Disentangling the Circumnuclear Environs of Centaurus A. III. An Inner Molecular Ring, Nuclear Shocks, and the CO to Warm H₂ Interface

Abstract

We present the distribution and kinematics of the molecular gas in the circumnuclear disk (CND, 400 pc × 200 pc) of Centaurus A with resolutions of ∼5 pc (0. 3) and shed light onto the mechanism feeding the Active Galactic Nucleus (AGN) using CO(3-2), HCO + (4-3), HCN(4-3), and CO(6-5) observations obtained with ALMA. Multiple filaments or streamers of tens to a hundred parsec scale exist within the CND, which form a ring-like structure with an unprojected diameter of 9 × 6 (162 pc × 108 pc) and a position angle P A 155• . Inside the nuclear ring, there are two leading and straight filamentary structures with lengths of about 30-60 pc at P A 120• on opposite sides of the AGN, with a rotational symmetry of 180• and steeper position-velocity diagrams, which are interpreted as nuclear shocks due to non-circular motions. Along the filaments, and unlike other nearby AGNs, several dense molecular clumps present low HCN/HCO + (4-3) ratios ( 0.5). The filaments abruptly end in the probed transitions at r 20 pc from the AGN, but previous near-IR H 2 (J=1-0)S(1) maps show that they continue in an even warmer gas phase (T∼1000 K), winding up in the form of nuclear spirals, and forming an inner ring structure with another set of symmetric filaments along the N-S direction and within r 10 pc. The molecular gas is governed primarily by non-circular motions, being the successive shock fronts at different scales where loss of angular momentum occurs, a mechanism which may feed efficiently powerful radio galaxies down to parsec scales.

ALMA Observations of the Physical and Chemical Conditions in Centaurus A

“…It is worth noting that the velocity field of the inner region of the CND seems to twist in the HCO + (1 − 0) moment 1 map ( Figure 5). This warping appears to decrease the position angle of the inner CND, consistent with the H 2 velocity map of Neumayer et al (2007) and CO maps of Espada et al (2017). However, in the H 2 velocity map, there appears to be another warp further in, not sampled by our data (due to the absorption).…”

“…The outer portions of the CND for these transitions are coincident with the edges of the CND that are seen in 13 CO(2 − 1). Regions C and D are most likely composed of a combination of nuclear filaments and the nuclear ring structure (Espada et al 2017).…”

“…Interior to the parallelogram structure is a 405 pc by 215 pc circumnuclear disk with a different position angle and inclination (Israel et al 1990(Israel et al , 1991Israel 1992;Rydbeck et al 1993;Neumayer et al 2007;Quillen et al 2010). Espada et al (2017) describe additional molecular components on much smaller scales within the circumnuclear disk, including CND filaments, a nuclear ring, nuclear filaments (probably shocks), a nuclear disk (∼30 pc in size), and absorption against the AGN. The nuclear disk is detected in ionized and molecular hydrogen (Marconi et al 2001(Marconi et al , 2006Neumayer et al 2007).…”

ALMA observations of AGN fuelling

“…Morganti 2010 for a review). Interestingly, recent ALMA observations of Centaurus A (Espada et al 2017), reveal a clumpy circumnuclear disk of molecular gas in its innermost 500 pc. In the centre of the disk, there is a lack of CO gas, while the warm H 2 is detected down to small radii (r 10 pc).…”

“…Indications that chaotic cold accretion can fuel an AGN have been found also in a handful of other sources. For example, in Centaurus A (Espada et al 2017), NGC 5044 (David et al 2014;Russell et al 2016;David et al 2017) and PKS 2322-12 (Tremblay et al 2016) cold gas clouds with non circular orbits are found in proximity of the AGN. While in NGC 3115 (Wong et al 2013) and M 87 (Russell et al 2015) similar clouds are likely condensating from the hot circumnuclear environment.…”