Dusty Starbursts Masquerading as Ultra-high Redshift Galaxies in JWST CEERS Observations

The superb sensitivity of the James Webb Space Telescope (JWST) coupled with its high angular resolution and its near-infrared (near-IR) detectors (Rigby et al. 2022) provides a unique view of the universe previously invisible to other telescopes, from nearby star-forming regions to the farthest, faintest galaxies ever found. In the field of extragalactic astronomy, JWST allows us to extend the Lyman-break galaxy (LBG) selection technique beyond z ≳ 11, the redshift at which the Lyman break is redshifted beyond the reach of Hubble Space Telescope (HST) coverage (Hubble serving as the previous workhorse instrument for the identification of such galaxies before the arrival of JWST; see reviews by Finkelstein 2016; Stark 2016; Robertson 2022 and references therein).

The identification of very high-redshift LBGs has strong implications for our understanding of galaxy formation and evolution. For example, the confirmation of large numbers of z > 11 galaxies can provide strong constraints on the formation epoch of the first galaxies and their star formation efficiencies. Their existence can shed light on the dark matter halo mass function in the early universe, particularly with the presence of very luminous sources found ≲400 Myr after the Big Bang (e.g., Behroozi et al. 2019).

In the first few days after the release of the first JWST observations, an increasing number of samples of LBG candidates at z ≳ 10 were identified (Adams et al. 2023; Atek et al. 2023; Castellano et al. 2022; Donnan et al. 2023; Finkelstein et al. 2022a; Harikane et al. 2022; Naidu et al. 2022; Yan et al. 2023). The abundance and masses of these sources start to be in tension with the predictions from most galaxy formation models (Boylan-Kolchin 2022; Finkelstein et al. 2022a; Lovell et al. 2023). Nevertheless, the observed colors for some of these very high-redshift candidates may be degenerated with other populations of galaxies at lower redshifts. This results from confusion between the Lyα forest break at z > 12 with the Balmer and the 4000 Å breaks combined with dust attenuation and/or strong nebular emission. This means that dusty star-forming galaxies (DSFGs) at significantly lower redshifts (z ≲ 6–7) can mimic the JWST/NIRCam colors of z ≳ 10 LBGs, particularly in the shortest-wavelength filters. While models tend to assume these galaxies are universally red in color, thus distinguishable from the typically very blue LBGs, the complex environments of the interstellar medium (ISM) within DSFGs plus contamination from nebular emission lines could lead to a mix of observed near-IR colors (Howell et al. 2010; Casey et al. 2014b), further obfuscating the secure identification of ultra-high redshift LBGs. The phenomenon of DSFGs contaminating high-redshift LBG searches is, in fact, not new to JWST, as often z ∼ 2–3 DSFGs were found to contaminate z ∼ 6–8 LBG samples selected by HST (e.g., Dunlop et al. 2007); here, both the contaminants (DSFGs at z ∼ 4–6) and LBG targets (z ∼ 10–20) for JWST have shifted to higher redshifts.

The secure identification of LBGs is thus important not only to quantify the contamination fraction in z ≳ 10 LBG samples (which could relax the observed tension between observations and model predictions) but also to constrain the volume density and physical properties of early massive quiescent galaxies and high-redshift DSFGs, an important step toward our ultimate goal of understanding galaxy formation and evolution. However, distinguishing these galaxies from other populations has proven challenging and requires spectroscopic or multiwavelength observations probing the older stellar populations (for the quiescent systems) or the dust thermal emission (for DSFGs).

Here, we use JCMT/SCUBA-2 850 μm and NOEMA 1.1 mm interferometric observations, in combination with the JWST data from the Cosmic Evolution Early Release Science (CEERS) Survey (Finkelstein et al. 2017; Bagley et al. 2022; S. Finkelstein et al. 2022, in preparation), to search for dust emission around z > 12 galaxy candidates and NIRCam dropout sources. We report on a galaxy, CEERS-DSFG-1, that is undetected in the NIRCam F115W and F150W filters, but whose photometric redshift is well constrained to be around z = 5 after including (sub)millimeter data. We also study the z ∼ 16.7 candidate, CEERS-93316, reported in Donnan et al. (2023), for which we find a tentative 2.6σ detection at 850 μm, and show that, if this emission is real and associated with this source, it would imply a lower-redshift solution around z ∼ 5. Finally, we examine all the available long-wavelength (mid-IR to millimeter) observations around the z ≈ 12 candidate known as Maisie's galaxy (Finkelstein et al. 2022a), finding no evidence of continuum emission.

