[Paper Review] Molecular line emission in NGC1068 imaged with ALMA. I An AGN-driven outflow in the dense molecular gas

S. García‐Burillo, F. Combes+23 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|May 28, 2014

Astrophysics and Star Formation Studies155 references177 citations

TL;DR

Using ALMA in Cycle 0, this study maps dense molecular gas in NGC 1068 at 0.3–0.5 arcsec resolution, revealing a massive, AGN-driven molecular outflow in the circumnuclear disk (CND) with an outflow rate of ~63 M☉ yr⁻¹, exceeding the star formation rate by an order of magnitude. The outflow is triggered by the AGN’s ionization cone sweeping across the CND, and its high momentum and kinetic power confirm AGN feedback as the driving mechanism, with a gas depletion timescale ≤1 Myr in the CND.

ABSTRACT

We investigate the fueling and the feedback of star formation and nuclear activity in NGC1068, a nearby (D=14Mpc) Seyfert 2 barred galaxy, by analyzing the distribution and kinematics of the molecular gas in the disk. We have used ALMA to map the emission of a set of dense molecular gas tracers (CO(3-2), CO(6-5), HCN(4-3), HCO+(4-3) and CS(7-6)) and their underlying continuum emission in the central r ~ 2kpc of NGC1068 with spatial resolutions ~ 0.3"-0.5" (~ 20-35pc). Molecular line and dust continuum emissions are detected from a r ~ 200pc off-centered circumnuclear disk (CND), from the 2.6kpc-diameter bar region, and from the r ~ 1.3kpc starburst (SB) ring. Most of the emission in HCO+, HCN and CS stems from the CND. Molecular line ratios show dramatic order-of-magnitude changes inside the CND that are correlated with the UV/X-ray illumination by the AGN, betraying ongoing feedback. The gas kinematics from r ~ 50pc out to r ~ 400pc reveal a massive (M_mol ~ 2.7 (+0.9, -1.2) x 10^7 Msun) outflow in all molecular tracers. The tight correlation between the ionized gas outflow, the radio jet and the occurrence of outward motions in the disk suggests that the outflow is AGN-driven. The outflow rate estimated in the CND, dM/dt ~ 63 (+21, -37) Msun yr^-1, is an order of magnitude higher than the star formation rate at these radii, confirming that the outflow is AGN-driven. The power of the AGN is able to account for the estimated momentum and kinetic luminosity of the outflow. The CND mass load rate of the CND outflow implies a very short gas depletion time scale of <=1 Myr.

Motivation & Objective

• To investigate the role of dense molecular gas in fueling and feedback processes in the Seyfert 2 galaxy NGC 1068.
• To determine whether gas accretion and AGN feedback can self-regulate via inflow and outflow mechanisms.
• To resolve the kinematics and distribution of dense molecular gas (n(H₂) ≥ 10⁵–10⁶ cm⁻³) in the central 2 kpc using high-resolution ALMA data.
• To constrain the geometry and properties of the putative AGN torus using ALMA dust continuum and multi-wavelength data.
• To identify signatures of gas inflow from the outer disk and outflow from the CND, and assess their roles in nuclear activity regulation.

Proposed method

• ALMA Cycle 0 observations at 0.3–0.5 arcsec resolution (20–35 pc at D = 14 Mpc) mapped molecular line emission from CO(3–2), CO(6–5), HCN(4–3), HCO⁺(4–3), and CS(7–6) tracers in the central 2 kpc of NGC 1068.
• Fourier decomposition of the CO(3–2) velocity field was used to disentangle rotational motions from radial inflows and outflows.
• Dust continuum fluxes from ALMA were combined with near-IR and mid-IR data to model the AGN torus using CLUMPY models.
• Molecular line ratios were analyzed to trace changes in gas excitation and ionization, indicating feedback effects from the AGN.
• Spatial and kinematic correlations between ionized gas nebulosity, radio jet, and molecular outflow signatures were used to infer the outflow’s origin.
• Outflow rate was estimated from the mass flux derived from velocity gradients and line-of-sight integration in the CND.

Experimental results

Research questions

• RQ1 Is the dense molecular gas in NGC 1068’s circumnuclear disk being driven by AGN feedback, and what is the outflow rate?
• RQ2 How does the AGN’s ionization cone interact with the molecular gas to launch the observed outflow?
• RQ3 What is the role of the circumnuclear disk in regulating gas inflow and AGN fueling?
• RQ4 Can the observed outflow and inflow kinematics explain the self-regulation of star formation and gas accretion in the nucleus?
• RQ5 What is the geometry and size of the putative AGN torus, as constrained by multi-wavelength dust continuum modeling?

Key findings

• ALMA resolved the dense molecular gas in NGC 1068 at 20–35 pc resolution, revealing a massive molecular outflow in the circumnuclear disk (CND) with a mass outflow rate of 63⁺²¹₋₃₇ M☉ yr⁻¹.
• The outflow rate exceeds the star formation rate in the CND by an order of magnitude, confirming that the outflow is AGN-driven rather than star formation-driven.
• The outflow is spatially and kinematically correlated with the ionized gas nebulosity and radio jet, indicating that it is launched when the AGN’s ionization cone sweeps across the CND.
• The CND has a very short gas depletion timescale of ≤1 Myr due to the high mass load rate of the outflow, implying rapid gas consumption.
• The dust continuum and multi-wavelength modeling suggest a torus radius of 20⁺⁶₋₁₀ pc, consistent with AGN unification models.
• Fourier decomposition of the CO(3–2) velocity field reveals inward radial flows in the bar and starburst ring, while the CND and bow-shock arc show outward radial motions superposed on rotation, consistent with outflow excitation.