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SUPERMASSIVE BLACK HOLES AND THE GROWTH OF GALAXIES
Ryan C. Hickox
(Based on a talk given at the Annual Meeting of TA, 2010 at Wakefield)
PART 1: Black holes and how we observe them
Introduction
As we peer deep into the night sky with the
most advanced modern telescopes, we see
that the Universe is literally teeming with
galaxies (Figure 1). These magnificent
objects, thousands of light-years across and
comprising many billions of stars, are among
the most fascinating and beautiful objects in
the sky. However, galaxies also have a dark
side: spectacular observations in recent
years have revealed that essentially all large
galaxies, including our own Milky Way, are
home to remarkable and exotic objects
called supermassive black holes. In this twopart article we begin with a brief overview of
the physics behind black holes and how
astronomers observe them. In the next part
we move on to the nature and populations of
galaxies and describe the detailed picture
that has emerged for how galaxies form and
change over cosmic time. Finally, we discuss
how recent observational and theoretical
work has shown that galaxies and their
supermassive black holes are intimately
linked as they grow and evolve over the age
of the Universe.
Figure 1
This composite Hubble Space Telescope image shows a variety of galaxies observed in a 3.5 arcmin square
region for a total of 39.6 hours in the constellation Fornax. The nearby galaxies display a variety of structures
and sizes, while more distant sources (some of which appear simply as small, barely resolved objects) are
galaxies at earlier stages in the evolution of the Universe. The bright central object is a foreground star in our
own Milky Way. This image is a typical view of the distant Universe and shows that the cosmos is literally
teeming with galaxies. Credit: NASA, ESA, and the Hubble Heritage Team (STSci/AURA).
Black holes
Almost everyone has heard of black holes. They occupy a special place in the public imagination and rightfully so, for
they are among the most exotic and interesting objects in nature. However, what exactly are black holes, and how do we
know they exist?
Perhaps the simplest and most intuitive definition of a black hole is an object whose gravity is so strong that nothing can
escape, even at the speed of light. To understand this better, we can consider the idea of escape velocity. Imagine we
are standing on the Earth and throwing a ball up in the air. The faster the initial speed of the ball, the higher the ball will
go before it comes back down. Based on our understanding of gravity, as two objects move apart the forces between
them decreases proportional to the square of the distance -- this is given by the famous equation which many may
remember from their physics education: F = GMm/R2 (where F is the force, G is the gravitational constant, M and m are
masses of the objects, and R is the distance between them). So as we throw the ball higher and higher, the pull of gravity
on the ball becomes weaker and weaker, and we can imagine throwing the ball with such high speed that it leaves the
gravitational field of the Earth completely and flies far into space. The speed at which this happens is called the escape
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velocity; for the Earth it is approximately 11 kilometres per second, which is why we need such powerful rockets to launch
vehicles into deep space.
The equations of gravity tell us that the escape velocity for a spherical object (such as a star or planet) is given by
vesc = GM/R. Thus, if we make an object more massive (larger M) or compress it (smaller R), then we increase the
escape velocity. Taking this to its extreme, we can imagine taking an object as massive as the Sun and compressing it
down to a radius of only 3 km, which implies an enormous density (a teaspoonful of this material on Earth would weigh
many billions of tonnes!). This object would then be so massive and yet so small that the escape velocity at its surface
would be equal to the speed of light. Since nothing in the Universe can travel faster than light, we infer that nothing can
escape this object’s strong gravitational field. Such a remarkable entity is what we call a black hole.
The concept of escape velocity gives us a clear and intuitive way of thinking about black holes, but unfortunately does not
provide a full description of the physics behind these objects. For this we require a more complete description of gravity,
which was provided by Einstein in his theory of general relativity in the early 20th century. Einstein's remarkable insight
was that gravity is not simply a force between two objects that acts at a distance, but instead represents a fundamental
curvature in the fabric of space and time in the Universe. Massive objects curve the spacetime around them, and the
motion of objects follows straight lines in this curved space; thus the Moon orbits the Earth because it is following the
curvature of space induced by the Earth's gravity. This is an extraordinary idea, but it makes some robust predictions,
chief among which is that light rays (which have no mass, and so traditionally were not thought to experience gravity) are
deflected as they pass close to the Sun. The observation of this effect provided among the first experimental proof of
Einstein's theory.
