# roberts by shuifanglj

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```									Black Holes and Active Galaxies

Doug Roberts, Ph. D.
Northwestern University

Basic concepts of gravity

• Gravity is created by mass.
• Gravity is always attractive.
• Gravitational attraction is proportional
to the sum of the masses of both
objects.
• Gravitational attraction increases as two
objects come closer to each other.

All due to Isaac Newton

• In 1665, the plague had shut down
Cambridge University where Newton
• He subsequently worked from home on
circular motion and other ideas.
• When the university reopened two
years later, Newton used Kepler‟s laws
and his own observations to derive the
Universal Law of Gravitation.
Newton‟s Universal Law of
Gravitation

F =GMm/r                   2

F: Gravitational attraction (Force)
G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2
M: Mass of central object
m: Mass of smaller object
r: Distance between the objects
Dark stars (a.k.a black holes)

• English geologist Rev John Michell realized that it
would be theoretically possible for gravity to be so
overwhelmingly strong that nothing – not even light
could escape.
• In his 1783 paper to the Royal Society Michell wrote:
If the semi-diameter of a sphere of the same density as the
Sun in the proportion of five hundred to one, and by
supposing light to be attracted by the same force in
proportion to its [mass] with other bodies, all light emitted
own proper gravity.

Dark stars (a.k.a black holes)

• At the time, the necessary conditions for “dark stars”
(as Michell called them) seemed physically impossible.
• In 1796, the great French mathematician, Pierre
Laplace proposed similar ideas to those of Michell in
his famous paper „Exposition du Systeme du Monde‟.
• In the early 1800‟s experiments on optical interference
led to the predominance of the wave theory of light
and the end of the corpuscular theory. Since light
waves were thought to be unaffected by gravitation,
interest in the hypothetical “dark stars” ceased.

General relativity

• 1915 Einstein published his General
Theory of Relativity.
• The General Theory was a new theory of
gravitation and one of its fundamental
predictions was the effect of gravity on
light.

General relativity

• According to the theory, matter causes
space-time to curve.
• The paths followed by light rays or
matter is determined by the curvature
of the space-time and allowed a
modern scientific proof of Mitchell‟s
hypothesis.

General relativity

• Soon after Einstein developed general
relativity, Karl Schwarzschild discovered a
mathematical solution to the equations of the
theory that described such an object.
• It was only much later, with the work of such
people as Oppenheimer, Volkoff, and Snyder
in the 1930‟s that people thought seriously
about the possibility that such objects might
actually exist in the universe.

General relativity

• Einstein himself vigorously denied their
reality, believing, as did most of his
contemporaries, that black holes were a
mere mathematical curiosity.
• He died in 1955, before the term “black
hole” was coined or understood and
observational evidence for black holes
began to mount.
General relativity

• Near a black hole, this distortion of
space-time is extremely severe and
causes black holes to have strange
properties.
• In particular, a black hole has an event
horizon, which is a spherical surface
that marks the boundary of the black
hole.
Event horizon

• You can think of the horizon as the place
where the escape velocity equals the velocity
of light.
• Outside of the horizon, the escape velocity is
less than the speed of light.
• But if you find yourself inside the horizon, the
escape velocity would be larger than the
speed of light, thus there is no escape.

Black holes have no hair

Black holes, unlike most objects
can only have three characteristics
•Mass
•Angular momentum or spin
•Electric charge

The event horizon and the
• For a nonrotating black hole, the horizon is
located at the Schwarzschild radius (Rs)

Rs =2GM/c                  2

G: Gravitational constant = 6.67 × 10-11 m3kg-1s-2
M: Mass of the black hole
c: Speed of light = 3 × 108 km s-1

The event horizon and the

• For a mass as small as a human
being, the gravitational radius is of
the order of 10-23 cm, much smaller
than the nucleus of an atom.
• For a typical star such as the Sun, it is

Black hole classifications

• Black holes are theorized to come in
three different sizes
– Small (“mini” or “primordial”)
– Medium (“stellar”)
– Large (“supermassive”).

Observations of black holes

• How can you check whether something
is a black hole or not?
• The first thing you‟d like to do is
measure how much mass there is in
that region.
• If you've found a large mass
concentrated in a small volume, and if
the mass is dark, then it's a good guess
that there's a black hole there.
Observations of stellar black holes

• One class of black-hole candidates are
stellar-mass black holes, which are
thought to form when a massive star
ends its life in a supernova explosion.

Stellar black holes

• Stellar evolution: low mass stars end up
as white dwarves
• Moderate mass stars end up as neutron
stars and pulsars
• The highest mass stars become black
holes

Observations of stellar black holes

• Another possibility is
that black holes
might form as a
merger of two
neutron stars.

Credit: Wai-Mo Suen, Malcolm Tobias,
Mark Miller, et. al.

Observations of stellar black holes

• Merger from loss of
gravitational

Credit: J. Faber & F. Rasio,
Northwestern U.

Observations of stellar black holes

• If such a stellar black hole were to be
off somewhere by itself, we wouldn't
have much hope of finding it.
• However, many (probably most) stars
come in binary systems – pairs of stars
in orbit around each other.

