Geoffrey V. Bicknell

We present NOEMA CO(2–1) observations of a nearby, young, low-luminosity radio source, B2 0258+35. Our earlier CO(1–0) study had shown the presence of strong jet-ISM interaction and a massive molecular gas outflow involving 75% of the circumnuclear gas. Our follow-up CO(2–1) observations have revealed even more complex gas kinematics, where the southern radio jet is driving out molecular gas in the form of a swiftly expanding bubble, with velocities up to almost 400 km s⁻¹. We found highly elevated CO(2–1)/CO(1–0) line ratios for the gas belonging to the bubble and also further away from the radio jets. Previous observations have shown that the active galactic nucleus (AGN) in the host galaxy, NGC 1167, is in a very low-accretion state. Thus, we attribute the high line ratios to the high gas excitation caused by the jet–ISM interaction. The radio jets, despite exhibiting a relatively low luminosity (1.3 × 10⁴⁴ erg s⁻¹), are solely responsible for the observed extreme gas kinematics. This is one of the clearest detections of an expanding cold gas bubble in such a type of source, showing that the jets are affecting both the kinematics and physicals conditions of the gas. Our study adds to the growing store of evidence that low-luminosity radio sources can also affect the kinematics and physical conditions of the cold gas, which fuels star formation, in their host galaxies to a significant extent. Hence, such sources should be considered in models seeking to quantify feedback from radio AGN.

The physical link between AGN activity and the suppression of star formation in their host galaxies is one of the major open questions of the AGN feedback scenario. The Spitzer space mission revealed a subset of powerful nearby radio galaxies with unusually bright line emission from warm (T ≥ 100 K) molecular hydrogen, while typical star-formation tracers such as polycyclic aromatic hydrocarbons (PAHs) or a dust continuum have been exceptionally faint or undetected. Here, we present JWST NIRSpec and MIRI MRS IFU observations of one of the best studied galaxies of this class, 3C 326 N at z = 0.09. We identified a total of 19 lines of the S, O, and Q series of ro-vibrational H₂ emission with NIRSpec at a 0.11″ spatial resolution, probing a small quantity (1.4 × 10⁴ M_⊙) of gas at temperatures of T ∼ 1000 K. We also mapped the rotational mid-infrared lines of H₂ 0–0 S(3), S(5), and S(6) at a spatial resolution of 0.4″ with MIRI/MRS, probing most of the 2 × 10⁹ M_⊙ of warm H₂ in this galaxy. The CO band heads show a stellar component consistent with a ‘slow-rotator’ that is typical of a massive (3 × 10¹¹ M_⊙) galaxy, offering a reliable systemic redshift of z = 0.08979 ± 0.0003. The extended line emission shows a bipolar bubble expanding through the molecular disk at velocities of up to 380 km s⁻¹, delineated by several bright clumps along the northern outer rim, potentially coming from gas fragmentation. Throughout the disk, H₂ is very broadly dispersed, with an FWHM of ∼100 − 1300 km s⁻¹ and complex, dual-component Gaussian line profiles. The extended [FeII]λ1.644 and Paα follow the same morphology, however, [NeIII]λ15.56 is more symmetric about the nucleus. We show that most of the gas (with the exception of [NeIII]λ15.56) is predominantly heated by shocks driven by the radio jets into the gas, both for the ro-vibrational and rotational H₂ lines. In addition, the accompanying line broadening is sufficient to suppress star formation in the molecular gas. We also compared the morphology and kinematics of the rotational and ro-vibrational lines, finding the latter to be a good proxy to the global morphology and kinematic properties of the former in strongly turbulent environments. This demonstrates the potential of using the higher frequency ro-vibrational lines in studying turbulent molecular gas. Provided they are bright enough, they would allow us to examine turbulence in galaxies during the early phases of cosmic history, while most rotational lines are red-shifted out of the MIRI bandpass for z ≥ 1.5.

Relativistic jets are believed to have a substantial impact on the gas dynamics and evolution of the interstellar medium (ISM) of their host galaxies. In this paper, we aim to draw a link between the simulations and the observable signatures of jet-ISM interactions by analyzing the emission morphology and gas kinematics resulting from jet-induced shocks in simulated disc and spherical systems. We find that the jet-induced laterally expanding forward shock of the energy bubble sweeping through the ISM causes large-scale outflows, creating shocked emission and high-velocity dispersion in the entire nuclear regions (∼2 kpcs) of their hosts. The jetted systems exhibit larger velocity widths (>800 km s⁻¹), broader Position-Velocity maps and distorted symmetry in the disc’s projected velocities than systems without a jet. We also investigate the above quantities at different inclination angles of the observer with respect to the galaxy. Jets inclined to the gas disc of its host are found to be confined for longer times, and consequently couple more strongly with the disc gas. This results in prominent shocked emission and high-velocity widths, not only along the jet’s path, but also in the regions perpendicular to them. Strong interaction of the jet with a gas disc can also distort its morphology. However, after the jets escape their initial confinement, the jet-disc coupling is weakened, thereby lowering the shocked emission and velocity widths.

