Disentangling the circumnuclear environs of Centaurus A: I. High resolution molecular gas imaging

We present high resolution images of the 12CO(2-1) emission in the central 1’ (1 kpc) of NGC 5128 (Centaurus A), observed using the SMA. We elucidate for the first time the distribution and kinematics of the molecular gas in this region with a resolution of 6’.0 x 2’.4 (100 pc x 40 pc). We spatially resolve the circumnuclear molecular gas in the inner 24’’ x 12’’ (400 pc x 200 pc), which is elongated along a position angle P.A. = 155 deg and perpendicular to the radio/X-ray jet. The SE and NW components of the circumnuclear gas are connected to molecular gas found at larger radii. This gas appears as two parallel filaments at P.A. = 120 deg, which are coextensive with the long sides of the 3 kiloparsec parallelogram shape of the previously observed dust continuum, as well as ionized and pure rotational H2 lines. Spatial and kinematical asymmetries are apparent in both the circumnuclear and outer gas, suggesting non-coplanar and/or non-circular motions. We extend to inner radii (r < 200 pc) previously studied warped disk models built to reproduce the central parallelogram-shaped structure. Adopting the warped disk model we would confirm a gap in emission between the radii r = 200 - 800 pc (12’’ - 50’’), as has been suggested previously. Although this model explains this prominent feature, however, our 12CO(2-1) observations show relevant deviations from this model. Namely, the physical connection between the circumnuclear gas and that at larger radii, brighter SE and NW sides on the parallelogram-shaped feature, and an outer curvature of its long sides. Overall it resembles more closely an S-shaped morphology, a trend that is also found in other molecular species. Hence, we explore qualitatively the possible contribution of a weak bi-symmetric potential which would naturally explain these peculiarities.

💡 Research Summary

This paper presents the first high‑resolution (6″ × 2.4″, ≈100 pc × 40 pc) imaging of the 12CO(2‑1) line in the central kiloparsec of NGC 5128 (Centaurus A) obtained with the Submillimeter Array (SMA). By mapping the CO emission with unprecedented spatial detail, the authors resolve the circumnuclear molecular gas (CMG) and its connection to the larger‑scale molecular structures previously inferred from dust continuum, ionized gas, and H₂ line observations.

The CMG occupies a region of roughly 400 pc × 200 pc (24″ × 12″) and is elongated along a position angle (PA) of 155°, i.e., roughly perpendicular to the well‑known radio/X‑ray jet. The CMG is not an isolated disk; its southeast (SE) and northwest (NW) lobes are physically linked to two parallel molecular filaments that extend outward at PA ≈ 120°. These filaments trace the long sides of the famous 3 kpc “parallelogram” seen in far‑infrared dust emission and are co‑spatial with ionized gas and pure‑rotational H₂ emission, indicating a multi‑phase interstellar medium (ISM) sharing the same geometry.

Kinematically, both the CMG and the outer filaments display pronounced asymmetries. The SE side of the CMG is brighter and exhibits a broader velocity range than the NW side. The outer filaments, while roughly parallel, are not straight; they show a gentle curvature that gives the overall molecular distribution an “S‑shaped” appearance rather than the straight, warped‑disk geometry previously assumed. Moreover, the velocity field deviates from pure circular rotation, suggesting non‑circular motions and possible vertical (out‑of‑plane) components.

The authors compare their observations with earlier warped‑disk models that were constructed to reproduce the large‑scale parallelogram. Those models predict a pronounced gap in CO emission between radii of 200–800 pc (12″–50″) and a relatively smooth, coplanar warp. While the SMA data confirm the existence of a low‑emission region, they also reveal a faint but continuous CO bridge linking the CMG to the outer filaments, contradicting the notion of a completely empty gap. In addition, the warped‑disk models cannot account for the observed brightness asymmetry, the S‑shaped curvature, or the subtle outer‑disk warp.

To explain these discrepancies, the paper qualitatively explores the influence of a weak bi‑symmetric (bar‑like) gravitational potential. A bar can induce streaming motions along its major axis, generate shocks that enhance CO brightness on one side, and produce an S‑shaped morphology as gas follows elongated, non‑circular orbits. The presence of a bar would naturally connect the inner CMG to the outer filaments, create the observed asymmetries, and reconcile the CO data with the previously reported structures seen in other molecular species (e.g., higher‑J CO transitions, HCN).

The study’s implications are twofold. First, it demonstrates that the molecular gas in the central kiloparsec of Centaurus A is more complex than a simple warped plane; instead, it likely experiences a combination of warping, vertical motions, and bar‑driven streaming. Second, the detailed CO kinematics provide essential constraints on how gas is funneled toward the active galactic nucleus (AGN) and how the AGN jet interacts with the surrounding ISM. The authors suggest that future observations with ALMA—offering higher sensitivity, finer angular resolution, and the ability to map multiple transitions simultaneously—will be crucial for quantifying the bar’s strength, measuring vertical motions, and testing dynamical models that incorporate both warps and non‑axisymmetric potentials.

In summary, this high‑resolution CO(2‑1) study resolves the circumnuclear molecular disk of Centaurus A, reveals its physical connection to larger‑scale filaments, identifies significant asymmetries and S‑shaped curvature, and argues that a weak bar-like potential is a plausible driver of these features, challenging earlier warped‑disk interpretations and advancing our understanding of gas dynamics in the immediate environment of a nearby radio galaxy.