PRIMER & JADES reveal an abundance of massive quiescent galaxies at 2 < z < 5

TL;DR

This work tackles the abundance and nature of massive quiescent galaxies at $2<z<5$, a regime where models struggle to reproduce early quenching. It builds a mass‑complete sample ($M_*>10^{10}\,M_\odot$) from PRIMER+JADES photometry, requires optical HST data alongside JWST imaging, and combines deep spectroscopy (EXCELS and archival DJA) and ALMA checks to quantify contamination. The study finds a modest total contamination of $12.9^{+4.0}_{-3.1}\%$, increasing the robustness of the derived number densities, which are three times higher than pre‑JWST estimates and broadly consistent with recent JWST results; simulations generally underpredict $z>3$ quiescent galaxies, though Magneticum overpredicts at lower redshifts. Emission‑line analyses of EXCELS galaxies reveal a high incidence of faint AGN activity (~50%), supporting maintenance‑mode feedback as a plausible quenching mechanism, and underscoring the need for larger optical coverage and spectroscopic samples to fully characterise early massive quiescent galaxy evolution.

Abstract

We select a mass-complete sample of 225 quiescent galaxies at $z>2$ with $M_* > 10^{10}\ \mathrm{M}_\odot$ from PRIMER and JADES photometry spanning a total area of $\simeq320$ sq. arcmin. We restrict our analysis to only area with optical coverage in three $HST$ ACS filters, and provide evidence that this is important for selecting the most complete and clean samples of $z>2$ massive quiescent galaxy candidates. We investigate the contamination in our sample via $JWST$ NIRSpec spectroscopic validation, $Chandra$ X-ray imaging, and ALMA interferometry, calculating a modest total contamination fraction of $12.9_{-3.1}^{+4.0}$ per cent. The removal of $HST$ data increases star-forming galaxy contamination by $\simeq10$ per cent and results in a $\simeq20$ per cent loss of candidates recovered from $HST$+$JWST$ data combined. We calculate massive quiescent galaxy number densities at $2<z<5$, finding values three times larger than pre-$JWST$ estimates, but generally in agreement with more-recent and larger-area $JWST$ studies. In comparison with galaxy evolution simulations, we find that most can now reproduce the observed massive quiescent galaxy number density at $2<z<3$, however they still increasingly fall short at $z>3$, with discrepancies of up to $\simeq 1$ dex. We place 14 of our $z>3$ massive quiescent galaxies on the BPT and WHaN diagrams using medium-resolution spectroscopic data from the EXCELS survey. We find a very high incidence of faint AGN in our sample, at a level of $\simeq50$ per cent, consistent with recent results at cosmic noon. This is interesting in the context of maintenance-mode feedback, which is invoked in many simulations to prevent quenched galaxies from re-igniting star formation. To properly characterise the evolution of early massive quiescent galaxies, greater coverage in optical filters and significantly larger spectroscopic samples will be required.

Figures (15)

• Figure 1: The F356W magnitudes and stellar masses of our Bagpipes simulated quiescent galaxy population (grey), constructed as described in Section \ref{['section:methods:masslimit']}. Our massive quiescent candidates resulting from our selection process described in Section \ref{['section:methods:selection']} are plotted as diamonds and coloured by their redshift bin. Our chosen stellar mass limit of $M_*=10^{10}\,\mathrm{M_\odot}$ is included as a dashed line, and the corresponding magnitude limit for $>99$ per cent completeness, $\mathrm{F356W}=26$, is included as a dotted line.
• Figure 2: Example SED fits to NIRCam imaging for candidates in our sample, along with posterior probability distributions for the redshift and sSFR parameters. We include robust candidates PRIMER-UDS-21154 (top), PRIMER-COSMOS-14115 (middle) and a non-robust candidate PRIMER-COSMOS-57642 (bottom). At the top of each example, galaxy imaging is shown in 2$\times$2 arcsec cutouts, where a dagger denotes an HST ACS filter and its absence denotes a NIRCam filter. The observed photometry points are shown in red and our posterior median model is shown in blue. The yellow rectangles cover the 16th to 84th percentiles of the photometry posteriors, and the dashed line in the sSFR posterior represents the sSFR threshold used in selection (see Section \ref{['section:methods:selection']}).
• Figure 3: A comparison between photometric and spectroscopic redshifts for the 83 of our massive quiescent galaxy candidates with robust spectroscopic redshifts (flag 3 or 4; see Section \ref{['section:data:spectroscopy']}). The value of $\sigma_z$ and the fraction of catastrophic outliers are provided in the legend. Our Bagpipes photometric redshifts can be seen to be highly robust.
• Figure 4: A representative sub-sample of the 10 objects from our photometric sample of massive quiescent galaxy candidates with both X-ray detections and spectroscopic data (see Section \ref{['section:sample:contamination:xray']}). Fluxes and wavelengths are displayed in the rest frame. We include the spectroscopic campaign, spectroscopic ID and spectroscopic redshift for each object. From top-down, these correspond to our photometric IDs: JADES-GOODSS-74806, JADES-GOODSS-55942, PRIMER-UDS-35941, PRIMER-UDS-116064. It is clear that some sources are stellar dominated in the rest-frame optical, despite having X-ray counterparts.
• Figure 5: The SFRs of a sub-sample of our massive quiescent galaxy sample, as derived from deep ALMA coverage (see Section \ref{['section:data:alma']}), which we define as $3\sigma$ sensitivity $<50\ \mathrm{M_\odot\ yr^{-1}}$ (see Section \ref{['section:sample:contamination:alma']}). This is available for 46 out of 225 objects. We include our 4 detections (defined as SFR $> 3\sigma$) as blue points and non-detections as red upper limits at their respective $3\sigma$ limits.