This Letter is organized as follows: Section 2 describes the new observations and the ancillary data sets. In Section 3 we describe the spectral energy distribution (SED) fitting methodology and the best-fit SED fitting for CEERS-DSFG-1 along with the inferred physical properties. Then, Section 4 introduces our search for potential contamination from other DSFGs in samples of z > 12 LBGs candidates in the CEERS field including CEERS-93316 and Maisie's galaxy. Finally, our conclusions are summarized in Section 5.

In this Letter, we assume H₀ = 67.3 km s⁻¹ Mpc⁻¹, Ω_λ =0.68, and Ω_M = 0.32 (Planck Collaboration et al. 2016).

2.1. NOEMA Observations

2.2. CEERS Data

2.3. Other Ancillary Data

The subarcsecond positional accuracy of the NOEMA observations allows us to directly identify the submillimeter-selected galaxy, CEERS-DSFG-1, in the JWST/NIRCam observations (see Figure 1) without any ambiguity. Interestingly, CEERS-DSFG-1 is well detected in F200W and redder bands but abruptly drops out in F150W and F115W and in all the HST filters, as can be seen in Figure 2. The dropout nature of this source satisfies some of the color criteria to identify z > 10 galaxy candidates. Indeed, it satisfies the criterion of m_150W − m_200W > 0.8 used in Yan et al. (2023), and some (but not all) of the criteria used in Donnan et al. (2023), with a 2σ nondetection in F115W and F150W, and >3σ detections in redder filters (see Figure 2 and Table 1). However, the identification of this source as a DSFG calls into question such a very high-redshift scenario, given that the highest-redshift dust continuum detections ever reported are at z ∼ 7–8 (Laporte et al. 2017; Strandet et al. 2017; Marrone et al. 2018; Tamura et al. 2019; Inami et al. 2022). Moreover, the (sub)millimeter emission would imply an extreme infrared (IR) luminosity in excess of ∼10¹³ L_⊙ and a large dust mass in tension with current models. This is thoroughly discussed in Appendix A, where we show that relatively bright (sub)millimeter sources are unlikely to lie at z > 10.

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Here we conduct a more thorough investigation as to the possible redshift of CEERS-DSFG-1 using JWST constraints alone, (sub)millimeter constraints alone, and a combination of both JWST and long-wavelength millimeter data. The results are highly dependent on the available photometric constraints and the inferred redshifts differ significantly, as discussed below in this section.

3.1. SED Fitting Procedure and Redshift Constraints

3.1.1.  EAZY

We first fit the SED of CEERS-DSFG-1 to JWST/NIRCam photometry alone using the eazy (Brammer et al. 2008) SED fitting code. The fitting was performed in an identical fashion as in Finkelstein et al. (2022a). To summarize, EAZY makes use of a user-supplied template set to generate linear combinations of stellar populations that fit the data and generate redshift probability distributions. The template set used in our case includes the "tweak_fsps_QSF_12_v3" set of 12 templates as well as 6 additional templates that span bluer colors (Larson et al. 2022). As shown in Figure 3, the redshift probability density distribution from EAZY shows two significant peaks at z ∼ 3 and z ∼ 5, and a nonnegligible probability (∼6%) at z ≈ 12–14. To put these fits in context with the (sub)millimeter data, we show in Figure 4 the best-fit SED from EAZY at z = 5.5, corresponding to the redshift with the maximum probability.

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3.1.2.  CIGALE

We also fit the photometry using CIGALE (Burgarella et al. 2005; Noll et al. 2009; Boquien et al. 2019) assuming a delayed star formation history (SFH): SFR(t) $\propto \,t\exp (-t/\tau )$ with stellar models from Bruzual & Charlot (2003). A Calzetti et al. (2000) law is also adopted for the dust attenuation of the stellar continuum. On the other hand, the nebular emission (continuum and lines) is attenuated with a screen model and an small magellanic cloud‐like extinction curve (Pei 1992). Finally, the dust emission reemitted in IR is modeled with Draine et al. (2014) models.