Observations of stellar black holes

• If one of the stars in such a binary
system becomes a black hole, we might
be able to detect it.
• In particular, in some binary systems
containing a compact object such as a
black hole, matter is sucked off of the
other object and forms an “accretion
disk” of stuff swirling into the black
hole.
Observations of stellar black holes

• The matter in the
accretion disk gets
very hot as it falls
closer and closer to
the black hole, and
it emits large
amounts of
Accretion from companion onto
Credit: Gamma-Ray Astronomy Program
Working Group, NASA.

Observations of stellar black holes

• Because of the intense heat created as
the mass falls into the accretion disk,
most of the radiation we observe is in
the X-ray part of the spectrum.
• Many such “X-ray binary systems” are
known, and some of them are thought
to be likely black-hole candidates.

Observations of stellar black holes

• In order to determine if an unseen compact object is
a black hole, you need to do is to estimate its mass.
• By measuring how fast the visible companion orbits
the center of mass of the system (together with a
few other things), you can figure out the mass of the
invisible companion.
• The technique is quite similar to the one for
supermassive black holes in galactic centers: the
faster the star is moving, the stronger the
gravitational force required to keep it in place, and so
the more massive the invisible companion.

Observations of stellar black holes

• If the mass of the compact object is found to
be very large very large, then there is no kind
of object we know about that it could be
other than a black hole.
• An ordinary star of that mass would be
visible. A stellar remnant such as a neutron
star would be unable to support itself against
gravity, and would collapse to a black hole.

Observations of stellar black holes

• The combination of such mass estimates and
detailed studies of the radiation from the
accretion disk can supply powerful
circumstantial evidence that the object in
question is indeed a black hole.
• Many of these “X-ray binary” systems are
known, and in some cases the evidence in
support of the black-hole hypothesis is quite
strong.

Stellar black holes: Cygnus X-1

• Cygnus X-1 was the name given to a
source of X-rays in the constellation
Cygnus, discovered in 1962 with a
primitive X-ray telescope flown on a
rocket.
• By 1971, the location of the X-ray
source in the sky had been measured
more precisely, using rocket and
satellite observations.
Stellar black holes: Cygnus X-1

•    A faint star appears to be the companion
to Cygnus X-1. Astronomers studying the
light of this companion star have found
two important facts:
1. HDE 226868 is a blue supergiant star – a
massive, normal star near the end of its life
2. the star is orbiting another massive object in a
5.6-day orbit.

Stellar black holes: Cygnus X-1

•    The explanation or “model” which best fits
these facts is that the companion is a black
hole of about 10 solar masses – the corpse
of a massive star which was once the
companion of the observed star.

Stellar black holes: Cygnus X-1

•    The X-rays are produced as gas from the
atmosphere of the blue supergiant star
falls into the collapsed object and is
heated.
•    The collapsed object cannot be a white
dwarf or neutron star, because these
objects can‟t have masses greater than 1.4
and 3 solar masses, respectively.
Similar stories for other X-ray binary
systems

•    LMC X-3
•    Nova Muscae 1991
•    V616 Mon (A0620-00)

Supermassive black holes

• Some point like sources of radio
emission are not related to any optical
star or galaxy.
• In 1963, the location of the radio
source (called 3C273) was identified
and coincided with a very distant star-
like object in the visual wavelengths.

Supermassive black holes
CHANDRA X-RAY

(Credit: NASA/HST, Jodrell Bank Observatory, & NASA/CXC/SAO/H. Marshall et al.)

Supermassive black holes

• They called these objects quasi-stellar
radio sources – quasars, for short –
because they looked like stars, and
produced large amounts of radio waves
as well as light.

Supermassive black holes

If you had radio eyes you would see these quasars and the centers of active
galaxies all over the sky in stead of stars. Credit: image courtesy of NRAO/AUI

Supermassive black holes

• Astronomers also realized that,
although quasars were rare, there were
many other objects – apparently
galaxies of stars – which showed less
extreme versions of the same
phenomenon: very large power from a
very small volume.

Supermassive black holes

• These objects
shared another
remarkable
property: jets of
high-energy
particles emitted
from their cores.

Credit: NASA.

Supermassive black holes

Credit: C.M. Urry & P. Padovani

Supermassive black holes

• More recent studies have confirmed that
QSO‟s lie at the hearts of galaxies which are
themselves too dim to be visible.
• QSO‟s are thought to be of the order of size of
our solar system, but radiate more than 1000
times as much energy as our entire galaxy.
• The current explanation is that they result
from a supermassive black hole which is
consuming matter from its surrounding galaxy.

Supermassive black holes

• The observed power output could be
explained if material the mass of our Sun
were to fall into the black hole each
year.
• This amount of material which could
easily come from the orbiting gas and
winds from massive stars near the core
of the galaxy.
Supermassive black holes

• The jets of particles in active galactic nuclei
are produced by material spiraling into a disk
around the black hole; jets are emitted from
the top and bottom of the disk.

at the Very Large Array.
Credit: Image courtesy of
NRAO/AUI

Supermassive black holes

• This explanation for the “central engine”
in an active galactic nucleus has been
strongly supported by images obtained
by the Hubble Space Telescope, Chandra
such as the Very Large Array.

