Transcript Chapter 27 Quasars, Active Galaxies, and Gamma

Announcements
• Grades for third exam are now available on
WebCT
• Observing next week counts on the third
exam.
Quasars and Active Galactic Nuclei
• Quasi-stellar objects
– Redshifts
– Variability
– Supermassive black holes
• Active Galaxies
• Reading 24.1-24.2, 23.2
Quasars
• Early radio telescopes found radio emission
from stars, nebulae, and some galaxies.
• There were also point-like, or star-like, radio
sources which varied rapidly these are the
`quasi-stellar’ radio sources or quasars.
• In visible light quasars appear as points, like
stars.
Optical
picture of a
quasar
Quasar optical spectrum
Hα unshifted
Redshift
shows this
quasar,
3C273, is
moving
away from
us at 16%
of the speed
of light
3C273
The quasar 3C273 is
2.6 billion light years
away.
It looks dim, but must
be extremely
luminous to be visible
as such distance.
The luminosity of
3C273 is more than
one trillion times the
entire energy output of
our Sun, or 100 times
the luminosity of our
entire galaxy.
PKS 2000-030 has
a distance of 23
billion light years
Quasars vary
Quasar size
Size places a limit on how fast
an object can change brightness.
Conversely, rapid variations
place a limit on the size of the
emitting object.
Quasars vary
The size of this quasar must be less than about one
light year.
Quasar engine
• Quasars have 100 times the luminosity of
our Galaxy
• The engine powering quasars is only a few
light years across
• The only known engine which is powerful
enough and compact enough is a black hole
• Quasars contain supermassive black holes
Quasar jets
Optical core 
Radio jet 
Quasars
•
•
•
•
•
•
•
•
object with a spectrum much like a dim star
high red shift
enormous recessional velocity
huge distance (from Hubble’s Law)
enormously luminous
compact physical size
powered by supermassive black hole
often produce huge jets
Quasars are the ultraluminous centers of
distant galaxies.
Quasars are often observed to be at the center of distant galaxies.
The wispy material is likely gas that has been pulled out of the hot
galaxy by gravitational interactions with nearby galaxies.
Quasars are the most extreme of a class
of galaxies known as active galaxies
M87 appears as an elliptical galaxy in visible light, but like a
dim quasar in radio. The nucleus of the galaxy contains a
weak quasar. This means that the galaxy harbors a
supermassive black hole.
Centaurus A
Optical
Radio
NGC 1566
Spiral
galaxies
also
sometimes
contain
active
cores.
Active galaxies lie at the center of
double radio sources.
Active galaxies lie at the center of double radio sources.
Active Galaxies come in several varieties
• Quasars
• Seyfert galaxies
– luminous, star-like nuclei with strong emission lines.
• BL Lacertae objects (BL Lacs)
– featureless spectrum with a brightness that can vary
by a factor of 15 times in a few months.
– Most commonly known as a Blazar.
• All contain supermassive black holes
Rotation Curves
Rotation curves of stars
near the centers of most
galaxies show the
presence of
supermassive black
holes with mass ranging
from 106 to 109 solar
masses.
Essentially all galaxies
contain supermassive
black holes and were
likely active galaxies at
some point in their lives.