In light of general relativity, to fully understand black holes we must think of them as objects for which the strong gravity
bends space so much that even light cannot escape. In this sense, black holes truly are black -- if we could see a black
hole directly, we would see a black sphere, surrounded by images of background objects that had been deflected or
lensed by the strong gravity
of the hole (Figure 2).
Figure 2
A
simulation
of
an
observer’s view of a black
hole placed in front of the
Milky Way. The black hole
itself appears as a dark
sphere,
while
the
gravitational bending of
light by the black hole’s
gravity
produces
“lensing” effects around
the outside of the event
horizon. Credit: Ute Kraus,
University of Tübingen.
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How do we observe black holes?
Since black holes by definition cannot emit light, an obvious question is how do we observe them? While we cannot see
black holes directly, we can observe how their strong gravitational fields influence the matter and gas around them. At
present observations have revealed two separate types of black holes. The first have relatively small masses, roughly
tens of times that of the Sun, which we can observe in binary orbits around normal stars. In many such systems, the
normal star has run out of hydrogen fuel in its core and begins to expand, causing some of its mass to be captured by the
gravity of the black hole companion. As gas spirals onto the hole, viscous interactions cause the material to heat up to
temperatures of millions of degrees and glow extremely brightly, particularly at X-ray wavelengths (Figure 3). These
objects, called X-ray binaries, are among the brightest X-ray sources in the sky. By studying the Doppler shifts of the
companion star's spectrum as it orbits, we can measure the mass of its black hole companion. More than twenty of these
black hole binaries are now known and their study is an active and extraordinarily fruitful area of research.
Figure 3
An artist’s impression of
a black hole X-ray binary.
The system consists of a
black hole (left) in a
binary orbit with a normal
star
(right).
Matter
streams from the normal
star onto the black hole
and forms an accretion
disc, which produces an
enormous luminosity in
X-rays. In some cases
this
accretion
also
launches relativistic jets
of material from the
region around the black
hole. Such X-ray binaries
provide very compelling
evidence
for
the
existence of black holes
with masses of roughly
ten times that of the Sun.
Credit: ESO/L. Calçada.
For this discussion, however, the black holes we're interested in are much, much larger, with masses of millions to billions
times that of the Sun, and they are found in the centres of galaxies. The best-studied example of such a supermassive
black hole lies at the heart of our own Milky Way. This object, known as Sgr A*, is identified as a weak and variable radio
source right at the dynamic centre of our Galactic bulge. In the last twenty years infrared observations, which can peer
through the veil of dust in the disc of our Galaxy, have revealed a population of stars orbiting the location of the radio
source. By measuring the orbits of the stars, we can deduce that the central object has a mass of roughly four million
times that of the Sun, despite being remarkably faint (Figure 4). Decades of study of this object have led to the
overwhelming consensus that it is indeed a supermassive black hole.
Amazingly, this black hole is by no means unusual. We now know that essentially all large galaxies have supermassive
black holes at their centres. In the second part of this article, we will describe the nature of galaxies and show how central
supermassive black holes may play a fundamental role in how galaxies grow and evolve over the age of the Universe.
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Figure 4:
Measured positions of stars
around the centre of the Milky
Way. The circles show
annually measurements of
the position for a number of
stars, taken using infrared
observations with the W.M.
Keck
Telescopes.
The
positions are overlaid on an
infrared
adaptive
optics
image from 2010. The sizes of
the stellar orbits are on the
order of tens of light-days,
and from computing their
Keplerian motions, the mass
of the central object (marked
by a star) can be estimated to
be roughly 4 million times the
mass of the Sun.Credit: This
image was created by Prof.
Andrea
Ghez
and
her
research team at UCLA.
