UNCOVER: Candidate Red Active Galactic Nuclei at 3<z<7 with JWST and ALMA

TL;DR

This study identifies a population of candidate reddened active galactic nuclei at redshifts 3 to 7 using JWST and ALMA data, revealing a potentially abundant, obscured AGN population that is much more numerous than UV-bright AGNs at similar luminosities.

Contribution

It presents the first identification of a large, obscured AGN population at high redshift using JWST and ALMA, highlighting their properties and potential abundance.

Findings

26 candidate sources with red NIRCam colors and point-source morphology.

None of the 20 sources with ALMA coverage are detected at 1.2 mm.

SED fits favor hot dust models over obscured star formation.

Abstract

The James Webb Space Telescope (JWST) is revolutionizing our knowledge of $z>5$ galaxies and their actively accreting black holes. Using the JWST Cycle 1 Treasury program Ultradeep NIRSpec and NIRCam ObserVations before the Epoch of Reionization (UNCOVER) in the lensing field Abell 2744, we report the identification of a sample of little red dots at $3 < z_{\rm{phot}} < 7$ that likely contain highly-reddened accreting supermassive black holes. Using a NIRCam-only selection to F444W$<27.7$ mag, we find 26 sources over the $\sim45$ arcmin$^{2}$ field that are blue in F115W$-$F200W$\sim0$ (or $\beta_{\rm UV}\sim-2.0$ for $f_{\lambda} \propto \lambda^\beta$), red in F200W$-$F444W = $1-4$ ($\beta_{\rm opt} \sim +2.0$), and are dominated by a point-source like central component. Of the 20 sources with deep ALMA 1.2-mm coverage, none are detected individually or in a stack. For the majority of the sample, SED fits to the JWST+ALMA observations prefer models with hot dust rather than obscured star-formation to reproduce the red NIRCam colors and ALMA 1.2-mm non-detections. While compact dusty star formation can not be ruled out, the combination of extremely small sizes ($\langle r_e \rangle\approx50$ pc after correction for magnification), red rest-frame optical slopes, and hot dust can by explained by reddened broad-line active galactic nuclei (AGNs). Our targets have faint $M_{\rm 1450} \approx -14\ \, {\rm to} -18$ mag but inferred bolometric luminosities of $L_{\rm bol} = 10^{43}-10^{46}$ erg/s, reflecting their obscured nature. If the candidates are confirmed as AGNs with upcoming UNCOVER spectroscopy, then we have found an abundant population of reddened luminous AGN that are at least ten times more numerous than UV-luminous AGN at the same intrinsic bolometric luminosity.