Including only JWST/NIRCAM photometry in the fit results in a similar redshift distribution as the one obtained with EAZY, with significant probability at z ≈ 3–5 and a moderate probability of ≈22% of being at z > 10. To illustrate how well the high-redshift solutions fit the available data, we include the best-fit SED at z = 13.5 in Figure 4.

In addition, we fit the JWST data along with SCUBA-2 and NOEMA detections (Herschel upper limits were not included in the fit) using the same CIGALE configuration described above. The addition of the long-wavelength data significantly impacts the results, narrowing down the redshift probability distribution of CEERS-DSFG-1 (see Figure 3). The best-fit photometric redshift when using all the available photometric constraints is $z={5.09}_{-0.72}^{+0.62}$, where the error bars encompass the 68% confidence interval. As shown in Figure 4, the fitted SED from this analysis is in good agreement with all the available photometric constraints, including upper limits.

3.1.3. MMPz

Finally, though the long-wavelength data on CEERS-DSFG-1 are somewhat limited, we are able to calculate an independent photometric redshift for the source based on long-wavelength data alone using the MMPz package (Casey 2020). MMPz presumes that sources with significant (sub)millimeter emission follow an empirically measured relationship between the rest-frame peak wavelength of emission, λ_peak, which is inversely proportional to the characteristic luminosity-weighted dust temperature of the ISM, and the total emergent IR luminosity, L_IR. This L_IR–λ_peak relation is fairly well constrained out to z ∼ 5 (Casey et al. 2018; Drew & Casey 2022) where more intrinsically luminous sources have warmer temperatures. MMPz generates a redshift probability distribution by computing the L_IR and λ_peak at all possible redshifts, and contrasts that against the empirical distribution of measured SEDs. By design, redshift solutions found using MMPz are very broad (due to the degeneracy between ISM dust temperature, constrained via λ_peak, and redshift). The best-fit redshift generated from the long-wavelength data alone (including the only two detections and all the nondetections) is most consistent with the joint CIGALE fit, but shifted to higher values with a best-fit redshift of $z={7.77}_{-1.69}^{+2.55}$ (see Figure 3).

3.1.4. The Moral of the Story

From the above analysis, it is clear that a single color (i.e., dropout) selection criteria to identify high-redshift (z ≳ 10) candidates might include contamination from lower-redshift sources, such as CEERS-DSFG-1. This contamination could be more severe in studies using preflight calibrations since they render the colors of some galaxies more akin to those expected for very high-redshift systems (see discussion by Adams et al. 2023). This is clearly illustrated in Figure 3, where we have also included the photometric redshift constraints from EAZY using a preflight calibration. The fit suggests a very high redshift of $z={18.2}_{-0.7}^{+1.2}$ with an almost negligible probability at z < 15. Careful selection criteria (with several conditions) are thus necessary to produce cleaner samples of high-redshift galaxies. Finkelstein et al. (2022b) and Harikane et al. (2022), for example, implemented a further criterion based on the significance of the high-redshift solution against secondary lower-redshift solutions (defined by the difference between the χ² values of the high-redshift and low-redshift solutions) to select robust candidates (see also Donnan et al. 2023). Similarly, other studies used a two-color criterion to minimize contaminants (e.g., Adams et al. 2023; Atek et al. 2023; Castellano et al. 2022; Harikane et al. 2022) that would have prevented the selection of CEERS-DSFG-1 as a very high-redshift candidate given its red colors at longer wavelengths (e.g., m_277W − m_444W > 1.26). Note, however, that despite these extra selection criteria, lower-redshift systems might still masquerade (and be misidentified) as very high-redshift galaxies as discussed in Section 4.

3.2. On the Physical Properties of CEERS-DSFG-1

As mentioned above, the joint fit of CIGALE using the JWST/NIRCam and the (sub)millimeter data provide tight constraints on the redshift of our target and its physical properties. Hence, here we adopt the these results as our fiducial values. The inferred physical properties are summarized in Table 2 and discussed below.