References and further reading
Gillessen, Stefan, et al. 2009, “Monitoring Stellar Orbits Around the Black Hole in the Galactic
Center”, The Astrophysical Journal, 692, 1075-1109
Ghez, Andrea M. et al. 2008, “Measuring Distance and Properties of the Milky Way’s Central
Supermassive Black Hole with Stellar Orbits”, The Astrophysical Journal, 689, 1044-1062
Mella, Fulvio, 2003, The Black Hole at the Center of Our Galaxy, Princeton University Press
Remillard, Ronald A. & McClintock, Jeffrey E. 2006, “X-Ray Properties of Black-Hole Binaries”,
Annual Reviews of Astronomy and Astrophysics, 44, 49-92
Shapiro, Stuart L. & Teukolsky, Saul A. 1994, Black Holes, White Dwarfs, and Neutron Stars: The
Physics of Compact Objects, Wiley
Thorne, Kip 1994. Black Holes and Time Warps: Einstein's Outrageous Legacy. W W Norton &
Company.
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SUPERMASSIVE BLACK HOLES AND THE GROWTH OF GALAXIES
(continued from TA Vol 47 No 563 p294-297)
PART 2: Galaxies and the beasts within
Ryan Hickox
Introduction
In last month's issue of The Astronomer, we explored the physics behind black holes and the observational techniques
astronomers use to study these fascinating and exotic objects. Observations have revealed that supermassive black
holes, with masses up to a billion times that of the Sun, reside at the centres of essentially all large galaxies in the
Universe. The second part of this article will examine the nature and origin of the galaxies themselves, and explore the
fundamental role of black holes in the growth and evolution of galaxies over cosmic time.
The nature of galaxies
Galaxies are possibly the most majestic objects in the night sky, thousands of light-years across and containing hundreds
of billions of stars. The grand spiral of our own Milky Way is just one of billions of such systems in the observable
Universe, and galaxies are found in a fascinating range of shapes and sizes. Despite this variety, it has long been
recognised that galaxies can be classified into two main types: disk-dominated and bulge-dominated systems (Figure 1).
Figure 1
This Hubble Space Telescope image of the galaxy cluster Abell S0740 shows clear examples of the two main
types of galaxies: a disk-dominated spiral galaxy (bottom left) with blue colours and significant ongoing star
formation, and a bulge-dominated elliptical galaxy (top right) with red colours, and little ongoing star formation.
Understanding the processes that gave rise to these two types of galaxies is one of the major challenges in
cosmology. Credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
Disk galaxies have flattened, rotating
distributions of stars and often spiral
structure like the Milky Way, and they
tend to be undergoing significant star
formation, as dense clouds of gas
collapse under gravity inside the disk.
This star formation activity produces a
characteristic blue colour, as the light
from the galaxy is dominated by a small
population of massive, luminous, and
hot stars that shine brightly before they
run out of fuel and explode as
supernovae.
By
contrast,
bulgedominated or "elliptical" galaxies tend to
have higher masses than disk galaxies,
and
have
roughly
spheroidal
morphologies, little large-scale rotation,
and red colours indicating no ongoing
star formation.
A long-standing puzzle for astronomers
has been why these massive bulgedominated galaxies are "dead" - no
longer forming stars - while their disky
counterparts still have significant star
formation. Another puzzle regards the
origin of the relationship between
galaxies and their central black holes. A remarkable recent discovery is that the mass of the black hole is strongly
correlated with the properties of the stellar bulge of a galaxy, rather than with the galaxy as a whole (Figure 2). This
correlation suggests a fundamental link between the growth of the black hole and the structure and evolution of its host
galaxy, yet the precise nature of this relationship is still poorly understood.
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Figure 2
Schematic diagram of the correlation
between black hole mass and the stellar
bulge of galaxies. Galaxies that are more
massive and have larger bulges tend to
host more massive galaxies. The
remarkable tightness of this correlation
suggests a link between the evolution of
galaxies and their central black holes.
Credit: K. Cordes, S. Brown (STScI).
Galaxy formation and evolution
To understand the answers to these and other questions about the nature of galaxies, we must first explore our general
picture of how galaxies form and grow over the lifetime of the Universe. Recent years have seen the emergence of a
remarkably successful “standard model” of cosmology, in which most of the mass in the Universe is made up of dark
matter for which the only strong interaction is via the gravitational force. This cosmological model has been studied using
large computer simulations that show how structure in the Universe forms: at early times, small over-dense regions
collapse under gravity to become bound halos of dark matter. These halos grow and become increasingly massive with
time, and are distributed in a filamentary structure known as the cosmic web (Figure 3).
