RUBIES confirms many massive quiescent galaxies at 2 < z < 5

What the study found: Massive quiescent galaxies, defined here as galaxies with log(M*/M⊙) > 10.3 and little or no ongoing star formation, are surprisingly common at 2 < z < 5. The study reports number densities of ≳10−5 Mpc−3 by 4 < z < 5.

Why the authors say this matters: The authors conclude that these are the most complete spectroscopic estimates available before cosmic noon, and they say the results are consistent with earlier JWST photometry-based studies and/or smaller survey areas. They also find that, at z > 3, most simulations do not produce enough massive quiescent galaxies, which the study suggests points to missing or incomplete treatments of feedback and/or early galaxy formation channels in many galaxy evolution models.

What the researchers tested: The researchers used JWST NIRSpec PRISM spectra from RUBIES, along with NIRCam photometry, to identify quiescent galaxy candidates and build a final sample. They applied principal component analysis, derived star formation histories through spectrophotometric fitting, and inverted the RUBIES selection function to correct for survey incompleteness before calculating number densities.

What worked and what didn't: The RUBIES data enabled robust inference of physical properties and number densities, and the resulting spectroscopic estimates support a high abundance of massive quiescent galaxies at these redshifts. When compared with six state-of-the-art cosmological galaxy formation simulations, most models at z > 3 underproduced these galaxies.

What to keep in mind: The abstract does not describe detailed limitations beyond the need to correct for survey incompleteness and the comparison to a specific redshift range and mass threshold. The findings apply to massive quiescent galaxies with log(M*/M⊙) > 10.3 at 2 < z < 5.

Key points

• The study reports that massive quiescent galaxies are surprisingly common at 2 < z < 5.
• By 4 < z < 5, their number density is reported as ≳10−5 Mpc−3.
• The analysis uses JWST NIRSpec PRISM spectra from RUBIES plus NIRCam photometry.
• The authors say the spectroscopic estimates are the most complete available before cosmic noon.
• At z > 3, most compared galaxy formation simulations underpredict these galaxies.