CO kinematics unveil outflows plausibly driven by a young jet in the Gigahertz Peaked Radio Core of NGC6328

Abstract: We report the detection of outflowing molecular gas in the center of the nearby (z=0.014) massive radio galaxy NGC6328. The radio core of the galaxy, PKS B1718-649, is identified as a Gigahertz Peaked Spectrum source with a compact (2 pc) double radio lobe morphology. We used ALMA CO(2-1) and CO(3-2) observations at 100 pc resolution to study the gas kinematics up to ~5 kpc from the galaxy center. While the bulk of the molecular gas is settled in a highly warped disk, in the inner 300 pc of the disk and along with the orientation of the radio jet, we identified high-excitation and high-velocity gas that cannot be attributed to any regular kinematic component based on our detailed 3D modeling of the ALMA data. The high-velocity dispersion in the gas also suggests that it is not part of an inflowing, shredding structure. These results suggest the presence of a molecular outflow of 3 to 8 solar masses per year. The outflow possibly originated from the interaction of the jet with the dense interstellar medium, even though the radio emission is detected closer to the center than the outflow. In this sense, this source resembles NGC1377, 4C31.04 and ESO420-G13, in which the outflows are linked to faint or past jet activity.

• The paper presents robust evidence of high-velocity molecular gas inconsistent with regular disk kinematics, indicating a significant outflow.
• It employs detailed 3D tilted-ring modeling of ALMA CO data to map the complex gas morphology and disentangle disk dynamics from jet-driven effects.
• The study estimates an outflow mass of 3–4×10⁷ M⊙ and rates of 3–8 M⊙/yr, supporting the notion of AGN feedback via a young radio jet.

Evidence of Molecular Outflows in NGC 6328

This paper (2111.00453) presents evidence for molecular outflows in the nuclear region of the massive radio galaxy NGC 6328, associating these outflows with the galaxy's Gigahertz Peaked Spectrum (GPS) radio core. Using ALMA CO(2-1) and CO(3-2) observations, the authors identify high-velocity molecular gas that cannot be explained by regular disk kinematics, suggesting the presence of an outflow driven by the galaxy's young radio jet. The research highlights the interplay between AGN feedback and the ISM in early-type galaxies.

Observational Data and Analysis

The study utilizes archival ALMA data of CO(2-1) and CO(3-2) transitions to map the molecular gas distribution and kinematics in NGC 6328. The data reduction involved standard CASA pipeline procedures, including continuum subtraction and imaging with natural weighting to optimize sensitivity. The resulting CO line cubes reveal a complex gas morphology, with an asymmetric, highly inclined disk extending up to 4.5 kpc. Kinematic analysis reveals two primary components: a central disk within 650 pc and a larger warped disk extending to 4.5 kpc. The velocity dispersion map identifies regions of high turbulence, particularly in the central disk and along the jet orientation, suggesting non-circular motions. The CO(3-2) emission shows similar distribution but also reveals a distinct inner clump with significant velocity shift.

Kinematic Modeling and Residual Analysis

To disentangle the complex kinematics, the authors developed a 3D tilted-ring model incorporating the gravitational potential of the SMBH, stellar bulge (Hernquist potential), and dark matter halo (NFW potential). The model parameters, including inclination, position angle, and rotational velocity, are optimized using a Markov Chain Monte Carlo (MCMC) sampler to fit the observed CO(2-1) emission and HI data. While the model successfully reproduces the overall disk kinematics, significant velocity residuals remain in the inner 300 pc, particularly along the jet axis. These residuals, characterized by high-velocity dispersion and a steep rise in velocity, cannot be explained by any plausible disk configuration, leading the authors to propose the presence of a molecular outflow.

Outflow Properties and Driving Mechanism

Based on the intensity of the high-velocity residuals and assuming a conversion factor αCO = 0.8 M⊙ (K km s−1 pc2)−1, the authors estimate the mass of the outflow to be 3-4 × 107 M⊙, with an outflow rate of 3-8 M⊙/yr. This corresponds to a kinetic power of 2-7 × 1040 ergs/s and a momentum rate of 1-7 × 1033 dyn. The authors argue that neither star formation nor the low-luminosity AGN can provide sufficient energy or momentum to drive the outflow. Instead, they suggest that the outflow is powered by the radio jet, which has a jet power of ~2 × 1043 ergs/s. This interpretation is supported by the spatial coincidence of the outflow with the jet orientation and the analogy to other galaxies where molecular outflows trace previously undetected radio jets.

Conclusions

The paper (2111.00453) presents compelling evidence for a molecular outflow in the GPS radio core of NGC 6328. By combining high-resolution ALMA observations with detailed kinematic modeling, the authors demonstrate that the observed high-velocity gas cannot be explained by regular disk motions. The outflow is likely driven by the AGN jet, providing further evidence for the role of AGN feedback in regulating gas content and star formation in galaxies. These findings contribute to a growing body of evidence linking AGN activity to the dynamics and evolution of the ISM in early-type galaxies.