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RUBIES spectroscopically confirms the high number density of quiescent galaxies from $\mathbf{2<z<5}$

Yunchong Zhang, Anna de Graaff, David J. Setton, Sedona H. Price, Rachel Bezanson, Claudia del P. Lagos, Sam E. Cutler, Ian McConachie, Nikko J. Cleri, Olivia R. Cooper, Rashmi Gottumukkala, Jenny E. Greene, Michaela Hirschmann, Gourav Khullar, Ivo Labbe, Joel Leja, Michael V. Maseda, Jorryt Matthee, Tim B. Miller, Themiya Nanayakkara, Katherine A. Suess, Bingjie Wang, Katherine E. Whitaker, Christina C. Williams

TL;DR

This paper delivers a spectroscopic census of massive quiescent galaxies at $2<z<5$ using JWST/NIRSpec PRISM data from the RUBIES program, applying a PCA-based, data-driven selection to robustly identify quiescent candidates. Through joint spectro-photometric modeling with Prospector, the authors infer SFHs and stellar properties, correct for targeting incompleteness, and compute the comoving number densities for $M_* > 10^{10.5}M_\odot$ across three redshift bins. They find that massive quiescent galaxies are relatively common by cosmic noon, with densities around $10^{-5}$–$10^{-4}$ Mpc$^{-3}$ and that the observed abundances at $z>3$ are significantly higher than predictions from six state-of-the-art simulations, even after accounting for measurement uncertainties and cosmic variance. The work emphasizes the importance of spectroscopic confirmation and robust selection functions to mitigate contamination in photometric samples, and it calls for larger-volume JWST surveys to reduce cosmic variance and to constrain quenching physics in the early universe.

Abstract

We present the number density of massive ($ \mathrm{ log (M_{*}/M_{\odot}) > 10.3} $) quiescent galaxies at $2<z<5$ using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES through principal component analysis and construct a final sample using star formation histories derived from spectro-photometric fitting of the NIRSpec PRISM spectra and NIRCam photometry. By inverting the RUBIES selection function, we correct for survey incompleteness and calculate the number density of massive quiescent galaxies at these redshifts, providing the most complete spectroscopic estimates prior to cosmic noon to date. We find that early massive quiescent galaxies are surprisingly common ($\gtrsim 10^{-5}$ Mpc$^{-3}$ by $4<z<5$), which is consistent with previous studies based on JWST photometry alone and/or in smaller survey areas. We compare our number densities with predictions from six state-of-the-art cosmological galaxy formation simulations. At $z>3$, most simulations fail to produce enough massive quiescent galaxies, suggesting the treatment of feedback and/or the channels for early efficient formation are incomplete in most galaxy evolution models.

RUBIES spectroscopically confirms the high number density of quiescent galaxies from $\mathbf{2<z<5}$

TL;DR

This paper delivers a spectroscopic census of massive quiescent galaxies at using JWST/NIRSpec PRISM data from the RUBIES program, applying a PCA-based, data-driven selection to robustly identify quiescent candidates. Through joint spectro-photometric modeling with Prospector, the authors infer SFHs and stellar properties, correct for targeting incompleteness, and compute the comoving number densities for across three redshift bins. They find that massive quiescent galaxies are relatively common by cosmic noon, with densities around – Mpc and that the observed abundances at are significantly higher than predictions from six state-of-the-art simulations, even after accounting for measurement uncertainties and cosmic variance. The work emphasizes the importance of spectroscopic confirmation and robust selection functions to mitigate contamination in photometric samples, and it calls for larger-volume JWST surveys to reduce cosmic variance and to constrain quenching physics in the early universe.

Abstract

We present the number density of massive () quiescent galaxies at using JWST NIRSpec PRISM spectra. This work relies on spectra from RUBIES, which provides excellent data quality and an unparalleled, well-defined targeting strategy to robustly infer physical properties and number densities. We identify quiescent galaxy candidates within RUBIES through principal component analysis and construct a final sample using star formation histories derived from spectro-photometric fitting of the NIRSpec PRISM spectra and NIRCam photometry. By inverting the RUBIES selection function, we correct for survey incompleteness and calculate the number density of massive quiescent galaxies at these redshifts, providing the most complete spectroscopic estimates prior to cosmic noon to date. We find that early massive quiescent galaxies are surprisingly common ( Mpc by ), which is consistent with previous studies based on JWST photometry alone and/or in smaller survey areas. We compare our number densities with predictions from six state-of-the-art cosmological galaxy formation simulations. At , most simulations fail to produce enough massive quiescent galaxies, suggesting the treatment of feedback and/or the channels for early efficient formation are incomplete in most galaxy evolution models.

