How A Black Hole Lights Up Its Surroundings

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How do the supermassive black holes that live at the centers of galaxies influence their environments? New observations of a distant active galaxy offer clues about this interaction.

Signs of Coevolution

M-sigma relation

Plot demonstrating the m-sigma relation, the empirical correlation between the stellar velocity dispersion of a galactic bulge and the mass of the supermassive black hole at its center. [Msigma]

We know that the centers of active galaxies host supermassive black holes with masses of millions to billions of suns. One mystery surrounding these beasts is that they are observed to evolve simultaneously with their host galaxies — for instance, an empirical relationship is seen between the growth of a black hole and the growth of its host galaxy’s bulge. This suggests that there must be a feedback mechanism through which the evolution of a black hole is linked to that of its host galaxy.

One proposed source of this coupling is the powerful jets emitted from the poles of these supermassive black holes. These jets are thought to be produced as some of the material accreting onto the black hole is flung out, confined by surrounding gas and magnetic fields. Because the jets of hot gas and radiation extend outward through the host galaxy, they provide a means for the black hole to influence the gas and dust of its surroundings.

Radio-loud AGN model

In our current model of a radio-loud active galactic nuclei, a region of hot, ionized gas — the narrow-line region — lies beyond the sphere of influence of the supermassive black hole. [C.M. Urry and P. Padovani]

Clues in the Narrow-Line Region

The region of gas thought to sit just outside of the black hole’s sphere of influence (at a distance of perhaps a thousand to a few thousand light-years) is known as the narrow line region — so named because we observe narrow emission lines from this gas. Given its hot, ionized state, this gas must somehow be being pummeled with energy. In the canonical picture, radiation from the black hole heats the gas directly in a process called photoionization. But could jets also be involved?

In a recent study led by Ákos Bogdán, a team of scientists at the Harvard-Smithsonian Center for Astrophysics used X-ray observations of a galaxy’s nucleus to explore the possibility that its narrow-line region is heated and ionized not only by radiation, but also by the shocks produced as radio jets collide with their surrounding environment.

Heating from Jets

Chandra Mrk 3

Chandra X-ray data for Mrk 3, with radio contours overplotted. Both wavelengths show S-shaped morphology of the jets, with the X-ray emission enveloping the radio emission. A strong shock is present in the west and a weaker shock toward the east. [Bogdán et al. 2017]

Bogdán and collaborators analyzed deep Chandra X-ray observations of the center of Mrk 3, an early-type galaxy located roughly 200 million light-years away. Chandra’s imaging and high-resolution spectroscopy of the galaxy’s narrow-line region allowed the team to build a detailed picture of the hot gas, demonstrating that it shows similar S-shaped morphology to the gas emitting at radio wavelengths, but it’s more broadly distributed.

The authors demonstrate the presence of shocks in the X-ray gas both toward the west and toward the east of the nucleus. These shocks, combined with the broadening of the X-ray emission and other signs, strongly support the idea that collisions of the jets with the surrounding environment heat the narrow-line-region gas, contributing to its ionization. The authors argue that, given how common small-scale radio jets are in galaxies such as Mrk 3, it’s likely that collisional ionization plays an important role in how the black holes in these galaxies impart energy to their surrounding environments.

Citation

Ákos Bogdán et al 2017 ApJ 848 61. doi:10.3847/1538-4357/aa8c76

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