[Paper Review] 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

Ryan Endsley, Daniel P. Stark+8 more|arXiv (Cornell University)|May 31, 2022

Galaxies: Formation, Evolution, Phenomena7 citations

TL;DR

This paper presents ALMA confirmation of a hyperluminous, radio-loud AGN at z=6.853 in the COSMOS field, hosting a dusty starburst with a 158 μm continuum luminosity of 9×10¹² L⊙ and a star formation rate of 1300 M⊙/yr. The AGN is heavily obscured (τ₉.₇≈2.3) with a bolometric luminosity of ~5×10¹³ L⊙, indicating a ~1.6×10⁹ M⊙ black hole accreting near Eddington, making it a rare, highly obscured quasar analog at z≈7.

ABSTRACT

We present band 6 ALMA observations of a heavily-obscured radio-loud ($L_{1.4\ \mathrm{GHz}}=10^{25.4}$ W Hz$^{-1}$) AGN candidate at $z_\mathrm{phot}=6.83\pm0.06$ found in the 1.5 deg$^2$ COSMOS field. The ALMA data reveal detections of exceptionally strong [CII]158$μ$m ($z_\mathrm{[CII]}=6.8532$) and underlying dust continuum emission from this object (COS-87259), where the [CII] line luminosity, line width, and 158$μ$m continuum luminosity are comparable to that seen from $z\sim7$ sub-mm galaxies and quasar hosts. The 158$μ$m continuum detection suggests a total infrared luminosity of $9 imes10^{12}$ $L_\odot$ with corresponding very large obscured star formation rate (1300 $M_\odot$/yr) and dust mass ($2 imes10^9$ $M_\odot$). The strong break seen between the VIRCam and IRAC photometry perhaps suggests that COS-87259 is an extremely massive reionization era galaxy with $M_\ast\approx1.7 imes10^{11}$ $M_\odot$. Moreover, the MIPS, PACS, and SPIRE detections imply that this object harbors an AGN that is heavily obscured ($τ_{_{\mathrm{9.7μm}}}=2.3$) with a bolometric luminosity of approximately $5 imes10^{13}$ $L_\odot$. Such a very high AGN luminosity suggests this object is powered by an $\approx$1.6 $ imes$ 10$^9$ $M_\odot$ black hole if accreting near the Eddington limit, and is effectively a highly-obscured version of an extremely UV-luminous ($M_{1450}\approx-27.3$) $z\sim7$ quasar. Notably, these $z\sim7$ quasars are an exceedingly rare population ($\sim$0.001 deg$^{-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 $z\gtrsim7$ objects similar to COS-87259, providing richer insight into the occurrence of intense obscured star formation and supermassive black hole growth among this population.

Motivation & Objective

• To identify and characterize extremely rare, hyperluminous, radio-loud AGN candidates at z≈7 in deep wide-area surveys.
• To determine the nature of COS-87259, a highly obscured, UV-bright source in the 1.5 deg² COSMOS field, through multi-wavelength follow-up.
• To measure the star formation rate, dust mass, and black hole mass in a high-redshift AGN-host system using ALMA and multi-band photometry.
• To assess the prevalence of obscured AGN and intense star formation in massive reionization-era galaxies through deep submillimeter and radio observations.
• To evaluate the potential of future wide-area surveys (e.g., Roman, Euclid) to detect similar extreme systems across larger volumes.

Proposed method

• Conducted deep band 6 ALMA observations at 1.3 mm to detect [CII]158 μm emission and dust continuum from COS-87259 at z=6.853.
• Combined ALMA data with multi-wavelength photometry from HST, Spitzer/IRAC, Herschel/PACS, SPIRE, and MIPS to model spectral energy distributions (SEDs).
• Used SED fitting to estimate dust obscuration (τ₉.₇=2.3), total infrared luminosity (9×10¹² L⊙), and star formation rate (1300 M⊙/yr).
• Inferred black hole mass (~1.6×10⁹ M⊙) from bolometric luminosity (~5×10¹³ L⊙) assuming Eddington-limited accretion.
• Assessed stellar mass via SED modeling, yielding M∗≈1.7×10¹¹ M⊙ under standard assumptions, though sensitive to star formation history.
• Evaluated the rarity of such systems by comparing to known quasar and submillimeter galaxy densities in the field.

Experimental results

Research questions

• RQ1 What is the nature of the highly obscured, radio-loud AGN candidate COS-87259 at z≈6.85 in the COSMOS field?
• RQ2 What are the star formation rate and dust mass in the host galaxy of this AGN, as inferred from ALMA and far-IR photometry?
• RQ3 What is the mass of the supermassive black hole powering this AGN, and is it accreting near the Eddington limit?
• RQ4 How common are such hyperluminous, obscured AGN with intense star formation in the early universe, and what does this imply for reionization-era galaxy evolution?
• RQ5 What is the potential of future wide-area surveys (e.g., Roman, Euclid) to detect similar extreme systems across larger areas?

Key findings

• ALMA detected strong [CII]158 μm emission at z=6.8532 and a 158 μm dust continuum, confirming the presence of a massive, dusty starburst host galaxy.
• The 158 μm continuum luminosity implies a total infrared luminosity of 9×10¹² L⊙ and a dust mass of 2×10⁹ M⊙.
• The star formation rate is estimated at 1300 M⊙/yr, consistent with extreme starburst activity in z≈7 submillimeter galaxies.
• The AGN is heavily obscured with τ₉.₇=2.3, and has a bolometric luminosity of ~5×10¹³ L⊙, indicating a black hole mass of ~1.6×10⁹ M⊙ if accreting near Eddington.
• Stellar mass estimates reach 1.7×10¹¹ M⊙, significantly exceeding local black hole-host scaling relations, suggesting extreme early growth.
• Such systems are exceedingly rare (~0.001 deg⁻²), but future wide-area surveys like Roman and Euclid are expected to detect many more, enabling a census of obscured AGN and star formation in massive early galaxies.