RUBIES: a complete census of the bright and red distant Universe with JWST/NIRSpec

TL;DR

RUBIES presents a ~60-hour JWST/NIRSpec program targeting the bright, extremely red sources revealed by JWST/NIRCam in the UDS and EGS fields, and a well-defined census of the z~2–7 galaxy population. It introduces a novel, quantified selection function based on three photometric quantities to enable robust completeness corrections and to optimize the observation of rare red sources. The study details the image data, mask-design, target prioritization, data reduction, and redshift measurements, reporting robust redshifts for the majority of red targets and a comprehensive assessment of flux and wavelength calibration across dispersers. The results reveal a heterogeneous red-source population spanning z≈1–9, including dust-obscured star-forming galaxies, massive quiescent systems, red AGN, and extreme emission-line galaxies, and demonstrate RUBIES’ ability to place rare objects in the context of the broader high-redshift galaxy population, with public data releases enhancing its legacy value.

Abstract

We present the Red Unknowns: Bright Infrared Extragalactic Survey (RUBIES), providing JWST/NIRSpec spectroscopy of red sources selected across ~150 arcmin$^2$ from public JWST/NIRCam imaging in the UDS and EGS fields. RUBIES novel observing strategy offers a well-quantified selection function: the survey is optimised to reach high (>70%) completeness for bright and red (F150W-F444W>2) sources that are very rare. To place these rare sources in context, we simultaneously observe a reference sample of the 2<z<7 galaxy population, sampling sources at a rate that is inversely proportional to their number density in the 3D space of F444W magnitude, F150W-F444W colour, and photometric redshift. In total, RUBIES observes ~3000 targets across $1<z_{phot}<10$ with both the PRISM and G395M dispersers, and ~1500 targets at $z_{phot}>3$ using only the G395M disperser. The RUBIES data reveal a highly diverse population of red sources that span a broad redshift range ($z_{spec}\sim1-9$), with photometric redshift scatter and outlier fraction that are 3 times higher than for similarly bright sources that are less red. This diversity is not apparent from the photometric SEDs. Only spectroscopy reveals that the SEDs encompass a mixture of galaxies with dust-obscured star formation, extreme line emission, a lack of star formation indicating early quenching, and luminous active galactic nuclei. As a first demonstration of our broader selection function we compare the stellar masses and rest-frame U-V colours of the red sources and our reference sample: red sources are typically more massive ($M_*\sim10^{10-11.5} M_\odot$) across all redshifts. However, we find that the most massive systems span a wide range in U-V colour. We describe our data reduction procedure and data quality, and publicly release the reduced RUBIES data and vetted spectroscopic redshifts of the first half of the survey through the DJA.

Figures (14)

• Figure 1: RUBIES footprint of 18 NIRSpec/MSA pointings in the UDS and EGS fields. Purple pointings correspond to the first half of observations in January-March 2024 and form the focus of the current data release. Background images show the NIRCam F444W image mosaics, primarily constructed from public imaging of the CEERS and PRIMER surveys. For the UDS we also show the outline of the PRIMER MIRI imaging footprint in pink.
• Figure 2: Distribution of targets in the RUBIES parent catalogue in different projections of the 3D parameter space of $\rm F150W-F444W$ colour, total $\rm F444W$ magnitude and best-fit photometric redshift. Black, grey and white contours enclose the 50th, 75th and 95th percentiles of all sources in the catalogue, respectively. The colour coding shows the average weight of sources in a bin; for bins containing fewer than 10 objects we show individual data points. Weights are computed for targets according to their number density in this 3D parameter space, such that the rarest sources receive the highest weight (see Section \ref{['sec:targets']}).
• Figure 3: Distribution of the RUBIES parent sample (spanning the full EGS and UDS NIRCam area of $\sim 300\,$arcmin$^2$) and of the targets selected for spectroscopic follow-up, shown in the space of parameters used to define the selection function: photometric redshift, F444W magnitude, and $\rm F150W-F444W$ colour. Top panels show absolute source counts, while bottom panels show the fraction of observed targets with respect to the full parent sample. The RUBIES selection is strongly biased (as intended) toward red sources and preferentially targets brighter sources at $z_{\rm phot}>3$, although the majority of the observed targets are still faint ($\rm F444W>26$) and relatively blue ($\rm F150W-F444W \sim 0$).
• Figure 4: Distribution of photometric redshifts and F444W magnitudes of RUBIES targets for the PRISM (top) and G395M (bottom) observations. Colour coding shows the spectroscopic completeness in each bin: on the left this is computed as the fraction of targets in the RUBIES NIRSpec footprint that are observed. On the right this is calculated as the fraction of observed targets from the full parent catalogue (i.e. the total PRIMER and CEERS area, approximately double the area covered by RUBIES). The RUBIES selection function achieves high ($>50\%$) spectroscopic targeting completeness for bright, high-redshift sources, even reaching $>70\%$ in the extremes of the parameter space.
• Figure 5: Distribution of F444W magnitudes and $\rm F150W-F444W$ colours of RUBIES targets for the PRISM (top) and G395M (bottom) observations. Symbols and colour scale are the same as in Figure \ref{['fig:zmag_completeness']}: the colour coding indicates the spectroscopic completeness computed for the RUBIES footprint alone or the full NIRCam area of the parent catalogue. The two sets of panels on the left show all sources, whereas those on the right only show sources with $z_{\rm phot}>3$. RUBIES reaches very high completeness for red sources, especially at $z_{\rm phot}>3$. Comparison of the two different measures of completeness shows that RUBIES is biased toward red sources, which is the result of our pointing location optimisation (Section \ref{['sec:pointings']}). Nevertheless, because the vast majority of sources in the parent sample are faint and blue (Figure \ref{['fig:sample_hists']}), the majority of RUBIES sources are also faint and blue, forming a critical comparison sample to place the rare, red sources in the context of the broader galaxy population.