NGC 6860, Mrk 915, and MCG -01-24-012. II. Inflowing and outflowing cold molecular gas and the connection with ionized gas in Seyfert galaxies

TL;DR

This study uses ALMA CO(2-1) observations at ~150–400 pc resolution to map cold molecular gas kinematics in three nearby Seyfert galaxies, aiming to diagnose inflows feeding the AGN and outflows driven by AGN feedback, and to connect these processes to the ionized gas traced by [O III]. The authors decompose CO line profiles into two Gaussian components to isolate rotation-dominated gas (c1) from disturbed gas (c2), construct 2D disk models to identify non-circular motions, and examine PV diagrams to infer inflow/outflow geometries, including potential circumnuclear disks. They find bar- or spiral-arm-driven inflows with rates up to $\dot{M}_{mol,in} \sim$ several $M_{\odot}\,\rm{yr^{-1}}$ and outflows with $\dot{M}_{mol,out} \sim$ $0.1$–$3\ M_{\odot}\,\rm{yr^{-1}}$, generally weaker than the accretion rate required for the observed AGN luminosities, suggesting that only a fraction of inflows reach the black hole and that molecular feedback, while present, is not yet strongly suppressive at these luminosities. The results imply a scenario where gas reservoirs near the nucleus may accumulate before fueling the AGN, with potential depletion over time due to feedback and multi-phase exchanges between molecular and ionized gas.

Abstract

We present a study of the cold molecular gas kinematics in the inner ~ 4-7 kpc (projected sizes) of three nearby Seyfert galaxies, with AGN luminosities of ~ 10$^{44}$ erg/s, using observations of the CO(2-1) emission line, obtained with the Atacama Large Millimeter/submillimeter Array (ALMA) at ~ 0.5-0.8$^{\prime\prime}$ (~ 150-400 pc) spatial resolutions. After modeling the CO profiles with multiple Gaussian components, we detected regions with double-peak profiles that exhibit kinematics distinct from the dominant rotational motion. In NGC 6860, a molecular outflow surrounding the bipolar emission of the [O III] ionized gas is observed extending up to $R_{out}$ ~ 560 pc from the nucleus. There is evidence of molecular inflows along the stellar bar, although an alternative scenario, involving a decoupled rotation in a circumnuclear disk (CND) can also explain the observed kinematics. Mrk 915 shows double-peak CO profiles along one of its spiral arms. Due to its ambiguous disk orientation, part of the CO emission can be interpreted as a molecular gas inflow or an outflow reaching $R_{out}$ ~ 2.8 kpc. MCG -01-24-012 has double-peak profiles associated with a CND, perpendicular to the [O III] bipolar emission. The CO in the CND is rotating while outflowing within $R_{out}$ ~ 3 kpc, with the disturbances possibly being caused by the passage of the ionized gas outflow. Overall, the mass inflow rates are larger than the accretion rate needed to produce the observed luminosities, suggesting that only a fraction of the inflowing gas ends up feeding the central black holes. Although we found signatures of AGN feedback on the cold molecular phase, the mass outflow rates of ~ 0.09-3 M$_\odot$/yr indicate an overall weak impact at these AGN luminosities. Nonetheless, we may be witnessing the start of the depletion and ejection of the molecular gas reservoir that has accumulated over time.

Figures (16)

• Figure 1: Color-composite images from DESI Legacy Survey Sky Viewer showing the local environment of NGC 6860 (left), Mrk 915 (middle) and MCG -01-24-012 (right), with squares marking the FoV of Fig. \ref{['fig:moments_6860']}, \ref{['fig:moments_mrk915']} and \ref{['fig:moments_mcg0124102']}. The spectroscopic redshifts of the brighter sources correspond to the preferred values from the NASA/IPAC Extragalactic Database (NED). north is up and east is left in this and the remaining figures along the paper.
• Figure 2: Maps of NGC 6860. (a) Spatial distributions of the CO(2-1) moments' maps of the observed spectral profile, with $M_0$ corresponding to the integrated flux, $M_1$ tracing the mean radial velocity and $M_2$ the velocity dispersion of the CO profile. (b) Maps the parameters of the two Gaussian components fitted the CO(2-1) profiles: c1 (top row) and c2 (bottom row), with the columns corresponding to distributions of the flux (left), the LoS velocity (middle), and the velocity dispersion (right). In the bottom row, contours outline the full CO(2-1) extent (in gray) and the double-peak region (in black). In the bottom left map, we added dashed lines representing the spiral arms (orange), the stellar bar (yellow), and the ring (sky-blue), as identified in dallagnol+25_paperI. The letters A -- E in the lower left panel show the locations of the spaxels used as examples of fits in Fig. \ref{['fig:fitgrid']}. For this and all other maps in the paper: north is up and east is left; coordinates are relative to ALMA millimeter continuum peak (black cross marks, see Table \ref{['tab:sample']}), assumed to be to the galactic nucleus; the gray ellipses correspond to the ALMA beam size of the observation; the inclined black dotted line is the major axis of the global kinematic model.
• Figure 3: Same as Fig. \ref{['fig:panel_ngc6860']}, but for Mrk 915.
• Figure 4: Same as Fig. \ref{['fig:panel_ngc6860']}, but for MCG -01-24-012.
• Figure 5: Example of the model multiple Gaussian models fitted to the CO(2-1) profiles of individual spaxels (0.076 -- 0.16 sizes), one row per object. The first four columns (A to D) show examples where two Gaussian components were preferred over a single component, as in the last column (E). The spatial positions (relative to the nucleus) of each spaxel are shown at the top of each panel, and are marked in Figs. \ref{['fig:fitmaps_ngc6860']}, \ref{['fig:fitmaps_mrk915']} and \ref{['fig:fitmaps_mcg0124012']} (blue letters in the lower left panels). The lines represent: original data (bluish-green); one Gaussian component models (in blue); two Gaussian models (in orange), along with their individual components (in black dashed). The residuals of both models are shifted downwards in the plots. The y-axes are in units of S/N, with the horizontal dotted line corresponding to S/N = 3 being highlighted, since this was the threshold used to discard components.