ALMA High-J CO Spectroscopy of High-Redshift Galaxies. II. 0.03" Resolution CO Kinematics Reveal Super-Eddington Accretion in a Dust-Obscured Galaxy at z=3.111

Abstract

We present ultra-high-resolution (0.03"~230 pc) Atacama Large Millimeter/submillimeter Array (ALMA) observations of the hyperluminous dust-obscured galaxy W2305-0039 at z=3.111, targeting the CO J=7-6 and J=11-10 lines. The CO(11-10) emission is extremely compact and exhibits anomalously high excitation relative to CO(7-6) within the central <500 pc. X-ray-dominated region models successfully reproduce this excitation, providing strong evidence for intense X-ray irradiation by a deeply obscured active galactic nucleus (AGN), while photodissociation-region models fail to match the observed ratio. Forward modeling of the nuclear CO(11-10) position-velocity diagram yields a dynamical black-hole mass of log(MBH/Msun) = 8.3${}_{-0.6}^{+0.7}$ and an intrinsic gas velocity dispersion of $277{}_{-14}^{+16}$ km s${}^{-1}$. Combined with the AGN luminosity from infrared spectral energy distribution decomposition, these measurements imply a highly super-Eddington accretion state with $\lambda {}_{\mathrm{Edd}}\gtrsim 4$. Our results provide dynamical evidence that the most rapid phases of black-hole growth can occur within a compact, heavily obscured nuclear region. Extending ALMA beyond its current 16 km maximum baselines will be essential for pushing such dynamical measurements to tens-of-parsec scales and resolving the black-hole sphere of influence in massive galaxies at $z\gtrsim 6$.

Published in ApJ. Figure 1 presents 0.03" resolution ALMA CO(11-10) and CO(7-6) maps. Companion catalog paper: arXiv:2602.23521

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