Title: AGN-driven outflows in clumpy media: multiphase structure and scaling relations
Authors: Samuel Ruthven Ward, Tiago Costa, Chris M. Harrison, Vincenzo Mainieri
First Author’s Institution: School of Mathematics, Statistics and Physics, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK
Status: Accepted for publication in MNRAS [open access]
At the center of nearly every massive galaxy lies an energetic powerhouse, commonly called a supermassive black hole (SMBH). These black holes are believed to play an essential role in the growth and evolution of galaxies. Studies have shown that there seems to be an apparent relationship between the properties of the central black hole and the large-scale properties of the galaxy, such as its size and mass. This relationship can possibly be established through outflows of gas powered by the central Active Galactic Nuclei (AGN), which can move across the entire galaxy. These outflows can transport mass and energy across the galaxy and thus likely affect the properties of the galaxy at considerable distances from the center.
AGN-powered outflows can occur over long timescales (about a hundred million years). They can move from the central regions to the edge and beyond of the galaxy (distances equivalent to traveling around the Earth about 10 billion times!). These numbers are most certainly beyond the scale of humans, making it difficult to understand the true nature of the effect of outflows on their host galaxies. So, we turn to simulations to help understand this. By injecting the physics of AGN-driven outflows in galaxy simulations, one can speed up timescales and better understand how outflows interact with the interstellar medium (ISM) of the galaxy.
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Today’s authors use a series of controlled numerical simulations to investigate how the ISM affects the propagation of AGN-driven outflows. They have an idealized gas disk with an AGN at the center, powering spherical outflows. The gas disk is set to mimic the ISM of galaxies and can be varied between a smooth and clumpy ISM (Figure 1). The clumpy ISM is made up of clumps of gas clouds of varying sizes and is likely to be similar to the true nature of the ISM. The authors aim to study how the outflow interacts with the ISM, characterize the nature and structure of the outflows, and determine several outflow properties. These include how much mass is carried by the outflow and the rates at which the outflow carries mass, momentum, and kinetic energy.
The authors determine that the clumpy ISM causes the outflow to have a multiphase nature, a hot phase (with a temperature greater than 106 K) caused by the wind moving through gaps in the disk, and a cool phase caused by the clouds (or clumps) in the medium. The hot phase of the outflow moves faster, with velocities greater than 1000s km/s, while the cold phase moves with the typical velocity of 100 km/s. The cold phase of the outflow carries more mass with it than the hot phase, indicating that the cold phase is likely more efficient in moving mass around in the galaxy. Both phases carry around the same amount of momentum, while the hot phase of the outflow carries more energy than the cold phase. Assuming a smooth ISM also produces outflows with hot and cool phases, but both tend to have similar velocities. Here, the cooler phase of the outflow carries more energy and momentum.
Combining predictions with observations!
These results also have significance in determining what possible challenges one may encounter as they interpret observations of AGN feedback. For example, they find that the outflows do not necessarily have a spherical shape (see Figure 2, top left panel), as assumed by most observational studies. This may lead to incorrect predictions about the strength of the outflow and how much energy and mass it can carry. Future observations of AGN feedback can help confirm these predictions and provide better constraints on feedback physics to help improve predictions from similar numerical simulations.
Astrobite edited by Karthik Yadavalli
Featured image credit: NASA, ESA, Joseph Olmsted (STScI)
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