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Arxiv: RUBIES: Evolved Stellar Populations with Extended Formation Histories at z \sim 7-8 in Candidate Massive Galaxies Identified with JWST/NIRSpec Published: 5/2/2024 8:06:30 PM Updated: 6/10/2024 8:10:39 PM

Paper abstract: The identification of red, apparently massive galaxies at z>7 in early JWSTphotometry suggests a strongly accelerated timeline compared to standard modelsof galaxy growth. A major uncertainty in the interpretation is whether the redcolors are caused by evolved stellar populations, dust, or other effects suchas emission lines or AGN. Here we show that three of the massive galaxycandidates at z=6.7-8.4 have prominent Balmer breaks in JWST/NIRSpecspectroscopy from the RUBIES program. The Balmer breaks demonstrateunambiguously that stellar emission dominates at \lambda_{\rm rest} =0.4\,\mum, and require formation histories extending hundreds of Myr into thepast in galaxies only 600--800 Myr after the Big Bang. Two of the threegalaxies also show broad Balmer lines, with H\beta FWHM >2500~{\rmkm\,s^{-1}}, suggesting that dust-reddened AGN contribute to, or evendominate, the SEDs of these galaxies at \lambda_{\rm rest}\gtrsim 0.6\,\mum.All three galaxies have relatively narrow [O III] lines, seemingly ruling out ahigh-mass interpretation if the lines arise in dynamically-relaxed, inclineddisks. Yet, the inferred masses also remain highly uncertain. We model thehigh-quality spectra using Prospector to decompose the continuum into stellarand AGN components, and explore limiting cases in stellar/AGN contribution.This produces a wide range of possible stellar masses, spanning M_* ~10^9 - 10^{11}\,{\rm M_{\odot}}. Nevertheless, all fits suggest a very earlyand rapid formation, most of which follow with a truncation in star formation.Potential origins and evolutionary tracks for these objects are discussed, fromthe cores of massive galaxies to low-mass galaxies with over-massive blackholes. Intriguingly, we find all of these explanations to be incomplete; deeperand redder data are needed to understand the physics of these systems.