Black Holes: Gravity's Relentless Pull

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Black Holes: Gravity's Relentless Pull
OBJECT: Cygnus A (Elliptical galaxy)
DISTANCE: 730 million lightyears
BLACK HOLE? Yes (supermassive)

This galaxy is the brightest radio source (as indicated by the letter A) in the constellation Cygnus (Swan). The supermassive black hole in its center is a billion times heavier than the Sun. Although the galaxy is relatively distant (300 times further away than the Andromeda galaxy), it appears to us as the second brightest radio source in the entire sky. This is because the black hole generates tremendous energy as it consumes large amounts of material. Nearby electrons are accelerated in this process, emitting strong radio waves as they spiral outward in magnetic fields.

Cygnus A is an elliptical galaxy, with billions of stars in its featureless oval. The dark streaks in the visible image are bands of dust blocking the starlight. In the radio image, thin jets of material lead from the black hole to two giant lobes on either side of the galaxy. These lobes extend ten times further from the galaxy center than the stars in the visible image (the images are not shown to the same scale). The galaxy is enclosed in a cocoon of hot gas that glows in X-rays. Radio and X-ray hot spots are visible where the jets collide with the surrounding gas.

Most large galaxies have a supermassive black hole in their center. But only in a few percent of galaxies does the black hole consume enough material to generate spectacular activity. Cygnus A is one such example. Both elliptical and spiral galaxies can be active in X-rays, but very bright radio emissions are generally seen only in elliptical galaxies.

X-Ray image of: Cygnus A (Elliptical galaxy)
Hot gas surrounds the galaxy. It is particularly bright in X-rays (two yellow spots) where the jets run into the gas.