Radio and far-IR emission associated with a massive star-forming galaxy candidate at z ≃ 6.8: a radio-loud AGN in the reionization era?. Issue 3 (21st March 2022)

• Record Type: Journal Article
• Authors: Endsley, Ryan; Stark, Daniel P; Fan, Xiaohui; Smit, Renske; Wang, Feige; Yang, Jinyi; Hainline, Kevin; Lyu, Jianwei; Bouwens, Rychard; Schouws, Sander
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 512:Issue 3(2022)
• Page Start: 4248
• Page End: 4261
• Publication Date: 2022-03-21
• Subjects: galaxies: evolution -- galaxies: high-redshift -- dark ages, reionization, first stars -- radio continuum: galaxies -- submillimetre: galaxies; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stac737
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: We report the identification of radio (0.144–3 GHz) and mid-, far-infrared, and sub-mm (24–850μm) emission at the position of one of 41 UV-bright ($\mathrm{M_{\mathrm{UV}}}^{ }\lesssim -21.25$ ) z ≃ 6.6–6.9 Lyman-break galaxy candidates in the 1.5 deg 2 COSMOS field. This source, COS-87259, exhibits a sharp flux discontinuity (factor >3) between two narrow/intermediate bands at 9450 and 9700 Å and is undetected in all nine bands blueward of 9600 Å, as expected from a Lyman alpha break at z ≃ 6.8. The full multiwavelength (X-ray through radio) data of COS-87529 can be self-consistently explained by a very massive ( M * = 10 10.8 M⊙ ) and extremely red (rest-UV slope β = −0.59) z ≃ 6.8 galaxy with hyperluminous infrared emission ( L IR = 10 13.6 L⊙ ) powered by both an intense burst of highly obscured star formation (SFR ≈ 1800 M⊙ yr −1 ) and an obscured ($\tau _{_{\mathrm{9.7\mu m}}} = 7.7\pm 2.5$ ) radio-loud (L1.4 GHz ≈ 10 25.4 W Hz −1 ) active galactic nucleus (AGN). The radio emission is compact (1.04 ± 0.12 arcsec) and exhibits an ultra-steep spectrum between 1.32 and 3 GHz ($\alpha =-1.57^{+0.22}_{-0.21}$ ) that flattens at lower frequencies ($\alpha = -0.86^{+0.22}_{-0.16}$ between 0.144 and 1.32 GHz), consistent with known z > 4 radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11×) galaxy overdensity, as common for systems hosting radio-loud AGN. While we find that low-redshift solutions to the optical + near-infrared data are not preferred, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 beyond any doubt. If confirmed to lie at z ≃ 6.8, the properties of COS-87259 would be consistent with a picture wherein AGN and highly obscured star formation activity are fairly common among very massive ( M * > 10 10 M⊙ ) reionization-era galaxies.