Table 2. Properties of CEERS-DSFG-1

Property	Value
Source ID	CEERSJ141938.19+525613.9
R.A. (J2000 [deg])	214.9091152
Decl. (J2000 [deg])	52.9371977
zCIGALE	${5.09}_{-0.72}^{+0.62}$
M⋆ (M⊙)	(2.1 ± 0.8) × 1010
LIR (L⊙)	(1.1 ± 0.3) × 1012
SFR (M⊙ yr−1)	110 ± 30
sSFR (Gyr−1)	5.2 ± 2.5
E(B − V) (mag)	1.6 ± 0.1
Age (Myr)	490 ± 240
Mass-weighted age (Myr)	170 ± 90

Note. The redshift and the listed physical properties were derived from the joint fit of CIGALE using the JWST/NIRCam data and the available (sub)millimeter constraints.

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Assuming the best-fit redshift of z = 5.1, the stellar mass of CEERS-DSFG-1 is constrained to be (2.1 ± 0.8) × 10¹⁰ M_☉. This is a factor of ∼4 smaller than the average mass of DSFGs detected by single-dish telescopes (e.g., da Cunha et al. 2015), but is aligned with expectations since our source was selected from one of the deepest SCUBA-2 850 μm surveys and has a fainter 850 μm flux density than typical galaxies identified in shallower single-dish telescope surveys. Indeed, the stellar mass of our target is in better agreement with other SCUBA-2 galaxies identified in this field, which have an average stellar mass of ≈5.6 × 10¹⁰ M_☉ (Cardona-Torres et al. 2022), and with the masses derived for galaxies identified in recent deeper Atacama Large Millimeter/submillimeter Array (ALMA) surveys (e.g., Gómez-Guijarro et al. 2022; see also Khusanova et al. 2021). Similarly, the star formation rate (SFR) of CEERS-DSFG-1 of 110 ± 30 M_☉ yr⁻¹ (averaged over the past 10 Myr) lie between those from submillimeter galaxies (SMGs) and fainter DSFGs identified in deeper ALMA observations (da Cunha et al. 2015; Zavala et al. 2018a; Aravena et al. 2020; Casey et al. 2021; Khusanova et al. 2021; Gómez-Guijarro et al. 2022). These properties imply a specific star formation rate of sSFR = 5.2 ± 2.5 Gyr⁻¹, meaning that CEERS-DSFG-1 lies on the main sequence of star-forming galaxies, similar to the so-called population of "HST-dark" galaxies ⁸⁸ (e.g., Wang et al. 2019).

At z = 5.1, the NIRCam photometry samples rest-frame wavelengths from 0.2 to 0.7 μm, allowing us to constrain the stellar dust attenuation. The red spectral shape in the NIRCam bands implies a strong dust attenuation (as typically found for this kind of galaxies; e.g., Simpson et al. 2017) with E(B − V) = 1.6 ± 0.04, which results in a dust luminosity of (1.1 ± 0.3) × 10¹² L_☉.

The SCUBA-2 observations from Zavala et al. (2017) partially overlap with the CEERS NIRCam survey and thus can be used to look for dust continuum emission around z > 10 candidates in the field. Here we focus on two recently reported high-redshift candidates: CEERS-93316 reported to be at z ≈ 16.7 (Donnan et al. 2023) and Maisie's galaxy at z ≈ 11.8 (Finkelstein et al. 2022a).

4.1. A Deeper Look into CEERS-93316

A 2.6σ tentative detection around the position of CEERS-93316 (Donnan et al. 2023; R.A. = 214.91450, decl. =52.943033) was found in the 850 μm SCUBA-2 map with a flux density of 0.65 ± 0.26 mJy (see Figure 5). Unfortunately, this source was not formally targeted by our NOEMA observations and, although it is only 26'' away from CEERS-DSFG-1 and within the coverage of the NOEMA map described above, it lies on the edge of the map, where the sensitivity is very low (with a primary beam response of ≲0.1, implying an rms of σ_{1.1 mm} ≳ 1 mJy beam⁻¹).

