Cold gas removal from the centre of a galaxy by a low-luminosity jet (2202.05222v1)

Abstract: The energy emitted by active galactic nuclei (AGN) may provide a self-regulating process (AGN feedback) that shapes the evolution of galaxies. This is believed to operate along two modes: on galactic scales by clearing the interstellar medium via outflows, and on circumgalactic scales by preventing the cooling and accretion of gas onto the host galaxy. Radio jets associated with radiatively-inefficient AGN are known to contribute to the latter mode of feedback. However, such jets could also play a role on circum-nuclear and galactic scales, blurring the distinction between the two modes. We have discovered a spatially-resolved, massive molecular outflow, carrying $\sim75\%$ of the gas in the central region of the host galaxy of a radiatively-inefficient AGN. The outflow coincides with the radio jet 540 pc offset from the core, unambiguously pointing to the jet as the driver of this phenomenon. The modest luminosity of the radio source ($L\rm_{1.4 GHz}=2.1 \times 10\rm^{23}~\rm W~\rm Hz^{-1}$) confirms predictions of simulations that jets of low-luminosity radio sources carry enough power to drive such outflows. Including kpc-scale feedback from such sources -- comprising of the majority of the radio AGN population -- in cosmological simulations may assist in resolving some of their limitations.

• The paper reveals that low-luminosity AGN jets can drive massive molecular outflows, as shown by CO(1-0) observations of B2 0258+35.
• The study employs detailed kinematic analysis and simulation comparisons, highlighting disrupted gas dynamics near jet-ISM interaction zones.
• Quantitative estimates indicate that the jet power exceeds the gas kinetic energy by nearly two orders of magnitude, emphasizing effective AGN feedback.

Low-Luminosity Jet Impact on Cold Gas in a Galaxy

This paper (2202.05222) presents observational evidence and simulation comparisons indicating that even low-luminosity radio jets from active galactic nuclei (AGN) can significantly impact the interstellar medium (ISM) of their host galaxies by driving massive molecular outflows. The paper focuses on the radio galaxy B2 0258+35, revealing a spatially resolved molecular outflow coincident with a radio jet.

Observational Evidence of Molecular Outflow

The research employs CO(1-0) observations from the NOrthern Extended Millimeter Array (NOEMA) to probe the distribution and kinematics of cold molecular gas in the nuclear region of B2 0258+35. The observations reveal a circumnuclear molecular gas structure spanning 3 kpc and a quiescent CO ring with a radius of 10 kpc. The kinematics of the gas in the central kiloparsecs deviate significantly from the regular rotation observed in the larger gas disc.

Figure 1: Distribution and velocity field of large-scale ring and nuclear region (left); zoom-in of nuclear region showing total intensity map (top right) and velocity dispersion map (bottom right); black contours represent radio continuum emission at 8.5 GHz.

The velocity dispersion map and position-velocity plots indicate that the gas exhibits minimal regular rotation, suggesting disruption of any pre-existing disc structure. The gas is particularly disturbed 540 pc southeast of the radio core, where the southern radio jet undergoes a sharp bend. This region displays high velocity dispersion, with gas outflowing at velocities blueshifted up to 500 km/s relative to the systemic velocity. Additionally, the CO emission is brighter in this region, forming a 'hotspot' indicative of either a higher excitation temperature or optically thin gas. This spatial offset from the radio core suggests the radio jet drives the outflow rather than the nucleus itself.

Figure 2: Position-velocity plot of large-scale gas disc and circumnuclear gas (left), highlighting kinematic differences; PV diagram of circumnuclear gas along the radio axis (right), showing outflow offset southeast of radio core.

Energetics and Jet-Driven Outflow

The estimated kinetic power of the gas associated with the molecular outflow ranges between 1.8 × 10<sup\>41</sup> erg s^-1 and 1.9 × 10<sup\>42</sup> erg s^-1. While NGC 1167 is classified as a low-luminosity optical AGN (LLAGN), the bolometric luminosity estimates range between 3 × 10<sup\>41</sup> erg s^-1 and 7 × 10<sup\>42</sup> erg s^-1. The paper argues that radiation pressure is unlikely to drive the observed outflow due to the outflow's offset from the nucleus and the moderate bolometric luminosity.

Conversely, estimates of the radio-jet power range between 8.2 × 10<sup\>43</sup> erg s^-1 and 1.3 × 10<sup\>44</sup> erg s^-1, exceeding the gas kinetic power by approximately two orders of magnitude. The paper acknowledges that these jet power estimates, based on various correlations, may underestimate the actual jet power for low-luminosity radio sources, suggesting the radio jet can drive the observed outflow even at low efficiency.

Comparison with Hydrodynamic Simulations

To further investigate the jet-ISM interaction, the observations are compared with a relativistic hydrodynamical simulation of jet-ISM interactions. The simulation, while not specifically tailored for B2 0258+35, qualitatively illuminates the underlying physics. Simulation D, involving a relativistic jet of power P_jet = 10<sup\>45</sup> erg s^-1 propagating through a clumpy galactic disc, reveals the evolution of the jet-disc system.

Figure 3: Mid-plane logarithmic density slices of simulation D showing the evolution of the jet-disc system.

The simulation shows that the dispersion of dense gas is strongest during the initial 100 kyr, and by 0.8 Myr, a substantial amount of jet plasma vents through chimneys perpendicular to the disc, reducing energy coupling between the jet and gas. The main jet streams interact directly with clumps, resulting in deflection or splitting, enhanced radio emission due to shocks, and strong gas velocity dispersions and outflows. Synthetic position-velocity diagrams from the simulation show enhanced velocity dispersions of the dense phase (gas with densities n > 100 cm^-3) in regions of jet-ISM interactions.

Figure 4: Synthetic position-velocity diagrams of dense gas in simulation D (top) with corresponding line-of-sight velocity dispersion and mean velocity (bottom).

Implications for AGN Feedback

The findings suggest that the galactic-scale impact of low-power radio galaxies may represent an important component for models of AGN feedback, particularly given the prevalence of low-luminosity radio sources in massive galaxies. The paper emphasizes that the radio emission's efficient coupling with the surrounding medium, as suggested by the results, is crucial. The paper also highlights the relevance of these AGN for cosmological simulations, particularly considering the recurrent impact of the AGN on the host galaxy over multiple cycles.

Conclusion

This paper provides compelling evidence for the role of low-luminosity radio jets in driving significant molecular outflows in galaxies. By combining detailed observations with hydrodynamical simulations, the research underscores the importance of considering the impact of low-power radio galaxies in models of AGN feedback and galaxy evolution. The results have implications for understanding the interplay between AGN activity and the ISM in galaxies, as well as the broader cosmological context of galaxy formation and evolution.