Torus feeding and outflow launching in the active nucleus of the Circinus galaxy

TL;DR

This work leverages high-resolution ALMA observations of the Circinus nucleus to dissect how material is fed from the circumnuclear disk into the pc-scale torus and how multi-phase outflows are launched. By mapping dust and ionised gas alongside multiple molecular tracers, the authors reveal spiral arms in the CND that drive inward transport toward the torus, with a feeding rate of 0.3–7.5 M⊙ yr−1 and a torus feeding timescale of 120 kyr–2.7 Myr. They simultaneously resolve the base of the pc-scale ionised outflow, showing it is launched or collimated by a warped accretion disk at r ~ 0.16 pc, and they argue for an anisotropic launching mechanism. Taken together, the results imply that more than 88% of the inflowing mass is expelled in outflows before reaching the accretion disk, tying torus feeding fluctuations to observed AGN variability and providing crucial constraints on AGN feeding and feedback timescales.

Abstract

Context: Most active galactic nuclei (AGN) are believed to be surrounded by a dusty molecular torus on the parsec scale which is often embedded within a larger circumnuclear disk (CND). AGN are fuelled by the inward transport of material through these structures and can launch multi-phase outflows that influence the host galaxy through AGN feedback. Aims: We use the Circinus Galaxy as a nearby laboratory to investigate the physical mechanisms responsible for feeding the torus and launching a multi-phase outflow in this Seyfert-type AGN, as these mechanisms remain poorly understood. Methods: We analysed observations from the Atacama Large Millimeter/submillimeter Array of the Circinus nucleus at angular resolutions down to 13 mas (0.25 pc). We traced dust and the ionised outflow using 86-665 GHz continuum emission, and studied the morphology and kinematics of the molecular gas. Results: We find that the Circinus CND hosts molecular and dusty spiral arms, two of which connect directly to the torus. We detect inward mass transport along these structures and argue that the non-axisymmetric potential generated by these arms is the mechanism responsible for fuelling the torus. We estimate a feeding rate of 0.3-7.5 M$_{\odot}$yr$^{-1}$, implying that over 88% of the inflowing material is expelled in a multi-phase outflow before reaching the accretion disk. The inferred torus feeding time scale (120 kyr - 2.7 Myr) suggests that variability in AGN activity may be driven by changes in torus feeding. On parsec scales, the ionised outflow is traced by optically thin free-free emission. The outflow is stratified, with a slightly wider opening angle in the molecular phase than in the dusty and ionised components. The ionised outflow is launched or collimated by a warped accretion disk at a radius of r ~ 0.16 pc, and its geometry requires an anisotropic launching mechanism.

Figures (16)

• Figure 1: Band 6 short (top) and long (bottom) baseline continuum maps at 259 GHz. Contour levels are drawn at $[2, 4, 8, 16,$$32, 64, 128, 256, 512, 1024]\,\times\,$rms, where the background rms equals $0.022\,\mathrm{mJy/beam}$ in the B6SB map and $0.013\,\mathrm{mJy/beam}$ in the B6LB map. The synthesised beam sizes are drawn in the bottom left as filled ellipses, and the cyan cross indicates the AGN position (as defined in Sect. \ref{['sec:Astrometry']}). The phoenix feature is indicated with a white arrow. The polar and equatorial directions mentioned throughout the text are shown in the upper panel.
• Figure 2: Spiral structures in the circumnuclear disk (as informed by the morphology in Figs. \ref{['fig:mol_morphology']} and \ref{['fig:mol_morphology_B7']}) overlaid on the band 7 CO(3-2) ($\nu_{\mathrm{rest}}=345.8\,\mathrm{GHz}$) map. The dashed cyan arrow represents the proposed inflow through the NW hook that feeds the torus as discussed in Sect. \ref{['sec:inflow']}. The synthesised beam size is drawn in the bottom left as a filled ellipse, and the cyan cross indicates the AGN position.
• Figure 3: Band 6 HCO$^+$(3-2) ($\nu_{\mathrm{rest}}=267.6\,\mathrm{GHz}$) and HCN(3-2) ($\nu_{\mathrm{rest}}=265.9\,\mathrm{GHz}$) short (top, covering the CND) and long (bottom, showing the resolved torus) baseline moment zero maps. Contour levels are drawn at $[-16, -8, -4, 2, 4, 8, 16]\,\times\,$rms, where the background rms equals 36, 23, 57 and $28\,\mathrm{mJy/beam\,km/s}$ in the HCO$^+$(3-2) B6SB and B6LB and HCN(3-2) B6SB and B6LB maps, respectively. White contours represent negative flux density values and indicate the position of an absorption hole that coincides with the continuum peak. The L-shape that contains the NW and SW hooks as discussed in the text is drawn as black dashed lines in the top panels. Cyan contours in the top panels indicate the position of the blue-shifted emission feature discussed in Sect. \ref{['sec:inflow']} and represent the $-30\,\mathrm{km\,s^{-1}}$ level in the right panels of Fig. \ref{['fig:BBAROLO_velfield']}. The synthesised beam sizes are drawn in the bottom left as filled ellipses, and the cyan cross indicates the AGN position.
• Figure 4: Spectral energy distribution of the Circinus AGN for the three apertures drawn in Fig. \ref{['fig:FF_dust_model']}. Also shown are four ATCA measurements at varying beam sizes Elmouttie_1998 in white. The dashed lines represent a power-law ($F_{\nu}\propto \nu^{\alpha}$) fit of the 300 mas nuclear aperture containing both a dust component with $\alpha=3.5$ and a free-free component with $\alpha=-0.1$. We also plot the nuclear dust model by Stalevski_2019 in orange.
• Figure 5: Band 6 (259 GHz) continuum decomposed into a free-free ($\alpha=-0.1$) and a dust ($\alpha=3.5$) component based on pixel-by-pixel SED-fitting. The common beam size between all maps used for these SED fits is drawn in the bottom left as a filled ellipse. In the left figure, we indicate the Phoenix and the two nuclear apertures used to obtain the spectral energy distribution shown in Fig. \ref{['fig:SED']}.