Cosmic Stillness: High Quiescent Galaxy Fractions Across Upper Mass Scales in the Early Universe to z = 7 with JWST

We present a detailed investigation into the abundance and morphology of high redshift quenched galaxies at $3 < z < 7$ using James Webb Space Telescope data in the NEP, CEERS and JADES fields. Within these fields, we identify 90 candidate passive galaxies using specific star formation rates modelled with the BAGPIPES SED fitting code, which is more effective at identifying recently quenched systems than the classical UVJ method. With this sample of galaxies, we find number densities broadly consistent with other works and a rapidly evolving passive fraction of high mass galaxies ($\log_{10}{(M_{\star}/M_{\odot})} > $ 9.5) between $3 < z < 5$. We find that the fraction of galaxies with low star formation rates and mass 9.5 $ < \log_{10}{(M_{\star}/M_{\odot})} < $ 10.5 decreases from $\sim$25% at $3 < z < 4$ to $\sim$2% at $5 < z < 7$. Our passive sample of galaxies is shown to exhibit more compact light profiles compared to star-forming counterparts and some exhibit traces of AGN activity through detections in either the X-ray or radio. At the highest redshifts ($z > 6.5$) passive selections start to include examples of ’little red dots’ which complicates any conclusions until their nature is better understood.

💡 Research Summary

This paper presents a comprehensive analysis of quiescent (passive) galaxies in the early universe, utilizing the unprecedented infrared imaging capabilities of the James Webb Space Telescope (JWST). The study focuses on the redshift range 3 < z < 7, corresponding to when the universe was between approximately 700 million and 2 billion years old.

The research team analyzed data from three major JWST deep imaging surveys: the Cosmic Evolution Early Release Science (CEERS) survey, the North Ecliptic Pole Time Domain Field (NEP-TDF, part of PEARLS), and the JWST Advanced Deep Extragalactic Survey (JADES). A consistent data reduction pipeline (EPOCHS) was applied to process the NIRCam multi-band images from these fields, ensuring uniform photometric measurements.

To identify galaxies that have ceased star formation, the authors employed spectral energy distribution (SED) fitting using the BAGPIPES code. Instead of relying on the traditional UVJ color-color selection, which can miss recently quenched systems, they selected quiescent candidates based on very low specific star formation rates (sSFR) derived from the SED models. This method yielded a robust sample of 90 candidate quiescent galaxies across the three fields.

The core findings of the study are multi-faceted:

1. Rapid Evolution of the Quiescent Fraction: The research reveals a dramatic decrease in the fraction of massive quiescent galaxies with increasing redshift. For galaxies with stellar masses between 10^9.5 and 10^10.5 solar masses, the quiescent fraction drops from about 25% at z~3-4 to merely 2% at z5-7. This indicates that the process of forming massive galaxies and subsequently shutting down their star formation was extremely efficient in the universe’s first few billion years.
2. Distinct Morphological Properties: The quiescent candidate galaxies were found to be systematically more compact than their star-forming counterparts at similar redshifts. They exhibit smaller half-light radii and more concentrated light profiles (higher Sersic indices), reminiscent of the spheroidal shapes of local elliptical galaxies. This suggests that the structural transformation of galaxies into “red and dead” systems occurs concurrently with, or shortly after, the quenching of star formation.
3. Potential AGN Connection: A subset of the quiescent candidates shows signs of Active Galactic Nuclei (AGN) activity, evidenced by detections in X-ray or radio wavelengths. This finding supports theoretical models where feedback from a supermassive black hole (AGN feedback) plays a crucial role in expelling or heating a galaxy’s gas reservoir, thereby rapidly quenching star formation.
4. Tension with Theoretical Models: The observed number densities of massive quiescent galaxies at z > 3 are higher than predictions from current state-of-the-art cosmological simulations like Illustris-TNG and semi-analytic models like SHARK. This discrepancy highlights a challenge for galaxy formation theory: models must now account for both the rapid assembly of large stellar masses and their equally rapid quenching at very early cosmic times.

The paper also notes a complicating factor at the highest redshifts (z > 6.5), where the quiescent selection begins to include objects known as “Little Red Dots.” The exact nature of these sources—whether they are dust-obscured starbursts or strong AGN—is still debated, and their inclusion cautions against over-interpreting the quiescent galaxy statistics at the epoch of reionization until their properties are better understood.

In conclusion, this JWST-based study provides compelling evidence that a significant population of evolved, quiescent galaxies existed much earlier than previously thought. The compact morphology and potential AGN links in these early quiescent systems offer vital clues about the physical mechanisms driving the cessation of star formation in the nascent universe, pointing towards the significant influence of internal processes like AGN feedback in shaping the first generations of massive galaxies.