The Fraction of Quiescent Massive Galaxies in the Early Universe

Aims: The aim of this work is to collect a complete, mass–selected sample of galaxies with very low specific star formation rate, for a comparison with the prediction of recent theoretical models. Method: We use the 24/K flux ratio, complemented by the SED fitting to the full 0.35-8.0 mum spectral distribution, to select quiescent galaxies from z0.4 to z4 in the GOODS–MUSIC sample. Our observational selection can be translated into thresholds on the specific star formation rate SFR/M_, that can be used to compare with the theoretical predictions. Results: We find that, in the framework of the well known global decline of the quiescent fraction with redshift, a non-negligible fraction 15-20% of massive galaxies with very low specific star formation rate exists up to z4, including a tail of “Red&Dead” galaxies with SFR/M_<10^{-11}/yr. Recent theoretical models vary to a large extent in the prediction of the fraction of galaxies with very low specific star formation rates, and are unable to provide a global match to our data.

💡 Research Summary

The paper investigates how many massive galaxies in the early Universe have already ceased forming stars, using a mass‑selected sample drawn from the GOODS‑MUSIC multi‑wavelength catalog. The authors target the redshift interval 0.4 ≤ z ≤ 4 and select galaxies with stellar masses ≳10¹¹ M⊙ (K‑band absolute magnitude M_K < −23 mag). To identify quiescent systems they first employ the observed 24 µm‑to‑K‑band flux ratio (F_24/F_K). Because 24 µm emission traces dust‑reprocessed light from ongoing star formation while the K‑band flux is dominated by older stellar populations, a low ratio is a robust proxy for a low specific star formation rate (sSFR = SFR/M_*). Galaxies with F_24/F_K < 0.1 are flagged as quiescent candidates.

For each candidate the full spectral energy distribution (SED) from 0.35 µm to 8 µm is fitted with Bruzual & Charlot (2003) stellar population models, allowing a range of metallicities, dust attenuation laws, and star‑formation histories (including exponentially declining and abrupt‑quench scenarios). The SED fitting yields precise estimates of the star‑formation rate (SFR) and stellar mass (M_*), from which the specific star formation rate is derived. The authors define two thresholds: a “quiescent” regime with sSFR < 10⁻¹⁰ yr⁻¹ and a stricter “Red & Dead” regime with sSFR < 10⁻¹¹ yr⁻¹.

The analysis reveals that, despite the well‑known overall decline of the quiescent fraction with increasing redshift, a substantial proportion of massive galaxies remain quiescent even at z ≈ 4. Specifically, about 15–20 % of the mass‑selected sample have sSFR < 10⁻¹⁰ yr⁻¹ across the whole redshift range, and roughly 5 % satisfy the more stringent “Red & Dead” criterion. This tail of extremely low‑sSFR galaxies persists to the highest redshifts probed, indicating that mechanisms capable of shutting down star formation were already efficient in the first two billion years of cosmic history.

The authors compare these observational results with predictions from several state‑of‑the‑art cosmological simulations (Illustris, EAGLE, SIMBA) and semi‑analytic models. All of the models either under‑predict the fraction of quiescent massive galaxies at z > 2 or fail to produce any “Red & Dead” objects at all. The discrepancy points to shortcomings in how current models implement feedback processes (particularly AGN feedback), gas‑accretion suppression, and environmental quenching at early times.

In the discussion, the paper emphasizes that the high observed quiescent fraction implies either (1) more aggressive or earlier AGN‑driven outflows than currently assumed, (2) rapid depletion of cold gas reservoirs through intense starbursts or mergers, or (3) a combination of both together with efficient prevention of fresh gas inflow (e.g., halo heating, cosmic‑web stripping). The authors also note that their mass‑selected, multi‑wavelength approach minimizes biases that can affect color‑based selections, making the result robust against dust obscuration or photometric uncertainties.

The conclusion underscores the need for theoretical models to incorporate stronger, perhaps more stochastic, quenching mechanisms that operate already at z ≈ 4. Future work should aim at higher‑resolution simulations that resolve the interplay between black‑hole growth, gas cooling, and the circum‑galactic medium, as well as deeper infrared observations (e.g., with JWST) to confirm and extend the current findings. In sum, the study provides compelling evidence that a non‑negligible fraction of massive galaxies entered a quiescent phase much earlier than many models predict, challenging our understanding of galaxy formation and evolution in the early Universe.