Radio and Far-IR Emission Associated with a Massive Star-forming Galaxy Candidate at z$\simeq$6.8: A Radio-Loud AGN in the Reionization Era?

Abstract

We report the identification of radio (1.4 and 3 GHz) and mid-infrared, far-infrared, and sub-mm (24-850𝜇m) emission at the position of one of 41 UV-bright (M UV −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 Å 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 multi-wavelength (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 (𝜏 9.7𝜇m = 7.7 ± 2.5) radio-loud (L 1.4 GHz ∼ 10 25.5 W Hz −1 ) AGN. The radio emission is compact (1.04±0.12 arcsec) and exhibits an ultra-steep spectrum between 1.4-3 GHz (𝛼 = −2.06 +0.27 −0.25 ) with evidence of spectral flattening at lower frequencies, consistent with known 𝑧 > 4 radio galaxies. We also demonstrate that COS-87259 may reside in a significant (11×) galaxy overdensity at 𝑧 6.6 − 6.9, as common for systems hosting radio-loud AGN. Nonetheless, a spectroscopic redshift will ultimately be required to establish the true nature of COS-87259 as we cannot yet completely rule out low-redshift solutions. 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.

A compact symmetric radio source born at one-tenth the current age of the Universe

“…The WISE colours 𝑊2−𝑊3 and 𝑊1−𝑊2 of J1606+3124 are 3.81 mag and 0.37 mag, respectively (Cutri et al 2013), clearly classifying it as a dust-obscured source. Most dustobscured high-redshift infrared AGN are found to be radio weak, such as WISE 𝐽224607.56−052634.9 (𝑧 = 4.6, Fan et al 2020) and COS-87259 (𝑧 = 6.8, Endsley et al 2021), whose radio emission is a mixture of weak jet, quasar-driven winds, and star formation activity (Gabányi et al 2021;Richards et al 2021). High-redshift galaxies with extremely high-power jets like J1606+3124 are rare.…”