Black Hole Jets

Merging X-ray data (blue) from NASA’s Chandra X-ray Observatory with microwave (orange) and visible images reveals the jets and radio-emitting lobes emanating from Centaurus A's central black hole. Credit: ESO/WFI (visible); MPIfR/ESO/APEX/A.Weiss et al. (microwave); NASA/CXC/CfA/R.Kraft et al. (X-ray). Click to Enlarge.

Centaurus A (also known technically as NGC 5128 and colloquially as Cen A) is a giant nearby elliptical galaxy that can be seen from Earth in the constellation Centaurus (but only in the Southern Hemisphere and the tropics of the Northern Hemisphere). Its distance has been measured to be between 3-5 megaparsecs from Earth. (A megaparsec is 3.262 x 106 light years). It is the fifth brightest galaxy. When viewed from radio telescopes Cen A is even brighter. According to the NASA press release which accompanies this image: “Seen in radio waves, Cen A is one of the biggest and brightest objects in the sky, nearly 20 times the apparent size of a full moon.”

This is because Cen A is one of the nearest radio satellite, meaning it has an active galactic nucleus (AGN). An AGN is a compact area at the center of a galaxy which is “brighter” all along the electro-magnetic spectrum than normal. This high luminosity is thought to be caused by the gathering (accretion) of matter by the gravity in an accretion disks around the black holes of spiral galaxies. The black hole at the center of Cen A is 55 million times our own sun’s mass. Cen A has a “relativistic jet” shooting from the face of its accretion disk. The jets shot from Cen A are nearly a million light years long.

In the June issue of Astronomy and Astrophysics is a letter about a new world-wide study of the AGN of Cen A: Cornelia Müller, et al., “Dual-frequency VLBI study of Centaurus A on sub-parsec scales: The highest-resolution view of an extragalactic jet,” 530 Astronomy & Astrophysics L11 (June 2011) (free access). The letter says that astonishingly high resolution of the jet-counter jet system have been obtained by the Southern Hemisphere VLBI monitoring program TANAMI (Tracking Active Galactic Nuclei with Austral Milliarcsecond Interferometry). VLBI observations of Cen A were made with the Australian Long Baseline Array (LBA) and associated telescopes in Antarctica, Chile, and South Africa. Müller and colleagues present the first dual-frequency images of Cen A along with the resulting spectral index map.

The composite image above shows the jets extending far from the center of the galaxy. The study described in the letter, however, according to the NASA release concerns a smaller part:

The new image shows a region less than 4.2 light-years across—less than the distance between our sun and the nearest star. Radio-emitting features as small as 15 light-days can be seen, making this the highest-resolution view of galactic jets ever made.

One current theory on the origin of these jets is that they involve distortions of magnetic fields inside the accretion disks. Müller says: “These jets arise as infalling matter approaches the black hole, but we don’t yet know the details of how they form and maintain themselves,”

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