Low-Luminosity AGNs: Fed More, Spit Up More
In the center of almost every major galaxy there exists a supermassive black hole. When matters fall onto the black hole, the gravitational potential energy can be released and radiated away via some physical processes that are still not well understood. This makes them bright sources called the active galactic nuclei (AGNs). However, it remains unclear how such processes work when the mass feeding rate (the accretion rate) is extremely low. THCA student Rui She led a project to study these AGNs in nearby galaxies and found that, in the low-luminosity regime, the more the AGNs are fed, the more they reject the in-fall materials in the form of outflow. The observed results are in good agreement with numerical simulations. The paper was accepted for publication in the Astrophysical Journal.
Low-luminosity AGNs were found to be radiating inefficiently. Compared with their high-luminosity cousins, the power that they can convert the potential energy into radiation is much lower. A remarkable case is the supermassive black hole in the center of the Milky Way, Sagittarius A*, which appears to be radiatively inefficient rather than being starved. There are several possibilities to account for this: advection of the accretion power locked in the hot ions onto the black hole, presence of convection, and loss of materials in outflows. Recent numerical simulations suggest that outflows are inevitable under such circumstance.
R. She and his collaborators assembled a large sample of X-ray AGNs in nearby galaxies, among which most of them are low-luminosity AGNs with an accretion rate lower than one-thousandth of the Eddington rate (the rate that can power a black hole to its theoretically maximum luminosity). They found that the absorption column density along the line of sight in the X-ray band is scaled with the AGN luminosity. The amount of materials needed to account for the absorption is well consistent with that lost in the outflow found by numerical simulations. This suggests that the outflow may be the source of absorption and plays an important role in reducing the radiation efficiency.
Contact: Hua Feng
Link to the paper: She, R., Ho, L. C., Feng, H., & Cui C. 2018 (arXiv:1804.07482)