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4.1.1. Caveats of a Marginal SCUBA-2 Detection

We emphasize that there are two primary reasons why this marginal detection may not conclusively imply that CEERS-93316 is a significant thermal dust emitter. The first concern is the significance of the signal itself and the possibility of being spurious. At 2.6σ, simulations of blind detections, single-dish submillimeter sources indicate false-positive rates as high as ∼30%–40% (Casey et al. 2013, 2014a). These rates of false positives are estimated by both searching SCUBA-2 maps for negative significance peaks at −2.6σ as well as conducting source injection tests on SCUBA-2 jackknife maps (following the same methodology as Casey et al. 2013, see their Figure 7). To complement these results, we test the reliability of these low signal-to-noise ratio peaks by creating a catalog of 2.5σ to 3.0σ SCUBA-2 sources and searching for counterparts in the deep VLA 3 GHz map (Dickinson, private communication). We find clear associations for at least 50% of the SCUBA-2 sources, ⁸⁹ implying a ∼50% fidelity rate. A similar result is obtained using the 24 μm map. Note, however, that this reliability fraction of 50% should be considered a lower limit since it is well known that a significant fraction (as high as 30%–40%) of submillimeter sources lack radio or mid-IR counterparts (particularly those at z > 3; Chapman et al. 2003; Pope et al. 2006; Barger et al. 2007; Dye et al. 2008). We thus conclude that the 2.6σ SCUBA-2 signal around CEERS-93316 has a ≳50% probability of being real.

The second significant concern is that even if the detection is real, the SCUBA-2 beam size is large enough that the 850 μm emission could arise from another galaxy at a close angular separation with CEERS-93316 on the sky. Figure 5 shows the neighboring sources within the beam size of the SCUBA-2 tentative detection, with contours overlaid for Spitzer 8 μm emission, 24 μm emission, and VLA 3 GHz continuum. Unfortunately, there is no secure emitter at these wavelengths to which we can definitively associate the 850 μm emission to unequivocally rule out association with CEERS-93316. Note that the lack of such a counterpart does not imply the tentative SCUBA-2 emission is spurious, since z > 3 galaxies are usually undetected in these bands (this is indeed the case for CEERS-DSFG-1). This lack of detection rather means that it is not implausible to associate the 850 μm emission with CEERS-93316, although it also does not confirm the association. Another possible counterpart could be the 8 μm emitter (with a ∼2.5σ significance) to the northwest that has a photometric redshift of z ∼ 5, though it is farther from the signal-to-noise peak in the SCUBA-2 map than CEERS-93316.

At present, we lack sufficient data to clearly associate the emission with CEERS-93316 or other neighboring sources. Follow-up interferometric observations would be necessary to provide both a confirmation of the emission and astrometric localization to CEERS-93316 or to a neighboring source. Nevertheless, given the remarkable properties of CEERS-93316 (being one of the highest-redshift candidates ever reported with a bright UV magnitude of M_UV = − 21.7), below we explore the impact that the submillimeter tentative detection might have on its redshift solution if the dust emission is real and associated with it.

4.1.2. Implications if Dust Emission Is Associated with CEERS-93316

First, we consider what the implications would be if CEERS-93316 had significant dust emission at its proposed redshift of z = 16.7. The observed 850 μm emission would probe the rest-frame ∼50 μm regime; in this scenario, the IR luminosity would be above 10¹² L_⊙ with a dust mass of ∼10⁸ M_⊙. A system with such high dust mass found ∼230 Myr after the Big Bang would surely be extraordinary, likely implausibly so (e.g., Dwek et al. 2014). This is further discussed in Appendix A, where we show the predicted IR luminosity and dust mass as a function of redshift for a hypothetical submillimeter detection with a flux density similar to that of the tentative emission discussed here.

We alternatively explore if a lower-redshift solution would be plausible given the JWST/NIRCam photometric constraints and the observed blue colors in these bands (which contrasts with those from CEERS-DSFG-1). To do that, we fit the JWST/NIRCam data ⁹⁰ along with the tentative 850 μm flux density with CIGALE.

While the redshift distribution from this fitting strongly favors a high-redshift solution in agreement with the Donnan et al. (2023) result (with z_CIGALE ≈ 16.4; see Figure 6), the best-fit SED does not satisfactorily reproduce the tentative submillimeter flux density that is underestimated by more than 1 order of magnitude (see Figure 7). Interestingly, the redshift probability distribution does show a secondary peak at z ∼ 4.8, although with a low integrated probability of less than 3% (see Figure 6). This peak is seen even without the inclusion of the long-wavelength emission and it is also seen in the redshift probability density distribution presented by Donnan et al. (2023, see their Figure A1). This lower redshift clearly dominates the probability distribution of the EAZY fitting when imposing a maximum redshift ⁹¹ of z = 10, as shown in the Figure 6.

