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ABSTRACT We present band 6 ALMA observations of a heavily obscured radio-loud (L1.4 GHz = 1025.4 W Hz−1) active galactic nucleus (AGN) candidate at zphot = 6.83 ± 0.06 found in the 1.5 deg2 COSMOS field. The ALMA data reveal detections of exceptionally strong [C ii]158 $$\mu$$m (z[C ii] = 6.8532) and underlying dust continuum emission from this object (COS-87259), where the [C ii] line luminosity, line width, and 158 $$\mu$$m continuum luminosity are comparable to those seen from z ∼ 7 sub-mm galaxies and quasar hosts. The 158 $$\mu$$m continuum detection suggests a total infrared luminosity of $$9\times 10^{12}\, \mathrm{ L}_\odot$$ with corresponding very large obscured star formation rate (1300 M⊙ yr−1) and dust mass ($$2\times 10^9\, \mathrm{ M}_\odot$$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization-era galaxy with $$M_\ast \approx 1.7\times 10^{11}\, \mathrm{ M}_\odot$$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbours an AGN that is heavily obscured ($$\tau _{_{\mathrm{9.7\,\mu m}}}=2.3$$) with a bolometric luminosity of approximately $$5\times 10^{13}\, \mathrm{ L}_\odot$$. Such a very high AGN luminosity suggests that this object is powered by an ≈1.6 × 10$$^9\, \mathrm{ M}_\odot$$ black hole if accreting near the Eddington limit, and is effectively a highly obscured version of an extremely ultraviolet (UV)-luminous (M1450 ≈ −27.3) z ∼ 7 quasar. Notably, these z ∼ 7 quasars are an exceedingly rare population (∼0.001 deg−2), while COS-87259 was identified over a relatively small field. Future very wide area surveys with e.g. Roman and Euclid have the potential to identify many more extremely red yet UV-bright z ≳ 7 objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population.
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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?
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 deg2 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* = 1010.8 M⊙) and extremely red (rest-UV slope β = −0.59) z ≃ 6.8 galaxy with hyperluminous infrared emission (LIR = 1013.6 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 ≈ 1025.4 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* > 1010 M⊙) reionization-era galaxies.
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- Award ID(s):
- 1908284
- PAR ID:
- 10365706
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Monthly Notices of the Royal Astronomical Society
- Volume:
- 512
- Issue:
- 3
- ISSN:
- 0035-8711
- Format(s):
- Medium: X Size: p. 4248-4261
- Size(s):
- p. 4248-4261
- Sponsoring Org:
- National Science Foundation
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