Unveiling the Nature of Infrared Bright, Optically Dark Galaxies with Early JWST Data

TL;DR

This study leverages JWST/NIRCam imaging from CEERS to identify 30 HST-dark galaxies via a F160W−F444W color cut, revealing a population of dust-obscured, massive star-forming systems at z ~ 2–8, including eight at z > 6. Using 12-band photometry and BAGPIPES SED fitting, the authors derive photometric redshifts, stellar masses around log(M*/M⊙) ≈ 9.5–10 and median A_V ≈ 2 mag, and SFRs generally below 50 M⊙/yr. These galaxies largely lie on the star-forming main sequence and, when integrated, contribute a non-negligible fraction to the cosmic star formation rate density up to z ~ 7, indicating substantial dust in the Epoch of Reionization. The results suggest that dusty, high-z galaxies were undercounted in previous rest-frame UV surveys and highlight JWST’s pivotal role in completing the census of massive galaxies in the early universe.

Abstract

Over the last few years, both ALMA and Spitzer/IRAC observations have revealed a population of likely massive galaxies at $z>3$ that was too faint to be detected in HST rest-frame ultraviolet imaging. However, due to the very limited photometry for individual galaxies, the true nature of these so-called HST-dark galaxies has remained elusive. Here, we present the first sample of such galaxies observed with very deep, high-resolution NIRCam imaging from the Early Release Science Program CEERS. 30 HST-dark sources are selected based on their red colours across 1.6 $μ$m to 4.4 $μ$m. Their physical properties are derived from 12-band multi-wavelength photometry, including ancillary HST imaging. We find that these galaxies are generally heavily dust-obscured ($A_{V}\sim2$ mag), massive ($\log (M/M_{\odot}) \sim10$), star-forming sources at $z\sim2-8$ with an observed surface density of $\sim0.8$ arcmin$^{-2}$. This suggests that an important fraction of massive galaxies may have been missing from our cosmic census at $z>3$ all the way into the Reionization epoch. The HST-dark sources lie on the main sequence of galaxies and add an obscured star formation rate density (SFRD) of $\mathrm{3.2^{+1.8}_{-1.3} \times 10^{-3} M_{\odot}/yr/Mpc^{3}}$ at $z\sim7$ showing likely presence of dust in the Epoch of Reionization. Our analysis shows the unique power of JWST to reveal this previously missing galaxy population and to provide a more complete census of galaxies at $z=2-8$ based on rest-frame optical imaging.

Figures (7)

• Figure 1: Colour-magnitude diagram (F160W-F444W vs F444W) used to select the HST-dark galaxies. The dashed line shows the location of $\mathrm{H = 27}$ mag, which corresponds to the average $3\sigma$ limit of the shallowest parts of the H-band in the HST data. The black line marks the adopted colour cut $\mathrm{H-F444W > 2.0}$ and the red points correspond to the galaxies included in our sample. Of the 39 HST-dark galaxies selected, 19 have lower limit fluxes in the H-band (grey arrows). The continuous lines show the theoretical evolution in galaxy colours at redshifts of $\mathrm{2<z<6}$ for three different levels of reddening $\mathrm{A_{v} = 2.0, \ 3.0, \ 4.0}$ mag (yellow, orange and purple lines respectively). The grey dots represent the density of sources from the total CEERS catalogue with detections in the F444W band ($\mathrm{\sim 26,000}$ sources).
• Figure 2: Examples of four HST-dark galaxies at $\mathrm{z \sim 3.3, \ 3.6, \ 4.6, \ 8}$ that represent the variety of physical properties, isolation/clustering and morphologies that we find in our sample (see also Nelson2022). (Top panels) Postage stamps in the optical/NIR for four examples of HST-dark galaxies, where the sources show no significant detections in the HST filters. The first row shows HST images with the bands F606W, F814W, F850LP, F125W, F140W, F160W and the Spitzer bands IRAC1, IRAC2 at $\mathrm{3.6 \ and \ 4.5 \mu m }$ respectively. The second row shows new NIRCam JWST images: F115W, F150W, F200W, F277W, F356W, F410M, and F444W. The extraordinary improvement in both depth and resolution of JWST at $3-5\,\mu$m compared to the IRAC images is obvious. Most of these sources would not have been easily selected in previous data and they would have potentially been missing from our cosmic census. (Bottom panels) Posterior spectral energy distributions (yellow; median with 16-84 percentile uncertainties) with the photometry (red dots). Orange points indicate the expected model fluxes from the posterior SEDs. The probability distribution function of the photometric redshift is shown in the lower right part of the panel. In general, HST-dark sources are found to be dust-obscured, massive, star-forming galaxies at $z\sim2-8$.
• Figure 3: Histograms of the main physical properties analysed: photometric redshift, SFR (first row), stellar masses and dust attenuation (second row). The histograms show the generally dusty high-z nature of the sample with most of the galaxies at $\mathrm{z > 3.3}$ and a median attenuation of $\mathrm{A_{v} \sim 2 mag}$. The SFRs are moderate, mostly at $\mathrm{SFR < 50 M_{\odot}/year }$ whereas the stellar masses are large with $\mathrm{log(M_{*}/M_{\odot}) \sim 9.5-10}$. Our analysis thus shows that previous stellar mass function estimates at $z>3$ were likely underestimated at high masses.
• Figure 4: UV against VJ rest-frame colours diagram. HST-dark galaxies are presented as big circles with the H-F444W colour on the third axis whereas the grey dots are the total CEERS sample. The black dashed lines show the usual classification of galaxies in quiescent, star-forming and dusty from Spitler2014. Our sample of HST-dark galaxies lies mostly in the dusty colour space with part of the sample in the star-forming galaxies region. There are no quiescent galaxies in our selection due to the colour cut, however, given the photometric uncertainties, we cannot exclude that some of the galaxies are quiescent.
• Figure 5: HST-dark galaxies compared with submillimetre galaxies (SMGs). (Top panel) The redshift histogram shows that HST-dark galaxies (red line) lie mostly at $\mathrm{ z > 3.2}$ and extend to higher redshift compared to SMGs (orange bars). (Bottom panel) Photometric redshifts against star formation rates. On average, HST-dark galaxies lie at higher redshift and have significantly lower SFRs (orange squares; ALESS sample from daCunha2015). The significantly smaller error bars in the photometric redshifts are due to the improvement coming from the deep, high-resolution JWST/NIRCam data.