Schrodinger's Galaxy Candidate: Puzzlingly Luminous at $z\approx17$, or Dusty/Quenched at $z\approx5$?

4 Aug 2022  ·  Rohan P. Naidu, Pascal A. Oesch, David J. Setton, Jorryt Matthee, Charlie Conroy, Benjamin D. Johnson, John R. Weaver, Rychard J. Bouwens, Gabriel B. Brammer, Pratika Dayal, Garth D. Illingworth, Laia Barrufet, Sirio Belli, Rachel Bezanson, Sownak Bose, Kasper E. Heintz, Joel Leja, Ecaterina Leonova, Rui Marques-Chaves, Mauro Stefanon, Sune Toft, Arjen van der Wel, Pieter van Dokkum, Andrea Weibel, Katherine E. Whitaker ·

$JWST$'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 $z\approx17$. This would make it an $M_{\rm{UV}}\approx-22$, $M_{\rm{\star}}\approx5\times10^{9}M_{\rm{\odot}}$ system that formed a mere $\sim220$ 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 $z\approx5$: all three of the galaxy's nearest neighbors at $<2.5$" have photometric redshifts of $z\approx5$. Further, we show that CEERS-1749 may lie in a $z\approx5$ protocluster that is $\gtrsim5\times$ overdense compared to the field. Intense line emission at $z\approx5$ 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 $z\approx5$ 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 ($\Delta z\lesssim0.1$), 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 $z\approx5$, 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.

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Astrophysics of Galaxies