Feeding the Black Hole
Our standard theory is that quasars and related AGNs turn on when there is matter to feed their
supermassive black hole engines at the center and turn off when there is no
longer fuel for the black hole. Recent Hubble Space Telescope observations indicate
that quasars can occur in galaxies that are interacting with each other. This
suggests the possibility that quasars that have turned off because they have
consumed the fuel available in the original galaxy may turn back on if the galaxy
hosting the quasar interacts with another galaxy in such a way to make more
matter available to the black hole.
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The Radio Source Centaurus A
Centaurus A is only about 10 million light years away.
It has radio lobes that span 10 degrees on the sky (20 times the Moon's diameter) and
a faint optical jet about 40 kpc in length, as well as a radio jet. This suggests
that violent activity at the center has been ejecting gas jets for some time,
thus producing the radio lobes. The logical conclusion is that there is a supermassive black hole
at the center of Centaurus A that has been triggered into activity by the collision of the parent
galaxy with another one.
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Centaurus A
The above image shows the nearest active galaxy to us.
This is the optical view of the giant elliptical galaxy NGC 5128,
which coincides with the strong radio source
Centaurus A. The
area outlined in blue in the
ground-based telescopic image shown in the upper left is expanded in the Hubble Space Telescope view in
the lower right.
This firestorm of star formation is thought to be the result of a collision between a smaller spiral
galaxy and a giant elliptical galaxy that has occurred within the last billion years.
The dark diagonal dust lanes are the remains of
the spiral galaxy, seen almost edge-on and encircling the core of the giant elliptical. The influx of
gas and dust from the collision has triggered rapid star formation, as indicated by the bright blue
colors along the edge of the dust rift. The softer glow around the dust rifts is due to the older
population of stars in the giant elliptical.
Turning the Black Hole On
Infrared observations suggest that behind the dark dust lanes lies a gas
disk 130 light years in diameter
that surrounds a black hole of a billion solar masses. Thus, Centaurus A may be a
relatively nearby example of a central black hole engine being turned on by a galactic collision that
channels gas and dust into it.
X-Ray Jets
This interpretation is supported by the adjacent left image, which shows a Chandra X-ray Observatory
false color X-ray map superposed on an optical image of Centaurus A.
The highest X-ray intensity is indicated by areas that are white or red in color. The Chandra data reveal
a bright central region (the white ball near the center) that probably surrounds the black hole, and a strong
jet oriented to the upper left that is about 25,000 light years in length. This jet is probably being
ejected on the polar axis of the central black hole.
In addition, there is a fainter jet oriented in the
opposite direction. It is thought that this jet is directed largely away from us, while the longer jet is
inclined toward us, making it easier to see. In addition, there are many smaller
X-ray sources (the red spots)
arrayed around the center. These presumably have something to do with the collision that formed Centaurus A,
but they are not yet understood.
