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(M$_{\rm BH}$/M$_{\odot}$) = 8.3$^{+0.7}_{-0.6}$ and an intrinsic gas velocity dispersion of $277~^{+16}_{-14}$ km s$^{-1}$. Combined with the AGN luminosity from infrared spectral energy distribution decomposition, these measurements imply a highly super-Eddington accretion state with $λ~_{\rm Edd}~\gtrsim 4$. 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 $z \gtrsim 6$.

Figures (8)

• Figure 1: Morphology and kinematics of W2305$-$0039 at $z=3.111$. ALMA maps of CO(7--6) integrated intensity, CO(11--10) integrated intensity, velocity field of the CO(11--10) line, and velocity dispersion. Contours are plotted every $2\sigma$, starting at $3\sigma$. The filled circle indicates the synthesized beam. The green lines indicate the orientation of the slit used to extract the PV diagram shown in Figure \ref{['fig:pv']}.
• Figure 2: Dust continuum emission in W2305$-$0039 at observed-frame 1.4 mm (Band 5; left) and 1.0 mm (Band 7; right). Contours are shown at 10, 20, 30, …, 80$\sigma$ (left) and 10, 20, 30, …, 120$\sigma$ (right).
• Figure 3: Azimuthally averaged visibility amplitudes as a function of uv distance for the continuum and line datasets. Data points show the binned amplitudes (error bars, $1\sigma$) for the individual ALMA projects, as indicated by the symbols. Dashed curves show the best-fitting two-component exponential-disk models, circularized for the azimuthal averaging. Shaded regions indicate the $1\sigma$ model uncertainty propagated from the fitted parameter uncertainties via Monte Carlo sampling.
• Figure 4: Radial profile of the CO(11--10)/CO(7--6) luminosity ratio derived from the visibility modeling. The shaded region indicates the $1\sigma$ uncertainty from Monte Carlo error propagation. The left axis shows the line ratio, while the right axis gives the corresponding incident X-ray flux inferred from XDR models Esposito2024.
• Figure 5: Position--velocity modeling of the nuclear CO(11--10) emission. Panels show the observed, best-fitting model, and residual PV diagrams extracted along the kinematic major axis. Contours indicate signal-to-noise levels of $3$--$13\sigma$ in steps of $2\sigma$.