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The Life History of Galaxies and Black Holes
• The life history of ‘normal’ galaxies
• Black holes
• How they are connected
Elaine Sadler, School of Physics, University of Sydney
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The Life History of Stars
“Colossal though they
may be, stars and galaxies
rank low on the scale of
complexity… A frog poses
a more daunting scientific
challenge than a star”.
Martin Rees (1997)
A star’s life history is
determined by its mass
at birth.
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The Life History of Galaxies
Galaxies are
‘cosmic
ecosystems’.
Complex interplay
between gas and stars
means there is no “HR
diagram” for galaxies.
A galaxy’s history has
to be deduced from
what we can observe.
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A galaxy’s appearance depends on its
star-formation history
Galaxy classification
scheme first proposed by
Hubble (1936)
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We live in a spiral galaxy….
The Milky Way galaxy imaged at far-infrared wavelengths by the COBE
satellite. How did our Galaxy form?
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Spiral galaxies
New stars are
forming in the
disk, which is
dominated by blue
light from
massive, luminous
young stars.
Older stars in
centre (bulge) and
halo.
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Dwarf galaxies
Small galaxies (106 to 109
stars, compared to 1010 to
1012 stars in giant
galaxies).
IC 5152
Leo dwarf
Often lack a nucleus, star
formation histories are
varied and often poorly
understood.
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Elliptical galaxies
No recent star formation available gas supply for
forming new stars has already
been used up, and light is
dominated by old, low mass
stars (K giants).
Last major episode of star
formation may have been as
long as 10 billion years ago.
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Galaxies can meet and collide...
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But gas and stars are not the whole
story...
Some galaxies are
powerful sources of radio
waves. These are always
giant elliptical galaxies,
never spirals or dwarfs.
Why??
PKS 2356-61 (ATCA: red: radio emission in red, blue: optical light).
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At the heart of a radio galaxy...
Radio telescopes
can image at
much higher
resolution than
optical telescopes.
Show us that the
‘central engine’ of
a radio galaxy is
very small (<0.1
light year) but
also very
powerful.
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Quasars and quasi-stellar objects (QSOs)
Very bright
nucleus, outshines
underlying galaxy
- so QSOs look
like stars when
seen with groundbased telescopes.
Luminosity can
equal over 100
‘normal’ galaxies.
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Imaging the sky at radio wavelengths
Molonglo Observatory Synthesis Telescope, University of Sydney
• Radio atlas of the whole southern sky 1997-2004 (SUMSS)
• Technology testbed for the Square Kilometre Array 2002-2007
“A machine for finding supermassive black holes…”
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Images of the optical and radio sky
Optical DSS B: Mostly nearby
Radio 843 MHz: Mostly very
galaxies (median z~0.1)
distant radio galaxies (median z~1)
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Spectral energy distribution for
galaxies (X-ray to radio)
Light dominated by stars, ‘black body
curve’ peaks near optical/IR
Different physical processes
dominate in normal and
‘active’ galaxies
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Synchrotron radiation
Produced by relativistic electrons spiralling in a magnetic
field - dominant mechanism for radio emission in active
galaxies (AGN)
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Galaxy Energetics
Object
Energy Output
Origin
Sun
3.8x1026 W
Thermal (nuclear fusion)
Milky Way
~1038 W
1011 stars, gas clouds etc.
Quasar
~1040 W
Emitted from a very small
region (maybe no larger than
What physical process
can achieve this??
our solar system)
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Accretion onto a central super-massive
black hole
Standard model:
• Black hole
• Accretion disk
• Collimated jets
Typical black hole
mass in radio galaxies,
QSOs : 107 - 1010 solar
masses
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M87 - a nearby radio galaxy with a jet
Synchrotron jet seen at wavelengths
from radio to X-ray
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What are Black Holes?
Regions of space from which nothing can escape, not even
light, because gravity is so strong.
First postulated in 1783 by English geologist John Michell,
term “black hole” coined in 1969.
The first conclusive
evidence that black
holes exist came in the
1990s (can’t observe a
BH directly, need to
observe its effects).
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Gravity bends light (1)
Distant galaxies being imaged
by the Abell cluster
Gravitational lensing by
the Abell galaxy cluster
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Gravity bends light (2)
.
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Black Hole Structure
• Schwarzschild radius
defines the event horizon can’t see inside this
(vesc=c).
• Inside the event horizon is
the singularity.