Supermassive black holes

2D MHD Simulation of Jet (Mach number=10, Jet density=0.01 of external
medium), Top: Gas Density; Bottom: Magnetic Pressure. Credit:
I. L. Tregillis, T. W. Jones & Dongsu Ryu.

Supermassive black holes

3D MHD Simulation of Jet (Mach number=6), Synchrotron (radio) surface
brightness. Credit: I. L. Tregillis, T. W. Jones & Dongsu Ryu.

How do supermassive black holes
form?

• Some theories hold that the first
generation of stars included a large
proportion of very massive stars, all of
which formed black holes which somehow
merged.
• Other theories hold that a single “seed”
black hole accreted stars and gas, growing
more and more massive with time.
Observations of supermassive black
holes

• Many galaxies have been observed to
contain such massive dark objects in
their centers.
• The masses of the cores of these
galaxies range from one million to
several billion times the mass of the
Sun.

Observations of supermassive black
holes
• The mass is measured by observing the speed which
stars and gas orbit around the center of the galaxy:
the faster the orbits, the stronger the gravitational
force required to hold them in their orbits.
• This is the most common way to measure masses in
astronomy. For example, we measure the mass of
the Sun by observing how fast the planets orbit it,
and we measure the amount of dark matter in
galaxies by measuring how fast things orbit at the
edge of the galaxy.

Observations of supermassive black
holes

• These massive dark objects in galactic
centers are thought to be black holes
for at least two reasons.
• First, it is hard to think of anything else
they could be: they are too dense and
dark to be stars or clusters of stars.

Observations of supermassive black
holes

• Secondly, the only promising theory to
explain the enigmatic objects known as
quasars and active galaxies suggests that
such galaxies have supermassive black holes
at their cores.
• If this theory is correct, then a large fraction
of galaxies – all the ones that are now or
used to be active galaxies – must have
supermassive black holes at the center.

Exotic technique: gravitational

• The existence of curved spacetime opens
up the possibility that ripples or waves
can exist in the spacetime continuum.
• These ripples are called gravitational
waves. Gravity waves could be detected
from colliding black holes, supernova
explosions and the black hole at the core
of our Galaxy.

Supermassive black holes: M87

• Hubble measurements show the disk at the
center of M87 is rotating very rapidly.
Scientists believe it contains a massive black
hole at its hub.
• Though the black hole weigh as much as 3
billion of our Suns, it is concentrated into a
space no larger than our solar system.
• A brilliant jet of high-speed electrons that
emits from the nucleus is believed to be
produced by the black hole‟s “engine.”

Supermassive black holes: M87

Credit: Image courtesy of NRAO/AUI.

Supermassive black holes: M87

• Hubble Space
Telescope image of
a spiral-shaped disk
of hot gas in the
core of active galaxy
M87.

Credit: STScI WFPC2.

Supermassive black holes: NGC 4261
A composite image of the
active galaxy NGC 4261,
emitting particles spurting
from the core of the galaxy.
A false-color image (right)
from the Hubble Space
Telescope, shows a dark,
doughnut-shaped structure
surrounding a possible
supermassive black hole.             Credit: Walter Jaffe, Leiden Observatory;
Holland Ford, STScI, NASA

Supermassive black holes: NGC 4258
NGC 4258 was found to have a system
of “water masers” near its nucleus.
Using the technique of very-long-
interferometry, researchers were able
to determine the motion of the gas
very accurately. From this they can
conclude that the massive object at the
center of this galaxy is less than half a
light-year in radius. It is hard to
imagine anything other than a black
hole that could have so much mass
concentrated in such a small volume.

Credit: Harvard Smithsonian CfA, NRAO/AUI.
Supermassive black holes: The Milky
Way

• There has been
growing evidence
that our own Galaxy
harbors a black hole
in its center.

Adler Planetarium & Astronomy Museum   Credit: Image courtesy of NRAO/AUI
Supermassive black holes: The Milky
Way

Credit: Image courtesy of NRAO/AUI

Supermassive black holes: The Milky
Way

Credit: Image courtesy of NRAO/AUI

Supermassive black holes: The Milky
Way

Credit: Image courtesy of NRAO/AUI

Proper motion of hot gas in the
Galactic Center

Credit: D. Roberts, F. Yusef-Zadeh & W.M Goss

Proper motion of stars in the
Galactic Center

Credit: R. Genzel, A. Eckart, T. Ott, MPE.

Proper motion of stars in the
Galactic Center

Credit: UCLA Galactic Center Group.

Supermassive black holes: The Milky
Way
• Research that
investigated the
motion of gas and
stars around the
center has shown
that the enclosed
mass is constant to
within a few times
the size of our solar
system.

Supermassive black holes: The Milky
Way

• The only known object that could produce
this effect is a supermassive black hole
(although this is the smallest supermassive
one known) that is about 2 million times
more massive than the sun.
• The big mystery in our Galaxy is why; with
such a large black hole in the center don‟t we
see our core as an active galactic nucleus.