Authors: Olga Borodina, Yueying Ni, Jake S. Bennett, Rainer Weinberger, Greg L. Bryan, Michaela Hirschmann, Marion Farcy, Julie Hlavacek-Larrondo, Lars Hernquist
First Author’s Institution: Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
Status: Accepted to ApJ [open access]
The central black hole of some galaxies is surrounded by an accretion disk of hot infalling gas. In cases when the accretion disk is massive and highly energetic, this central system forms an Active Galactic Nucleus (AGN) that outshines the rest of the galaxy. AGN feedback is the process of energetic interactions between the central AGN and its surroundings. AGN are essential for galaxy quenching, and correctly modeling their contribution is important for accurate cosmological simulations.
AGN release the energy from their accretion via winds, which are widespread outflows, and jets. Jets are collimated, fast outflows of baryonic matter which can spread out from hundreds to tens of thousands of parsecs from their origin. There’s a big range in power for jets, ranging from a low end of around 1038 erg/s to a high end of 1046 erg/s. For context, our Sun emits at about 1033 erg/s.
As jets pass through their host galaxy, they interact with the gas and dust of the interstellar medium (ISM). There isn’t a lot of observational evidence for what goes on in that interaction, as the scale of resolution needed is so small – on the order of parsecs within the galaxy. Cosmological simulations don’t model scales small enough to accurately simulate these interactions, and rely on feedback models that implement the macroscale effects. This means we could be missing part of the picture. Today’s paper considers galaxy-scale numeric models for jets passing through a turbulent ISM to study how the jet is affected by the ISM.
To do this, the authors used the Arepo code, which has an adaptive mesh setup that improves the resolution in areas that need it. They set up a 2 kpc cubed box full of gas with properties based on real galaxies’ ISMs. This study is unique compared to other works because they use a filamentary structure to the ISM, while previous studies used a clumpy structure. This filamentary structure reflects our actual Milky Way’s ISM, and creates more low-density cavities for the jet to interact with. They studied three jet powers on the low to intermediate end, of 1038, 1040, and 1043 erg/s. This is also unique to the study, as many works look at higher powered jets, which produce large-scale radio galaxies, despite lower powers being more common. The jets were launched along the x axis, and had a tracer set up to track their positions through time.
This study found three different outcomes for their three jet powers, and these general conclusions held up even when they changed the initial randomized structure of the ISM.
The most powerful of the three jet systems, 1043 erg/s, were easily able to pass through the turbulent environment and reach the outer boundary of the simulation. They mostly travelled along the axis that the jet was launched along, with some lateral expansion.
The intermediate power jets, 1040 erg/s, were highly disrupted and redirected from the axis they were launched along. The jets bent, filled the low density cavities between the filaments, and produced bubble-like shapes. These jets did make it all the way to the simulation boundary, but at later times and not along the axis they were initialized on. These intermediate power jets don’t have enough ram pressure–the pressure against the gas due to the jet’s motion–to push through the turbulent structure of the ISM, and have to fill pre-existing cavities rather than plowing their own. The authors developed an analytic model for the minimum energy needed for jets to be able to plow through the ISM, at approximately 1041 erg/s.
The lowest power case, 1038 erg/s, was stalled out by the ICM within the first kpc of travel. It couldn’t penetrate the higher density regions of the ICM the way the 1040 erg/s case could, and couldn’t make it to lower density cavities to expand into.
All three of these cases were slower and shorter than the same power of jets run in a non-turbulent simulation, with the most significant disruption in the lowest powered jets. This work demonstrates that a turbulent ISM – jet interaction can be the sole cause of observed asymmetric and bent AGN jets. By improving our understanding of this small-scale interaction, the authors are contributing to making better AGN feedback models in cosmological simulations.
Astrobite edited by Ansh Gupta
Featured image credit: NASA (background), Warner Brothers (Gandalf)
