Transcript Hierarchical galaxy formation

Do black holes really exist?
Dr Marek Kukula, Royal Observatory Greenwich
Now have strong evidence for
two classes of black hole
Stellar-mass black holes:
few times the mass of the sun.
Found throughout our own Galaxy.
Supermassive black holes:
up to 10 billion times the
mass of the sun. Found
only in the centres of
large galaxies.
What is a black hole?
• A region of space with such intense
gravity that not even light can
escape.
• First suggested in the 18th Century by
Laplace.
• Idea confirmed by Einstein’s General
Theory of Relativity.
Escape velocity
Strength of gravity
depends on:
•Mass of object
•Distance from
centre of mass
If enough mass is
concentrated into a small
enough volume its gravity
will be so strong that even
light will not be able to
escape.
A Black Hole
Event horizon
Background light distorted
by intense gravitational field
close to the black hole.
Singularity: all
matter inside the
event horizon is
crushed to a point
of ZERO SIZE and
INFINITE DENSITY.
Event horizon: escape velocity = speed of light
Nothing can escape the gravitational pull inside
this radius.
How might black holes form?
Where should we look for them?
To be sure that we’ve found a black hole
astronomers need to demonstrate the object has:
(
 very large mass
 very small volume
)
Physicists and mathematicians might
also like to see evidence for:
 an event horizon
 a singularity
Stellar mass black holes
Nuclear reactions in the
stellar core support a
star against the inward
force of its own gravity.
When the star’s nuclear
fuel runs out it should
begin to collapse…
Is this a way to form a black hole?
Everything depends on the mass of the star…
Death of a star like the sun
• When the the Sun’s helium fuel is
exhausted it will have no further source of
energy
• The outer layers of the star are gently
expelled into space, forming a glowing
“planetary nebula”
• The hot, dense stellar core is left behind to
cool slowly over billions of years – a White
Dwarf star
White Dwarf star
The mass of the sun in a volume the size of a planet.
Composed of “degenerate matter”.
… but it’s not a black hole
Planetary nebulae
Stars more massive than the Sun end
their lives in Supernova explosions:
Much of the star’s
mass is lost in the
explosion
A dense, compact core
is left behind.
If the remaining stellar core has a mass less
than 3 times the mass of the sun it will form
a Neutron Star:
Neutron star: a ball of subatomic particles
supported by nuclear forces
Mass: 1.4  3 times the Sun
Radius: 10 km
Density: Ben Nevis per teaspoonful!
Do neutron stars really exist?
Lovell radio telescope, Jodrell Bank
Radio signals from the
centre of supernova
remnants:
“pulsars”
The discovery of pulsars
Jocelyn Bell-Burnell & Anthony Hewish 1967
Such rapid radio pulsations
could only come from a very
small, dense object with an
intense magnetic field.
 Exactly the properties
expected for a rapidly spinning
neutron star.
But this still isn’t a black hole!
For really massive stars (> 10 solar masses) the remaining stellar
core will have a mass more than 3 times that of the sun.
even neutrons cannot support this amount of mass.
The core is crushed down to a point of INFINITE DENSITY with
a gravitational field so intense that even light cannot escape…
A Black Hole
How can we detect them?
Can’t see the black hole directly
But can try to observe the effects of
its gravity on its surroundings…
Binary star systems
Many stars occur in
binary pairs, orbiting
each other.
If one of the stars goes supernova, the
collapsed core of the star will remain in
orbit around its companion.
X-ray Binary Systems
The collapsed stellar core is too small to be directly
detected but we can infer its presence from its effect on
the visible companion star.
Gas is stripped from
the companion star
and heated as it
spirals in towards the
neutron star or black
hole.
This gas emits huge
amounts of X-rays.
Anatomy of an X-ray binary system
Accretion disc: shines in X-rays
Jets of material ejected at
high speed, giving off radio
waves
Gravity of compact object
pulls matter off companion star
Measuring mass in X-ray
Binaries
Binary orbit around common centre of
mass causes a wobble in the position
of the visible star:
Speed of wobble gives mass of
invisible compact companion.
If the mass of the compact
companion is greater than
3 times the mass of the sun
it CANNOT be a neutron star.
The object must be a black hole.
Cygnus-X1: the best candidate for a
stellar-mass black hole
X-ray source associated with
a binary star. 1 billion times
more luminous in X-rays
than the Sun.
From the ‘wobble’ of the visible
star we can weigh the mass of
the companion to be ~10 solar
masses. Astronomers are 95%
certain that Cyg-X1 is a black
hole.
8 such black hole candidates are now known,
with masses estimated at >3 solar masses
The case for stellar-mass black holes looks good
The evidence for stellar
mass black holes
• Intense X-ray emission from gas
falling onto an extremely compact
object (< 3km across)
• Wobble of companion star indicates
a mass of over 3 times the mass of
the Sun
Physics suggests such an object can
only be a black hole
Supermassive
Black Holes
Quasi-stellar radio sources (Quasars)
• 1963: radio source 3C273 associated with a blue star-like
object.
• Implied distance is 2 billion light years.
 Optical luminosity 250 times brighter than the milky way.
3C273
Many similar objects soon discovered, all with highly unusual properties.
Imaging quasars with Hubble
 Quasars lie at the centres of distant galaxies
Quasar properties
Luminous at all wavelengths
Jets  compact, stable
energy source
Rapid variability
 object is small
Powering quasars
• Extremely luminous
• Extremely small
Only plausible energy source is an accretion
disc around a black hole with millions of
times the mass of the Sun.
The black hole’s accretion disc is only the size of the
solar system, yet it emits more light than the 100 billion
stars in the Milky Way.
X-rays from iron atoms
• High temperatures cause iron atoms to give off X-rays
• High speeds close to the black hole change the
frequency of these X-rays  “Doppler Shift”
• Gas moving with velocities up to
100,000 km/s - exactly the speed
we’d expect at the Event Horizon
• Broad “emission tail”  evidence
for gravitational redshift predicted
by General Relativity close to a BH
X-Ray frequency 
More evidence from the Hubble Space Telescope
Hubble finds signs of dormant black holes in most
large galaxies, not just quasars
Stellar
velocities:
very massive,
very compact
object in
galaxy centre.
Is there a Supermassive Black Hole
in the Milky Way?
Sag A*
Radio image of the Galactic Centre
Infrared images
Reveal the central star cluster:
The La Silla Observatory Chile
The SHARP-1 Camera
(Speckle-Interferometry)
Special technique counteracts atmospheric blurring
to give accurate positions for the stars in the
Galactic centre.
High resolution infrared imaging
of the galactic centre
1994
1997
2000
 can track the motions of individual stars
Stellar motions in the Galactic centre
 mass of central object = 3 million suns
Chandra launch, July 23 1999
Measure X-ray emission from the Galactic centre
Our black hole takes a snack
Before:
After:
What does the black hole look like?
The Evidence for
Supermassive Black Holes
 Energy source for quasars
 Quasar variability
 Stability of radio jets
 X-rays from iron atoms at the
Event Horizon
 Motion of gas in nearby galaxies
 Stellar motions in centre of Milky Way
 only plausible explanation is a black hole
So do black holes really exist?
We have found:
• Extremely compact stellar-mass
objects in X-ray binary systems
• Extremely massive compact objects
in the centres of most galaxies
Their properties are exactly what we’d expect
if they are powered by black holes
(BUT we still haven’t seen a black hole directly!)
Answer: yes
(probably)
The End