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To further explore the feasibility of this alternative redshift solution, we rerun CIGALE but fix the redshift to z = 4.8. The resulting SED is shown in Figure 7 along with the best-fit z ∼ 16 SEDs from EAZY and CIGALE, for comparison. In the low-redshift scenario, the strong break seen between F200W and F277W in CEERS-93316 is attributable to strong [OIII] and Hβ emission in the F277W band (see Figure 7). Similarly, the excess flux in F356W above the continuum, which produces a blue F356W-F410M color, would be attributable to Hα emission. The measured NIRCam photometry would thus require a young starburst with strong nebular line emission to satisfy a z ∼ 4.8 solution, but this would be within the realm of expectation for an early-stage DSFG in formation at these redshifts.

The z ≈ 4.8 best-fit SED would imply an SFR averaged over 10 Myr of 20 ± 10 M_☉ yr⁻¹ and a stellar mass equal to (1.4 ± 0.5) × 10⁹ M_☉, with a dust attenuation (for both continuum and lines) of E(B − V) = 0.5 ± 0.1 and a dust luminosity of (1.7 ± 0.8) × 10¹¹ L_☉. These properties are in broad agreement with those derived for the relatively faint population of z ∼ 7 dusty galaxies in the REBELS survey (Bouwens et al. 2020; Inami et al. 2022). In addition, the line fluxes required to reproduce the given NIRCam photometry range from ∼1 × 10⁻¹⁸–1 × 10⁻¹⁷ erg s⁻¹ cm⁻², which are within the range of those predicted for CEERS-DSFG-1 .

While deep interferometric observations at millimeter wavelengths are required to confirm or refute dust continuum emission in this high-redshift candidate, here we show (see also Appendix A) that a z ∼ 4.8 scenario associated with a DSFG with strong nebular emission is plausible for CEERS-93316 and highly likely if the submillimeter emission is confirmed, despite its blue near-IR colors that are usually associated with the emission of dust-free systems (e.g., Finkelstein 2016, and references therein). If this lower-redshift solution is true, it would contrast with the low probability of being at z < 15 inferred from the different redshift probability distributions shown in Figure 6 (see also Donnan et al. 2023; Finkelstein et al. 2022b). The reason for this low probability might be related to the low significance of the tentative SCUBA-2 detection in the case of CIGALE or with the adopted templates and the fitting approach for EAZY. Interestingly, Pérez-González et al. (2022), who used a novel 2D fitting approach with a new set of SED templates, found a best-fit redshift of 4.59 ± 0.03 for this source (known as nircam2-2159 in Pérez-González et al. 2022) with a low probability of being at z > 10.

4.2. A Deeper Look into Maisie's Galaxy

Given that the recently reported z = 11.8 galaxy candidate from Finkelstein et al. (2022a) lies close to the two galaxies described above (∼78'' and ∼65'' away from CEERS-DSFG-1 and from CEERS-93316, respectively), we carefully examine the available long-wavelength observations to investigate any possible detection of dust emission.

Because this source is not covered by our NOEMA observations, we started by looking at the deep SCUBA-2 850 μm map (Zavala et al. 2017). As shown in Figure 8 no significant detection is found (with a measured flux density of S₈₅₀ = −0.40 ± 0.25 mJy at the position of the source). We also search for significant emission in the Spitzer 8 μm and 24 μm map; Herschel 100, 160, 250, 350, and 500 μm imaging; and SCUBA-2 450 μm observations, finding only nondetections. We thus conclude that a lower-redshift scenario for Maisie's galaxy in which the source is rather associated with a DSFG is unlikely. Given the lack of far-infrared-to-submillimeter detections, the best-fit SEDs and their associated redshift probability distributions for Maisie's galaxy would be similar to those reported in Finkelstein et al. (2022a). Hence, to avoid duplication, they are not included in this Letter.

