ALMA FIR View of Ultra High-redshift Galaxy Candidates at $z\sim$ 11-17: Blue Monsters or Low-$z$ Red Interlopers?

Abstract

We present ALMA Band~7 observations of a remarkably bright galaxy candidate at $z_{\mathrm{phot}}$=$16.7_{-0.3}^{+1.9}$ ($M_{\mathrm{UV}}$=$-21.6$), S5-z17-1, identified in JWST Early Release Observation data of Stephen's Quintet. We do not detect the dust continuum at 866~$\hat{I}¼$m, ruling out the possibility that \targb\ is a low-$z$ dusty starburst with a star-formation rate of $\gtrsim 30$~$M_{\odot }$~yr${}^{-1}$. We detect a 5.1$\ddot{I}ƒ$ line feature at $338.726\pm 0.007$~GHz exactly coinciding with the JWST source position, with a 2\% likelihood of the signal being spurious. The most likely line identification would be [OIII]52$\hat{I}¼$m at $z=16.01$ or [CII]158$\hat{I}¼$m at $z=4.61$, whose line luminosities do not violate the non-detection of the dust continuum in both cases. Together with three other $z\gtrsim$ 11--13 candidate galaxies recently observed with ALMA, we conduct a joint ALMA and JWST spectral energy distribution (SED) analysis and find that the high-$z$ solution at $z\sim$11--17 is favored in every candidate as a very blue (UV continuum slope of $\simeq -2.3$) and luminous ($M_{\mathrm{UV}}\simeq [-$24:$-21]$) system. Still, we find in several candidates that reasonable SED fits ($\hat{I}”$ $\ddot{I}{‡}^2\lesssim 4$) are reproduced by type-II quasar and/or quiescent galaxy templates with strong emission lines at $z\sim 3$--5, where such populations predicted from their luminosity functions and EW([OIII]+H${\hat{I}}^2$) distributions are abundant in survey volumes used for the identification of the $z\sim$11--17 candidates. While these recent ALMA observation results have strengthened the likelihood of the high-$z$ solutions, lower-$z$ possibilities are not completely ruled out in several of the $z\sim$11--17 candidates, indicating the need to consider the relative surface densities of the lower-$z$ contaminants in the ultra high-$z$ galaxy search.

25 pages, 8 figures, 7 tables. ApJ in press