ALMA Confirmation of an Obscured Hyperluminous Radio-Loud AGN at $z=6.853$ Associated with a Dusty Starburst in the 1.5 deg$^2$ COSMOS Field

Abstract

We present band 6 ALMA observations of a heavily-obscured radio-loud (𝐿 1.4 GHz = 10 25.4 W Hz −1 ) AGN candidate at 𝑧 phot = 6.83 ± 0.06 found in the 1.5 deg 2 COSMOS field. The ALMA data reveal detections of exceptionally strong [CII]158𝜇m (𝑧 [CII] = 6.8532) and underlying dust continuum emission from this object (COS-87259), where the [CII] line luminosity, line width, and 158𝜇m continuum luminosity are comparable to that seen from 𝑧 ∼ 7 sub-mm galaxies and quasar hosts. The 158𝜇m continuum detection suggests a total infrared luminosity of 9 × 10 12 𝐿 with corresponding very large obscured star formation rate (1300 𝑀 /yr) and dust mass (2 × 10 9 𝑀 ). The apparent strong Balmer break seen between the VIRCam and IRAC photometry suggests that COS-87259 is an extremely massive reionization era galaxy with 𝑀 * ≈ 1.7×10 11 𝑀 . Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbors an AGN that is heavily obscured (𝜏 9.7𝜇m = 2.3) with a bolometric luminosity of approximately 5 × 10 13 𝐿 . Such a very high AGN luminosity suggests this object is powered by an ≈1.6 × 10 9 𝑀 black hole if accreting near the Eddington limit, and is effectively a highly-obscured version of an extremely UV-luminous (𝑀 1450 ≈ −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.

Star formation histories of UV-luminous galaxies at $z \simeq 6.8$: implications for stellar mass assembly at early cosmic times

“…We also note that our sample has a relatively high spectroscopic confirmation rate of 33 per cent with redshifts ranging from 𝑧 = 6.538 − 6.883, generally consistent with our targeted colour selection redshift interval. Ten of our 36 candidates have been detected in [C ] (Smit et al 2018;Bouwens et al 2021a;Endsley et al 2022) (with one also having a Ly 𝛼 detection; Endsley et al 2021a,b), and another two sources have Ly 𝛼 detections only (Endsley et al 2021b). Notably, the most red galaxy in our sample (COS-87259) is spectroscopically confirmed via [C ] and likely hosts a heavily obscured active galactic nucleus (Endsley et al 2022), leading to possible uncertainties on its age and stellar mass.…”

“…Ten of our 36 candidates have been detected in [C ] (Smit et al 2018;Bouwens et al 2021a;Endsley et al 2022) (with one also having a Ly 𝛼 detection; Endsley et al 2021a,b), and another two sources have Ly 𝛼 detections only (Endsley et al 2021b). Notably, the most red galaxy in our sample (COS-87259) is spectroscopically confirmed via [C ] and likely hosts a heavily obscured active galactic nucleus (Endsley et al 2022), leading to possible uncertainties on its age and stellar mass. However, we investigate the influence of COS-87259 on our inferred age distribution and find that the distribution is only minimally affected by its inclusion (see Section 6).…”

A Simple Condition for Sustained Super-Eddington Black Hole Growth

“…6 The prediction that highly super-Eddington sources would be heavily obscured is bolstered by the expectation that a large supply of dense gas is required to fuel such elevated accretion rates, as this gas would further attenuate any radiation that is not trapped in the accretion flow. Consistent with this, recent observations of highly obscured quasars at z  7 that may be powered by BHs accreting at super-Eddington rates (Fujimoto et al 2022;Endsley et al 2022) strengthen the possibility that upcoming next-generation facilities such as the Rubin Observatory (Ivezić et al 2019), Euclid (Euclid Collaboration et al 2019) and the recently launched James Webb Space Telescope (Gardner et al 2006) may uncover evidence that the progenitors of the BHs powering such quasars are grown through sustained super-Eddington accretion.…”

“…Indeed, no observed BHs lie beyond the limit in our Figure 2. If they are unveiled by future observations, this would add to the evidence for significant obscured BH growth taking place in the early universe (e.g., Treister et al 2013;Comastri et al 2015;Pezzulli et al 2017a;Endsley et al 2022;Fujimoto et al 2022) and provide strong evidence that sustained super-Eddington growth may explain the emergence of the most massive BHs inferred to power high-z quasars. The expectation that highly super-Eddington BHs would be heavily obscured not only due to the trapping of radiation in the accretion flow, but also due to attenuation by the surrounding dense gas from which they are accreting is, in fact, broadly consistent with the recent finding of Gilli et al (2022) that 80%-90% of supermassive BHs at z  6 may be hidden from view by the dense gas in their host galaxies.…”

What powers galactic outflows: nuclear starbursts or AGN?

“…Large amounts of dust, with a median dust mass of M d ∼ 10 9 M , are also observed in a sample of DSFG at 1.9 < z < 6.9 (Reuter et al 2020). A huge dust mass of M d ∼ 2 × 10 9 M is inferred in a dusty SB/obscured AGN candidate at z ∼ 6.8 (Endsley et al 2022), while a significant dust mass of M d 10 8 M is also reported for a dustenshrouded SB at z ∼ 7.2 (Fujimoto et al 2022). These observations suggest a rapid dust enrichment in the early Universe, possibly due to early supernovae and grain growth (Leśniewska & Micha lowski 2019).…”