This research has been conducted by the University of Cambridge and was published on July 14, 2024.
This research, conducted by the University of Cambridge and published on July 14, 2024, explores the intriguing properties of galaxies in the early Universe using data from the JWST NIRSpec PRISM.
In this study, they presented spectra showing both a Balmer break and emission lines, which are significant findings in the context of recent observations of (mini-)quenched galaxies at by JWST. Previous studies have noted that these galaxies often display Balmer breaks without accompanying emission lines. By focusing on galaxies with high metallicity and low ionisation, they found that those at z>5.5z > 5.5 typically have a Balmer break. This sets them apart from galaxies with higher O32 ratios and the general star-forming galaxy population.
Their analysis suggests that these unique emission line ratios are linked to the presence of an older stellar population. Using CLOUDY modelling, they concluded that this is due to the chemically enriched and disrupted interstellar medium (ISM) produced by these older stars. Essentially, these galaxies host both an older stellar population (around 100 million years old) that produces the Balmer break and a younger stellar population (less than 10 million years old) that drives the strong nebular emission lines.
To understand the star formation history (SFH) behind these observations, they used a non-parametric SFH modelling approach with the SED-fitting code PROSPECTOR. Their results indicate that both the galaxy YD4 and their sample stack are best described by a rejuvenating SFH. This means the young stellar population is less than a tenth of the total stellar mass, and the observed emission lines suggest this fraction is close to 5%. Further constraints using SPHINX20 simulations showed that a rejuvenating SFH can match the observed spectral features, including the Balmer break strength and emission line ratios.
They found that while gradual declines in star formation could theoretically produce the observed features, they don’t fit well with the rapid quenching timescales seen in other early Universe galaxies. Their comprehensive modelling and analysis support the idea that these galaxies experience rejuvenation, characterized by chemically enriched, disrupted ISMs and multiple bursts of star formation. This bursty star formation might also explain the excess of UV-bright galaxies observed in the early Universe, possibly due to rapid stellar mass accumulation through galaxy mergers.
Their findings suggest that YD4 and the galaxies in their sample experienced a quiescent period of about 25 million years, followed by rejuvenation due to gas reaccretion or feedback processes. The short quenching timescales indicate that rapid gas consumption or strong AGN feedback are unlikely, a conclusion supported by the lack of evidence for AGN activity. Instead, disruptive feedback from star formation or weak AGN feedback seems more probable.
Overall, these results highlight the importance of considering complex SFHs when modelling early Universe galaxies. They also point out the fragility of using the Balmer break strength as a tracer for old stellar populations, since it can be easily overshadowed by young stars. Therefore, the presence of a Balmer break confirms an old stellar population, but its absence does not necessarily mean there isn’t one. Stochastic SFHs can mask the optical continuum of old stars, leading to potential underestimation of the age and stellar mass of these ancient galaxies.