We use the results of relativistic hydrodynamic simulations of jet-interstellar medium (ISM) interactions in a galaxy with a radio-loud AGN to quantify the extent of ionization in the central few kpcs of the gaseous galactic disc. We perform post-process radiative transfer of AGN radiation through the simulated gaseous jet-perturbed disc to estimate the extent of photo-ionization by the AGN with an incident luminosity of 10⁴⁵ erg s⁻¹. We also map the gas that is collisionally ionized due to shocks driven by the jet. The analysis was carried out for simulations with similar jet power (10⁴⁵ erg s⁻¹) but different jet orientations with respect to the gas disc. We find that the shocks from the jets can ionize a significant fraction (up to 33 |${{\ \rm per\ cent}}$|⁠) of dense gas (⁠|$n\gt 100\, \mathrm{cm^{-3}}$|⁠) in the disc, and that the jets clear out the central regions of gas for AGN radiation to penetrate to larger distances in the disc. Jets inclined towards the disc plane couple more strongly with the ISM and ionize a larger fraction of gas in the disc as compared to the vertical jet. However, similar to previous studies, we find that the AGN radiation is quickly absorbed by the outer layers of dense clouds in the disc, and is not able to substantially ionize the disc on a global scale. Thus, compared to jet–ISM interactions, we expect that photo-ionization by the AGN radiation only weakly affects the star-formation activity in the central regions of the galactic disc (≲ 1 kpc), although the jet-induced shocks can spread farther out.

We study the ionization and excitation structure of the interstellar medium in the late-stage gas-rich galaxy merger NGC 6240 using a suite of emission-line maps at ∼25 pc resolution from the Hubble Space Telescope, Keck/NIRC2 with Adaptive Optics, and the Atacama Large Millimeter/submillimeter Array (ALMA). NGC 6240 hosts a superwind driven by intense star formation and/or one or both of two active nuclei; the outflows produce bubbles and filaments seen in shock tracers from warm molecular gas (H₂ 2.12 μm) to optical ionized gas ([O iii], [N ii], [S ii], and [O i]) and hot plasma (Fe XXV). In the most distinct bubble, we see a clear shock front traced by high [O iii]/Hβ and [O iii]/[O i]. Cool molecular gas (CO(2−1)) is only present near the base of the bubble, toward the nuclei launching the outflow. We interpret the lack of molecular gas outside the bubble to mean that the shock front is not responsible for dissociating molecular gas, and conclude that the molecular clouds are partly shielded and either entrained briefly in the outflow, or left undisturbed while the hot wind flows around them. Elsewhere in the galaxy, shock-excited H₂ extends at least ∼4 kpc from the nuclei, tracing molecular gas even warmer than that between the nuclei, where the two galaxies’ interstellar media are colliding. A ridgeline of high [O iii]/Hβ emission along the eastern arm aligns with the southern nucleus’ stellar disk minor axis; optical integral field spectroscopy from WiFeS suggests this highly ionized gas is centered at systemic velocity and likely photoionized by direct line of sight to the southern active galactic nucleus.

MeerKAT radio continuum and XMM-Newton X-ray images have recently revealed a spectacular bipolar channel at the Galactic Center that spans several degrees (∼0.5 kpc). An intermittent jet likely formed this channel and is consistent with earlier evidence of a sustained, Seyfert-level outburst fueled by black hole accretion onto Sgr A* several Myr ago. Therefore, to trace a now weak jet that perhaps penetrated, deflected, and percolated along multiple paths through the interstellar medium, relevant interactions are identified and quantified in archival X-ray images, Hubble Space Telescope Paschen α images and Atacama Large Millimeter/submillimeter Array millimeter-wave spectra, and new SOAR telescope IR spectra. Hydrodynamical simulations are used to show how a nuclear jet can explain these structures and inflate the ROSAT/eROSITA X-ray and Fermi γ-ray bubbles that extend ± 75° from the Galactic plane. Thus, our Galactic outflow has features in common with energetic, jet-driven structures in the prototypical Seyfert galaxy NGC 1068.

We apply a turbulence-regulated model of star formation to calculate the star formation rate (SFR) of dense star-forming clouds in simulations of jet–interstellar medium (ISM) interactions. The method isolates individual clumps and accounts for the impact of virial parameter and Mach number of the clumps on the star formation activity. This improves upon other estimates of the SFR in simulations of jet–ISM interactions, which are often solely based on local gas density, neglecting the impact of turbulence. We apply this framework to the results of a suite of jet–ISM interaction simulations to study how the jet regulates the SFR both globally and on the scale of individual star-forming clouds. We find that the jet strongly affects the multiphase ISM in the galaxy, inducing turbulence and increasing the velocity dispersion within the clouds. This causes a global reduction in the SFR compared to a simulation without a jet. The shocks driven into clouds by the jet also compress the gas to higher densities, resulting in local enhancements of the SFR. However, the velocity dispersion in such clouds is also comparably high, which results in a lower SFR than would be observed in galaxies with similar gas mass surface densities and without powerful radio jets. We thus show that both local negative and positive jet feedback can occur in a single system during a single jet event, and that the SFR in the ISM varies in a complicated manner that depends on the strength of the jet–ISM coupling and the jet break-out time-scale.