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Using the available data sets from the JWST CEERS survey in combination with NOEMA and SCUBA-2 observations, we have demonstrated that DSFGs at z ∼ 4–6 can drop out in the bluest JWST/NIRCam filters while being well detected in the redder filters. This kind of galaxies could even show a significant probability of being at high redshifts when performing SED fittings. This is illustrated by studying the source CEERS-DSFG-1, a 850 μm-selected galaxy with robust interferometric observations at 1.1 mm by NOEMA that is undetected in the F115W and F150W bands. A joint SED fitting analysis including the NIRCam constraints and the long-wavelength (sub)millimeter data implies a photometric redshift of ${5.09}_{-0.72}^{+0.62}$, with physical properties that resemble other DSFGs: M_⋆ = (2.1 ± 0.8) × 10¹⁰ M_⊙; SFR =110 ± 30 M_⊙ yr⁻¹; L_dust = (1.1 ± 0.3) × 10¹² L_☉. Hence, searches of z > 10 LBGs that rely only on a dropout selection could introduce significant contaminants from lower-redshift systems. This could be minimized by adopting multicolor selection criteria or by defining alternative conditions (such as a minimum redshift probability or χ² goodness-of-fit; e.g., Adams et al. 2023; Castellano et al. 2022; Donnan et al. 2023; Finkelstein et al. 2022a; Harikane et al. 2022).

Taking advantage of the available submillimeter data in the field, we extended the search for dust continuum emission to two close z > 10 LBG candidates recently reported, CEERS-93316 at z ≈ 16.7 (Donnan et al. 2023) and Maisie's galaxy at z ≈ 11.8 (Finkelstein et al. 2022a). We found a tentative 2.6σ detection at 850 μm around the position of CEERS-93316. A confirmation of this flux density measurement and a firm spatial association requires higher-resolution submillimeter imaging. This is particularly important given its high probability of being spurious and the large beam size (≈146) of the SCUBA-2 observations that encompass several galaxies.

While additional observations are required to corroborate this identification, we use this possible association to illustrate that z ∼ 5 DSFGs can also exhibit blue colors in the JWST/NIRCam bands when strong nebular emission lines are present (with line fluxes on the order of ∼10⁻¹⁸–10⁻¹⁷ erg s⁻¹ cm⁻²), and conclude that (sub)millimeter emission in samples of z > 10 LBGs likely implies misidentifications of DSFGs at lower redshifts (z ≲ 7). Indeed, if CEERS-93316 is confirmed to be a dust emitter, our analysis suggests that it would rather lie at z ∼ 5.

This work has illustrated both the importance and potential of combining JWST observations with submillimeter/millimeter data, a synergy that allows us to identify and characterize populations of galaxies that were previously unreachable, including both z ≳ 5 DSFGs as well as ultra-high redshift z > 10 LBGs. In particular, it will become crucial for searches of ultra-high redshift LBGs to closely consider contamination from lower-redshift (z ∼ 4–7) dusty sources with significant nebular line emission that can mimic the colors of a higher-redshift Lyman break.

Despite sitting at lower redshift, new discoveries and characterizations of z ∼ 5 DSFGs will also shed new light on an otherwise mysterious population, where fewer than a few dozen systems are currently known. Such discoveries will enable a major step forward in our understanding of massive galaxy formation in the first ∼1 Gyr of the universe's history.

We thank the reviewer for a constructive report that improved the clarity of our results. We also thank Jim Dunlop for helpful discussions.

V.B. and D.B. thank the Programme National de Cosmologie et Galaxies and CNES for their support. We thank Médéric Boquien and Yannick Roehlly for their help. C.M.C. thanks the National Science Foundation for support through grants AST-1814034 and AST-2009577 and additionally the Research Corporation for Science Advancement from a 2019 Cottrell Scholar Award sponsored by IF/THEN, an initiative of Lyda Hill Philanthropies. I.A. acknowledges support from CONACyT CB-382947. We acknowledge support from STScI through award JWST-ERS-1345.

This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. These observations are associated with program #1345 and can be accessed in a raw format via doi:10.17909/4abm-k128. This work is based on observations carried out under project number W20CK with the IRAM NOEMA Interferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

Facilities: JWST - James Webb Space Telescope, NOEMA - , JCMT. -

Continuum observations at submillimeter and millimeter wavelengths probe galaxies' dust thermal emission for a wide range of redshifts. Here, adopting typical dust SEDs and relationships between dust continuum emission and other physical properties, we estimate the IR luminosity, SFR, and dust mass as a function of redshift implied by a dust continuum detection similar to the one reported in this work. Then, we compare these quantities with the expected galaxies' properties at z > 10 to assess whether or not they lie within the realm of high-redshift galaxies.

