Probing the Physics of Mechanical AGN Feedback with Radial Elongations of X-ray Cavities. (arXiv:2007.04521v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Guo_F/0/1/0/all/0/1">Fulai Guo</a>
Mechanical active galactic nucleus (AGN) feedback plays a key role in massive
galaxies, galaxy groups and clusters. However, the energy content of AGN jets
that mediate this feedback process is still far from clear. Here we present a
preliminary study of radial elongations $tau$ of a large sample of X-ray
cavities, which are apparently produced by mechanical AGN feedback. All the
cavities in our sample are elongated along the angular (type-I) or jet
directions (type-II), or nearly circular (type-III). The observed value of
$tau$ roughly decreases as the cavities rise buoyantly, confirming the same
trend found in hydrodynamic simulations. For young cavities, both type-I and II
cavities exist, and the latter dominates. Assuming a spheroidal cavity shape,
we derive an analytical relation between the intrinsic radial elongation
$bar{tau}$ and the inclination-angle-dependent value of $tau$, showing that
projection effect makes cavities appear more circular, but does not change
type-I cavities into type-II ones, or vice versa. We summarize radial
elongations of young cavities in hydrodynamic simulations, and find that
$bar{tau}$ increases with the kinetic fraction of AGN jets. While mild jets
always produce type-II cavities, thermal-energy-dominated strong jets produce
type-I cavities, and kinetic-energy-dominated strong jets produce type-II
cavities. The existence of type-I young cavities indicates that some AGN jets
are strong and dominated by thermal energy (or cosmic rays). If most jets are
dominated by non-kinetic energies, our results suggest that they must be
long-duration mild jets. However, if most jets are strong, they must be
dominated by the kinetic energy.
Mechanical active galactic nucleus (AGN) feedback plays a key role in massive
galaxies, galaxy groups and clusters. However, the energy content of AGN jets
that mediate this feedback process is still far from clear. Here we present a
preliminary study of radial elongations $tau$ of a large sample of X-ray
cavities, which are apparently produced by mechanical AGN feedback. All the
cavities in our sample are elongated along the angular (type-I) or jet
directions (type-II), or nearly circular (type-III). The observed value of
$tau$ roughly decreases as the cavities rise buoyantly, confirming the same
trend found in hydrodynamic simulations. For young cavities, both type-I and II
cavities exist, and the latter dominates. Assuming a spheroidal cavity shape,
we derive an analytical relation between the intrinsic radial elongation
$bar{tau}$ and the inclination-angle-dependent value of $tau$, showing that
projection effect makes cavities appear more circular, but does not change
type-I cavities into type-II ones, or vice versa. We summarize radial
elongations of young cavities in hydrodynamic simulations, and find that
$bar{tau}$ increases with the kinetic fraction of AGN jets. While mild jets
always produce type-II cavities, thermal-energy-dominated strong jets produce
type-I cavities, and kinetic-energy-dominated strong jets produce type-II
cavities. The existence of type-I young cavities indicates that some AGN jets
are strong and dominated by thermal energy (or cosmic rays). If most jets are
dominated by non-kinetic energies, our results suggest that they must be
long-duration mild jets. However, if most jets are strong, they must be
dominated by the kinetic energy.
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