SUMSS

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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 stars 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
galaxys history has to be deduced from what we
can observe.
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A galaxys 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). Often
lack a nucleus, star formation histories are
varied and often poorly understood.
IC 5152
Leo dwarf
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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 (lt0.1 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
ground-based 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 galaxies (median
z0.1)
Radio 843 MHz Mostly very distant radio galaxies
(median z1)
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Spectral energy distribution for galaxies (X-ray
to radio)
Different physical processes dominate in normal
and active galaxies
Light dominated by stars, black body curve
peaks near optical/IR
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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
our solar
system)
What physical process can achieve this??
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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 (cant 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 -
  cant see inside this (vescc).
• Inside the event horizon is the singularity.
• Singularities are points of infinite gravity, or
  more accurately, infinite space-time 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
(Ghez)

• 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

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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 Time Since Big Bang Fraction
of . z (in Gyr109 yr)
current age 1400 250,000
yr 0.0019
. 20 0.1 Gyr
1.0
. 10 0.3
2.7
. 5 0.9
6.8
. 3 1.6
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. 2 2.5
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. 1 4.6
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. 0.5 7.1
54
. 0.3 8.8
67
. 0.2 9 .9
76
. 0.1 11.3
87
. 0 13.0
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 z1 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 - end-product 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 dont 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.