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

Abstract

Lyman-break galaxy (LBG) candidates at z ≳ 10 are rapidly being identified in James Webb Space Telescope (JWST)/NIRCam observations. Due to the (redshifted) break produced by neutral hydrogen absorption of rest-frame UV photons, these sources are expected to drop out in the bluer filters while being well detected in redder filters. However, here we show that dust-enshrouded star-forming galaxies at lower redshifts (z ≲ 7) may also mimic the near-infrared (near-IR) colors of z > 10 LBGs, representing potential contaminants in LBG candidate samples. First, we analyze CEERS-DSFG-1, a NIRCam dropout undetected in the F115W and F150W filters but detected at longer wavelengths. Combining the JWST data with (sub)millimeter constraints, including deep NOEMA interferometric observations, we show that this source is a dusty star-forming galaxy (DSFG) at z ≈ 5.1. We also present a tentative 2.6σ SCUBA-2 detection at 850 μm around a recently identified z ≈ 16 LBG candidate in the same field and show that, if the emission is real and associated with this candidate, the available photometry is consistent with a z ∼ 5 dusty galaxy with strong nebular emission lines despite its blue near-IR colors. Further observations on this candidate are imperative to mitigate the low confidence of this tentative submillimeter emission and its positional uncertainty. Our analysis shows that robust (sub)millimeter detections of NIRCam dropout galaxies likely imply z ∼ 4–6 redshift solutions, where the observed near-IR break would be the result of a strong rest-frame optical Balmer break combined with high dust attenuation and strong nebular line emission, rather than the rest-frame UV Lyman break. This provides evidence that DSFGs may contaminate searches for ultra-high redshift LBG candidates from JWST observations.

JWST Insight into a Lensed HST-dark Galaxy and Its Quiescent Companion at z = 2.58

“…As shown in Figure 5, at z ∼ 4 analog galaxies would drop out in all SW images and become "3 or 4 μm only" objects in the deep UNCOVER survey. As concluded by Zavala et al (2023) and Naidu et al (2022), these 2 μm dropouts could masquerade as the z > 15 candidates and have a severe impact on studies in the most distant universe. Strikingly, at z ∼ 6 similarly dusty objects would be completely invisible in any JWST bands, and become JWST-dark.…”

A Comprehensive Study of Galaxies at z ∼ 9–16 Found in the Early JWST Data: Ultraviolet Luminosity Functions and Cosmic Star Formation History at the Pre-reionization Epoch

“…the F200W band, consistent with a z = 16.3 galaxy, although there are some discussions about a possible solution of a dusty line emitter at z ∼ 5 (Naidu et al 2022a;Zavala et al 2023).…”

“…Our color selection criteria avoid model tracks of these z = 0-8 galaxies, and most of our candidates are located far from these model tracks. In the right panel of Figure 10, we also plot a dusty starburst galaxy at z ∼ 5, which may appear as a z > 15 galaxy discussed in Zavala et al (2023). Such a dusty interloper is also removed from our sample due to the red continuum color (F277W − F444W > 1.0).…”

“…The black arrow indicates a shift of the colors with dust extinction of ΔE(B − V ) = + 0.1. The magenta circle in the right panel is a dusty starburst galaxy at z ∼ 5, which may appear as a z > 15 galaxy discussed in Zavala et al (2023). Our color selection criteria avoid these low-redshift interlopers at z ∼ 0-8.…”

Broad Emission Lines in Optical Spectra of Hot, Dust-obscured Galaxies Can Contribute Significantly to JWST/NIRCam Photometry

“…JWST/NIRSpec has proven capable of spectroscopically detecting the rest-frame optical light from galaxies up to z ∼ 9-10 (Carnall et al 2023;Roberts-Borsani et al 2022), but this might not be well suited for rapidly validating redshifts in statistical samples. A complimentary approach, born from similar rest-frame optical colors of z > 10 Lymanbreak galaxies (LBGs), dusty galaxies (Howell et al 2010;Casey et al 2014), and in some cases similar optical/near-IR apparent magnitudes (Zavala et al 2023), is to use far-IR/ submillimeter follow-up observations of cold dust continuum (Zavala et al 2023) and/or far-IR cooling lines (Fujimoto et al 2022) to identify or rule out z < 6 IR-luminous galaxies lurking within z > 10 candidate catalogs.…”