Paper Structure

This paper contains 18 sections, 5 equations, 9 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data
  3. RUBIES NIRSpec/MSA Spectroscopy
  4. NIRCam photometry in the EGS and UDS
  5. Method
  6. PCA Selection
  7. Spectro-Photometric Fitting
  8. The Selection of Quiescent Galaxies
  9. The Selection and Classification of the RUBIES Quiescent Galaxy Sample
  10. How Would This Sample Have Been Selected with Rest-frame Colors?
  11. Correcting for Targeting Incompleteness with the RUBIES Selection Function
  12. Quiescent Galaxy Number Densities
  13. The Number Density of Massive Quiescent Galaxies in the RUBIES
  14. Comparing to Other Observational Results
  15. There Are More Massive Quiescent Galaxies in the Early Universe Than Predicted by Simulations
  16. ...and 3 more sections

Figures (9)

  • Figure 1: Demonstration of the PCA analysis of NIRSpec/PRISM spectra from the DJA: The top row includes super color (SC) distributions (left and center) and derived eigenspectra (right panel). Each SC corresponds to the normalization of an eigenspectrum for each individual source, thus SC space location maps to spectral types. The bottom rows highlight representative examples: Dusty Star-Forming Galaxies (brown triangle), Little Red Dots (yellow circle), Old Quiescent Galaxies (red hexagon), Young Quiescent/Post-Starburst Galaxies (green star), Evolved/Napping Star-Forming Galaxies (blue cross), and Young Star-Forming Galaxies (purple cross). The de-redshifted original spectrum is shown in grey. The SpectRes-resampled spectrum used in PCA is shown as black dots.
  • Figure 2: Example spectro-photometric fits of a $z\sim4$ post-starburst galaxy (ID: RUBIES-UDS-12594) and an old quiescent galaxy (ID: RUBIES-EGS-42328). For each row, the upper left panel shows the SED of observed NIRCam photometry and uncertainty (orange circle and error bar), observed NIRSpec PRISM spectrum and uncertainty (red solid line and pink bands), best-fit model photometry and its 68% confidence interval (black square and errorbar), and best-fit model spectrum and its 68% confidence interval (black solid line and grey band). The bottom left panel shows the residuals of our fits to the observed photometry (orange squares) and spectrum (red solid line). The inset shows the NIRCam/F444W image of the target galaxy. The red rectangle traces the central MSA micro-shutter used to compute slit-like aperture photometry, and the red circle traces the circular photometric catalog aperture. The right panels show the median (lines) and $16-84\%$ intervals (bands) for the inferred SFHs and $t_{90}$ measurements. These fits allow us to robustly determine the physical properties of these galaxies.
  • Figure 3: Left panel: Stellar mass versus sSFR for SC-selected quiescent galaxy candidates. Robust quiescent galaxies (Criterion 1) are shown in red, and marginally quiescent cases (Criterion 2) are shown in pink. Galaxies with the 50th percentile $\mathrm{ sSFR} < 10^{-10}\, \mathrm{yr}^{-1}$ are shown as upper limits. One galaxy (ID: RUBIES-EGS-61168) is not in the range of this plot due to its extremely low sSFR. Middle panel: stellar mass versus $\mathrm t_{90}$ for all galaxies. We additionally divide our sample into young quiescent galaxies ($\mathrm t_{90} < 0.8$ Gyr, red or pink) and old quiescent galaxies ($\mathrm t_{90} > 0.8$ Gyr, black). Right panel: Stellar mass versus spectroscopic redshift of the finalized sample galaxies. The old quiescent population emerges at $z \sim 3$.
  • Figure 4: Left panel: Rest-frame UVJ colors of all ($\mathrm{F444W<24}$) galaxies at $2<z<5$ in EGS and UDS. We show the SC-selected RUBIES galaxies as hexagon symbols, whose color coding follows previous convention. We show the remaining galaxies in RUBIES as black dots. Finally, the distribution of any EGS or UDS galaxies not in RUBIES are shown by 2D histogram bins, in which the darker color indicates higher density. Middle panel: Rest-frame $u_s$$g_s$$i_s$ colors of the same set of galaxies. The characteristic uncertainties in these rest-frame colors are shown by the error bars in the upper left corners. All rest-frame color selections shown here suffer from various degrees of impurity and incompleteness, which we discuss in detail in the text. Right panel: EAZY-derived photometric redshifts versus spectroscopic redshifts of all RUBIES massive quiescent galaxies. These photometric redshifts are largely consistent with the redshifts recovered from PRISM spectroscopy, though they are the main contribution to uncertainties in rest-frame colors.
  • Figure 5: The parent catalog F444W magnitude versus F150W-F444W color of this quiescent sample, RUBIES surveyed sources, and all parent catalog sources in PRIMER/UDS and CEERS/EGS in three redshift bins. These panels illustrate the survey completeness of RUBIES as a function of magnitude, color, and redshift. The boxes outlined by grey dashed lines represent the area in color-magnitude space in which we perform the completeness calculation in each redshift bin. The survey completeness is close to $100\%$ for sources under the footprint in the color-magnitude space occupied by massive quiescent galaxies at $z>4$.
  • ...and 4 more figures