• Singularities are points of
infinite gravity, or more
accurately, infinite spacetime curvature, where space
and time end.
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How much energy from a black hole?
Energy output is set by
the accretion rate onto
the black hole. The
Eddington limit is the
maximum rate at which
gas can be accreted.
Above this, the
luminosity is so high that
radiation pressure
prevents further inflow.
Eddington limit is higher
for more massive black
holes.
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Types of Black Holes
• Primordial – can be any size, including very small
If Earth were a BH it would have mass 6x1024 kg and radius ~1cm.
• Stellar Mass – must be at least 3 solar masses
(~1031 kg)
• Intermediate Mass – a few thousand to a few tens
of thousands of solar masses; possibly the
agglomeration of stellar mass holes
• Supermassive – millions to billions of solar
masses; located in centres of galaxies
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Cygnus X-1 - a nearby “stellar-mass” black
hole
• Cygnus X-1, X-ray
binary system
• Mass determined by
Doppler shift
measurements of
optical lines
• Measured mass is 16
(+/- 5) solar masses.
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The Galactic Centre
• Nearest supermassive
black hole: 2.6x106 M
• Black hole mass can be
measured accurately from
the 3D orbits of stars
which pass close to the
centre:
– Proper motions &
radial velocities
(Ghez/Genzel)
– Measurements in IR
because of dust
(Ghez)
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NGC 4258 - weighing the central black
hole via masers
Black hole mass measured as 3 x 107 Msun
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Using gas dynamics to ‘weigh’ the central
black hole in M87
(Harms
et al. 1994)
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Bigger galaxies have bigger black holes
Black hole mass-bulge
mass correlation implies
that formation of galaxy
and central black hole
are intimately related
coupled.
Explains how radio
galaxies and quasars
‘know’ what kind of
galaxy they live in.
(Kormendy & Richstone 95)
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Where do black holes come from?
Collapse of individual
stars - 1-10 Msun BHs
Black holes grow by
black-hole mergers
or…
Black holes grow by
swallowing gas (QSOs)
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When did the galaxy-black hole connection arise?
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Redshift and look-back time
.
.
.
.
.
.
.
.
.
.
.
.
Redshift
z
1400
20
10
5
3
2
1
0.5
0.3
0.2
0.1
0
Time Since Big Bang
(in Gyr=109 yr)
250,000 yr
0.1 Gyr
0.3
0.9
1.6
2.5
4.6
7.1
8.8
9 .9
11.3
13.0
Fraction of
current age
0.0019%
1.0 %
2.7 %
6.8 %
13 %
19 %
35 %
54 %
67 %
76 %
87 %
100 %
CMB
Peak of
Galaxy
formation?
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The Hubble Deep Field (HST)
Finding black holes is
easy. Studying the
galaxies they live in is
hard.
Our deepest view of the
Universe in optical
light:
Median redshift of z~1
implies galaxies appear
as they were when the
Universe was a third of
its current age.
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High-redshift radio galaxies
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The star-formation history of the Universe
(Baugh et al. 1998)
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The rise and fall of quasars
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Galaxies and black holes both grow
when galaxies collide and merge
• Galaxy mergers trigger extra
star formation, feed gas to
nucleus.
• Accretion rate onto black hole
rises, BH grows, star formation
also makes galaxy more
luminous.
The antennae: Two nearby merging
galaxies - star formation is triggered
by shocks from the interaction.
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The Antennae, gas and stars
(HST)
Star formation is most intense
near the centre (unlike Milky
Way).
(NRAO VLA)
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Nearby radio galaxy Centaurus A - endproduct of a galaxy merger?
A typical large galaxy
has probably had at
least 10 interactions
or mergers over its
lifetime. Most
galaxies are probably
‘assembled’ in this way
rather than forming at
a single epoch.
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Circinus galaxy - star formation around
an accreting black hole
Well-studied nearby
galaxy: HST image
shows the active nucleus
surrounded by two
starburst rings.
The dust-enshrouded
star-forming regions are
the dominant energy
source in the radio and
infrared regions of the
spectrum.
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Summary
• Super-massive black holes (106 to 109 solar masses)
probably lie at the centres of most bright galaxies.
• The process by which these black holes form
appears to be tightly related to the process of galaxy
formation, in ways we don’t yet understand fully.
• Massive black holes are the central engines of active
galactic nuclei (radio galaxies and quasars) though
the level of activity has varied over cosmic time.
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