation of these red colors, with possibilities including evolved stellar populations, dust effects, emission lines, or AGN
- Analysis of three massive galaxy candidates at $z=6.7-8.4$ using JWST/NIRSpec spectroscopy revealed prominent Balmer breaks in their spectra
- The presence of broad Balmer lines with FWHM exceeding $2500~{\rm km\,s^{-1}$ suggests dust-reddened AGN contributing to or dominating their SEDs at longer wavelengths
- All three galaxies displayed relatively narrow [O III] lines, which may contradict a high-mass interpretation if these lines arise in dynamically-relaxed inclined disks
- Uncertainties remain regarding the inferred masses of these galaxies despite sophisticated modeling techniques yielding a wide range of possible stellar masses
- The study explores potential origins and evolutionary paths for these enigmatic objects but notes that deeper and redder data are necessary for a complete understanding

## Summary

A recent study has identified red massive galaxies at $z>7$ in early James Webb Space Telescope (JWST) photometry. This suggests an accelerated timeline of galaxy growth compared to standard models. However, the interpretation of these red colors remains uncertain. Possibilities include evolved stellar populations, dust effects, emission lines or active galactic nuclei (AGN). To shed light on this issue, three massive galaxy candidates at $z=6.7-8.4$ were analyzed using JWST/NIRSpec spectroscopy from the RUBIES program. The results revealed prominent Balmer breaks in the spectra of these galaxies, indicating that stellar emission dominates at $\lambda_{\rm rest} = 0.4\,\mu$m. This finding implies formation histories extending hundreds of millions of years into the past for galaxies that are only 600-800 million years old after the Big Bang. Furthermore, two out of the three galaxies exhibited broad Balmer lines with H$\beta$ Full Width at Half Maximum (FWHM) exceeding $2500~{\rm km\,s^{-1}}$, suggesting the presence of dust-reddened AGN contributing to or even dominating their Spectral Energy Distributions (SEDs) at longer wavelengths ($\lambda_{\rm rest}\gtrsim 0.6\,\mu$m). Interestingly, all three galaxies displayed relatively narrow [O III] lines which seemingly contradicts a high-mass interpretation if these lines arise in dynamically-relaxed inclined disks. Despite these intriguing findings, uncertainties remain regarding the inferred masses of these galaxies. By employing sophisticated modeling techniques and decomposing the spectra into stellar and AGN components using Prospector software, a wide range of possible stellar masses ranging from $10^9$ to $10^{11}\,{\rm M_{\odot}}$ were obtained. Nonetheless, all fits indicate an early and rapid formation scenario followed by a truncation in star formation. The study also delves into potential origins and evolutionary paths for these enigmatic objects - from being located in the cores of massive galaxies to existing as low-mass galaxies hosting over-massive black holes. However, it is noted that none of these explanations provide a complete understanding; hence deeper and redder data are deemed necessary to unravel the underlying physics governing these systems.