Schrodinger's Galaxy Candidate: Puzzlingly Luminous at $z\approx 17$, or Dusty/Quenched at $z\approx 5$?

Abstract

$JWST$'s first glimpse of the $z>10$ Universe has yielded a surprising abundance of luminous galaxy candidates. Here we present the most extreme of these systems: CEERS-1749. Based on $0.6-5\mu$m photometry, this strikingly luminous ($\approx$26 mag) galaxy appears to lie at $z\approx 17$. This would make it an $M_{\mathrm{UV}}\approx -22$, $M_{\star }\approx 5\times 10^9{M}_{\odot }$ system that formed a mere $\sim 220$ Myrs after the Big Bang. The implied number density of this galaxy and its analogues challenges virtually every early galaxy evolution model that assumes $\Lambda$CDM cosmology. However, there is strong environmental evidence supporting a secondary redshift solution of $z\approx 5$: all three of the galaxy's nearest neighbors at $<2.5$" have photometric redshifts of $z\approx 5$. Further, we show that CEERS-1749 may lie in a $z\approx 5$ protocluster that is $\gtrsim 5\times$ overdense compared to the field. Intense line emission at $z\approx 5$ from a quiescent galaxy harboring ionized gas, or from a dusty starburst, may provide satisfactory explanations for CEERS-1749's photometry. The emission lines at $z\approx 5$ conspire to boost the $>2\mu$m photometry, producing an apparent blue slope as well as a strong break in the SED. Such a perfectly disguised contaminant is possible only in a narrow redshift window ($\Delta z\lesssim 0.1$), implying that the permitted volume for such interlopers may not be a major concern for $z>10$ searches, particularly when medium-bands are deployed. If CEERS-1749 is confirmed to lie at $z\approx 5$, it will be the highest-redshift quiescent galaxy, or one of the lowest mass dusty galaxies of the early Universe detected to-date. Both redshift solutions of this intriguing galaxy hold the potential to challenge existing models of early galaxy evolution, making spectroscopic follow-up of this source critical.

Submitted to ApJL. Fig. 1 shows the "superposed" state, Fig. 2 presents the potential z~5 protocluster, Fig. 3 shows some atypical z~5 SEDs, Fig. 4 studies the morphology, Fig. 5 summarizes the extraordinary situation at z~17, and Fig. 6 argues for why such interlopers may not be a major issue for z>10 searches. Comments warmly welcomed

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