For these calculations, we adopt a modified blackbody distribution with a dust emissivity index of β = 1.8 for the dust SED (e.g., Casey 2012). Two different dust temperatures of 35 and 75 K are explored. Then, the IR luminosity at a given redshift is estimated by, first, scaling the redshifted SED to the 850 μm flux density and, second, integrating over 8–1000 μm (in the rest frame). The cosmic microwave background (CMB) effects on the observed flux density are also taken into account following da Cunha et al. (2013). The inferred IR luminosity as a function of redshift for a S_{850 μm} = 1 mJy dust detection is shown in the left panel of Figure 9. The corresponding dust-obscured SFR estimated directly from the IR luminosity (Kennicutt & Evans 2012) is also indicated on the right axis. Then, we calculate the dust mass as follows. At a given redshift, we estimate the rest-frame 850 μm flux density, S_{850 μm,rest}, from the scaled SED described above (which takes into account the CMB effects) and use the following equation:

$$\begin{eqnarray}&&{M}_{{\rm{d}}}=\displaystyle \frac{{S}_{850\,\mu {\rm{m}},\mathrm{rest}}\,{D}_{L}^{2}}{(1+z){\kappa }_{\mathrm{ref}}\,B({\nu }_{\mathrm{ref}},{T}_{{\rm{d}}})},\end{eqnarray} \tag{ A1 }$$

where κ_ref represents the dust mass absorption coefficient at a reference wavelength and B(ν_ref, T_d) represents the Planck function evaluated at the same frequency. We adopt κ(850 μm) = 0.043 m² kg⁻¹ (Li & Draine 2001) for this calculation. The implied dust mass as a function of redshift is plotted in the right panel of Figure 9.

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As clearly seen in Figure 9, a dust continuum detection of S_{850 μm} ∼ 1 mJy (or S_{1.1 mm} ∼ 0.4 mJy) at z > 10 would imply a SFR in excess of ∼100 M_⊙ yr⁻¹, rapidly reaching ∼1000 M_⊙ yr⁻¹ at z ∼ 15 (depending on the adopted temperature). This SFR is significantly higher than what is measured in any z ≳ 8 object and around 2 orders of magnitude higher than the SFRs inferred for JWST-selected candidates. In the right panel of Figure 9, the inferred dust mass is compared with the maximum mass limit allowed by a ΛCDM universe (which depends on redshift and survey volume ⁹² ). To estimate this limit, we use the halo mass function from the Harrison & Hotchkiss (2013) scaling; first, the halo mass down by a factor of 20 (following Marrone et al. 2018; see also Casey et al. 2021) to approximate the corresponding galaxy ISM mass, and, second, by a factor of 100, which corresponds to the ISM-to-dust ratio typically measured in massive galaxies (e.g., Magdis et al. 2012; Rémy-Ruyer et al. 2014; Scoville et al. 2016). As shown in the figure, within the volume probed by the CEERS observations, a submillimeter detection at z ≳ 6 start to be in tension (at 1σ level) with the maximum mass limit inferred from the halo mass function (when adopting T_d = 35 K). This could be slightly alleviated if the dust temperature is higher. Nevertheless, Scoville et al. (2016) argue that, even when the luminosity-weighted dust temperature could be higher at higher redshifts (e.g., Faisst et al. 2017; Bakx et al. 2020; Sommovigo et al. 2022), the mass-weighted temperature is usually cold (≈25–35 K). Furthermore, even adopting the results from this relatively high dust temperature at face value, the implied dust masses exceed the expected mass limit at z ≳ 12, implying that such a system is unlikely to exist.

While these estimates represent zero-order approximations and depend strongly on the adopted assumptions (which might not be valid at very high redshifts), it is clear that dust continuum detections (on the order of S_{850 μm} ∼ 1 mJy) strongly disfavor high redshifts, z > 10, solutions for galaxies discovered in small surveys such as those conducted to date by the JWST. Submillimeter/millimeter surveys can thus be used to efficiently identify lower-redshift interlopers (i.e., dusty, star-forming galaxies) in samples of very high-redshift galaxy candidates.

The photometry used during the SED fitting procedure on CEERS-93316 is listed in Table 3. Our fluxes are systematically brighter than those reported by Donnan et al. (2023) in all the detected bands, although the difference is small (with an average magnitude difference of −0.09 mag). This could be related to the different processes used to reduce the data and the applied correction factors as discussed in Finkelstein et al. (2022b).