How many galaxies have a super-massive black holes (SMBH) in their centers?
All galaxies, except for some dwarf galaxies, have supermassive black holes at their centers.
How many of these are active black holes?
Most likely all the supermassive black holes are swallowing some matter. However only a small fraction ( about 10%) are VERY active and are called AGN.
What are the typical masses for the super-massiveblack holes in centers of galaxies?
106 – 109 Msun
What distinguishes an active galaxy, that is, one with an active galactic nucleus, from a normal galaxy?
See pages 113-115, jets, strong emission over a broad range in wavelengths (nonthermal spectra), some are 100 timesbrighter than the rest of the galaxy;AGN can produce the equivalent of the power of an entire galaxy within the size of the solarsystem.
On what time scale does the brightness of an AGN vary? What does that say about the size of the emitting region?
The variability is on the order of months, hence they must be very compact (see page 115).
What causes the large lobes seen in radio waves?
Jets and their interaction with the intergalactic gas. See page 111 and several pages afterwards.
How is an AGN spectrum different from a black body spectrum?
An AGN has emission at all wavelengths! See figure on page 116.
What is the unification scheme?
See figure on page 123.
What is the typical age of an AGN?
100 million years, 1% age of the universe, since 1% are seen to be active at any one time.
All galaxies, except for some dwarf galaxies, have supermassive black holes at
their centers. Since we only see quasars in the past, and yet most galaxies
have black holes in their centers, we can conclude that
all galaxies may have passed through a quasar-like stage earlier in
time.
What are common/general characteristics of AGN?
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All have some sort of “star-like” object at their very centers, that dominates the galaxies’ luminosities.
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They are all quite a bit more luminous than normal galaxies (by factors of 10-1000) and are therefore all thought to involve central, supermassive black holes.
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All show radiation over a wide range of wavelengths (not like typical galaxies which have thermal radiation).
A large variety of AGN have been discovered (Seyfert, Quasars, Bl Lac objects). What powers the active galactic nuclei?
Their radiation is not created by stars, but is powered by the
central supermassive black hole and the radiation is
concentrated in the nucleus. The activity comes from matter falling into the
black hole.
What is the source of the difference between normal galaxies and active galaxies?
The accretion disk fuels the central power source in AGN, and if there is no
fuel or not enough fuel, then the galaxy will not be seen as an AGN.
When we look at AGN in detail, it appears that many of them show evidence for
the host galaxy absorbing another galaxy.
Where does the material that forms an accretion disk around a supermassive black hole come from?
Since most AGN are undergoing some sort of galaxy interactions, the gas in the
galaxy is disrupted and it can funnel to the core, providing fuel to power the
supermassive black hole.
Can an AGN run out of fuel?
Yes, as galaxies age, less gas is available because stars are forming and
using up the available material. If there is no more material available around
the central supermassive black hole, the AGN will no longer be active and will
then, look like a regular galaxy.
Can activity restart?
Yes, if the nucleus gets new supply of material, usually through mergers with
other galaxies.
What is the “era of quasars?”
We do not observe any quasars nearby (in other words in the current age of the
universe). However, observations suggests that quasars were a thousand
times more common when the universe was ~2 billion years old. This
implies that a quasar may actually be a phase in the evolution of galaxies.
During the quasar era galaxies were closer together, and would experience more
encounters with other galaxies. Such encounters can disrupt the gas within
galaxies and funnel it to the center where they would feed the central
“engine” of the quasar.
What are some of the distinctive features that can indicate the presence of
a supermassive black hole in the center of a galaxy?
Observe two or more of these features to find a black hole:
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Gravitational deflection of light, by an amount requiring
black hole masses and sizes.
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X-ray and/or g-ray emission from ionized gas falling into
the black hole.
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Orbital motion of nearby stars or gas clouds that can be
used to infer the mass of a black hole.
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Motion close to the speed of light, or apparently greater
than the speed of light (“superluminal motion”).
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Extremely large luminosity that cannot be explained easily
by normal stellar energy generation.
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Direct observation of a large, massive accretion disk.