[Paper Review] Rising from the ashes: evidence of old stellar populations and rejuvenation events in the very early Universe

Callum Witten, William McClymont+13 more|arXiv (Cornell University)|Jul 10, 2024

Scientific Research and Discoveries4 citations

TL;DR

This study presents the first spectroscopic evidence of a $z\sim7.9$ galaxy, A2744-YD4, hosting both a mature $\sim80$ Myr stellar population and a recent rejuvenation event following a $\sim20$ Myr quiescent phase. The Balmer break is preserved despite a young stellar population contributing $\sim30\%$ of the total mass, demonstrating that bursty star formation histories can obscure old stellar population signals in early galaxies.

ABSTRACT

While JWST has observed galaxies assembling as early as $z\sim14$, evidence of galaxies with significant old stellar populations in the Epoch of Reionisation (EoR) -- the descendants of these earliest galaxies -- are few and far between. Bursty star-formation histories (SFHs) have been invoked to explain the detectability of the earliest UV-bright galaxies, but also to interpret galaxies showing Balmer breaks without nebular emission lines. We present the first spectroscopic evidence of a $z\sim7.9$ galaxy, A2744-YD4, which shows a Balmer break and emission lines, indicating the presence of both a mature and young stellar population. The spectrum of A2744-YD4 shows peculiar emission line ratios suggesting a relatively low ionisation parameter and high gas-phase metallicity. A median stack of galaxies with similar emission line ratios reveals a clear Balmer break in their stacked spectrum. This suggests that a mature stellar population ($\sim 80$ Myr old) has produced a chemically enriched, disrupted interstellar medium. Based on SED-fitting and comparison to simulations, we conclude that the observed young stellar population is in fact the result of a rejuvenation event following a lull in star formation lasting $\sim 20$ Myr, making A2744-YD4 and our stack the first spectroscopic confirmation of galaxies that have rejuvenated following a mini-quenched phase. These rejuvenating galaxies appear to be in an exceptional evolutionary moment where they can be identified. Our analysis shows that a young stellar population of just $\sim 30 \%$ of the total stellar mass would erase the Balmer break. Hence, 'outshining' through bursty SFHs of galaxies in the early Universe is likely plaguing attempts to measure their stellar ages and masses accurately.

Motivation & Objective

• To identify and characterize old stellar populations in high-redshift galaxies using JWST spectroscopy.
• To resolve the ambiguity in Balmer break detection caused by competing effects of young stars and nebular emission.
• To test whether bursty star formation histories can mask the presence of mature stellar populations in early galaxies.
• To investigate the physical mechanisms behind short-timescale quenching and rejuvenation events in the early Universe.
• To assess the impact of stochastic star formation on stellar mass and age estimates in high-redshift galaxy SED fitting.

Proposed method

• Acquired JWST/NIRSpec spectroscopy of A2744-YD4, a high-redshift galaxy at $z\sim7.9$, to measure emission line ratios and detect the Balmer break.
• Performed SED fitting using PROSPECTOR to model the galaxy's star formation history and infer stellar population ages and metallicities.
• Constructed a median stack of galaxies with similar emission line ratios to enhance signal-to-noise and detect the Balmer break in the stacked spectrum.
• Compared observed spectra and SEDs to cosmological simulations (SPHINX 20, PROSPECTOR) to validate the inferred quenching and rejuvenation timescales.
• Used emission line diagnostics (e.g., [O III]/H$\beta$, [N II]/H$\alpha$) to infer low ionisation parameter and high gas-phase metallicity.
• Quantified the impact of young stellar populations on Balmer break strength by modeling the contribution of $\sim30\%$ stellar mass in young stars.

Figure 1: (Left) Positions of the three NIRSpec/JWST slit positions used to observe A2744-YD4, the central slit of each observation is indicated in green. For scale, the NIRSpec slit is approximately $0.2^{\prime\prime}\times 0.46^{\prime\prime}$ . The NIRSpec observations cover the galaxy YD4 while

Experimental results

Research questions

• RQ1 Can a Balmer break be preserved in a high-redshift galaxy with a significant young stellar population?
• RQ2 What is the physical origin of the observed Balmer break and emission line ratios in A2744-YD4?
• RQ3 How do bursty star formation histories affect the detectability of old stellar populations in early galaxies?
• RQ4 What are the timescales and mechanisms behind short-lived quenching and rejuvenation events in $z\sim8$ galaxies?
• RQ5 To what extent do stochastic star formation histories bias stellar mass and age estimates in high-redshift galaxy SED fitting?

Key findings

• A2744-YD4 exhibits a clear Balmer break at $z\sim7.9$, indicating a mature stellar population of $\sim80$ Myr, despite strong emission lines from a recent burst.
• The galaxy's emission line ratios suggest a low ionisation parameter and high gas-phase metallicity, consistent with a chemically enriched interstellar medium.
• A median stack of galaxies with similar line ratios reveals a statistically significant Balmer break, confirming the presence of old stellar populations in a broader population.
• SED fitting and simulation comparisons indicate a $\sim20$ Myr quiescent phase followed by a rejuvenation event, making A2744-YD4 the first spectroscopically confirmed rejuvenated galaxy at $z\sim8$.
• A young stellar population contributing $\sim30\%$ of the total stellar mass is sufficient to erase the Balmer break, demonstrating its fragility as a diagnostic.
• The results imply that bursty star formation histories can severely bias age and mass estimates in early galaxies, leading to systematic underestimation of stellar populations.

Figure 2: The Balmer break strength, $F_{\nu,4225}/F_{\nu,3565}$ , using the definition of Roberts-Borsani et al. ( 2024 ) , showing the break strength in YD4 (star) and our two stacks (triangles). These are compared to the median break strength found in the SPHINX 20 simulations (dashed line; Rosda