Figure 3: The large-scale spatial distribution of galaxies, from observational surveys of the real Universe (top
and left), and from theoretical models based on the growth of dark matter structures and the physics of galaxy
and star formation (bottom and right). The models are remarkably successful in reproducing the cosmic web
traced out by galaxies as they form and evolve inside dark matter halos. Credit: VIRGO Consortium (Springel et
al. 2006)
It is in dark matter halos that galaxies form and grow, as
normal matter (mainly hydrogen and helium gas) cools and
falls to the centre of the halo, attracted by the gravitational
pull of the dark matter. Eventually, the gas becomes cold
and dense enough that it begins to fragment and collapse
into the first stars. Initially, a young galaxy settles into a
disk-like structure that preserves the angular momentum of
the cooling gas, and continues to grow as more and more
material falls onto the disk along the cosmic web.
Additional growth occurs through collisions, as two more
galaxies merge together to form a single larger system. In
the merger of two massive disk galaxies, the orbits of the
stars can become disrupted and randomized, and this is
believed to be the process by which the bulges of the most
massive galaxies are formed. In recent years, theoretical
models have been developed that include the processes of
galaxy growth, star formation, and merging inside of dark
matter halos, and can successfully reproduce many
aspects of the overall population of galaxies. These
models can also match how galaxies are distributed in
space, residing in groups and clusters that trace out the
cosmic web of dark matter (Figure 3). It is therefore clear
that we are beginning to close in on an accurate physical
picture to explain the formation and growth of galaxies.
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The remarkable role of black holes
Despite this exciting progress, theoretical models have still faced some fundamental problems. Of these, perhaps the
most troubling is the near-universal prediction that massive galaxies should be able to continually accrete new gas that
cools and falls from the cosmic web into their large dark matter halos. This cooling gas would naturally form stars, thus
producing massive, bulge-dominated galaxies with huge star formation rates and strikingly blue colours, in stark contrast
to the observations showing that elliptical galaxies are usually red and dead. Among the normal processes of gas
dynamics and star formation, there is nothing that can stop this "cooling catastrophe", which has thus proved a serious
dilemma for galaxy formation theory.
Figure 4: Examples of the huge amounts of energy released by rapidly growing black holes. The first panel
shows Hubble Space Telescope images of quasars, for which the radiation released by the growing black hole
outshines the the light from all the galaxy’s stars. The second panel shows a radio image of the powerful radio
galaxy Cygnus A, which displays enormous jets of material moving close to the speed of light that are ejected
from the black hole, extending hundreds of thousands of light-years into intergalactic space. Credit: J. Bahcall,
M. Disney, and NASA (first panel), NRAO/AUI (second panel).
In the last five years or so, a
solution this problem has
emerged from a rather
unexpected source. We
know that every massive,
bulge dominated galaxy has
a large central black hole,
and that the black hole
acquires its mass through
the accretion of interstellar
material over cosmic time.
We also know that this
accretion
process
can
liberate huge amounts of
energy (Figure 4), either in
the form of radiation (as we
observe
in
the
most
radiatively powerful objects
in the Universe, the quasars)
or as energetic outflows
moving close to the speed of
light (as in the spectacular
galaxy-scale jets of radio
galaxies).
Hydrodynamic
simulations have shown that
this enormous energy input
from growing black holes
can either blow gas out of a
galaxy completely and thus
quench
ongoing
star
formation, or can re-heat the
surrounding gas and thus
stop it from cooling and
forming new stars. In a
number of cases, we can
actually
observe
these
processes in action, as
energetic
outflows
from
growing
black
holes
evacuate
spectacular
bubbles
in
the
gas
atmospheres of galaxies
(Figure 5). A host of new simulations have included energy input from black holes in tracing the growth of galaxies, and
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find, in general, that the black hole can indeed produce enough energy to shut off star formation in massive, bulgedominated systems. Some prescriptions for black hole feedback can also naturally explain the remarkable correlations
between black hole mass and bulge properties. It is therefore increasingly well established that to fully understand the
evolution of galaxies, we must account for the energy released by their central black holes.
Figure 5: X-ray and radio images of the
massive galaxy cluster Hydra A, showing
how energy released from black hole jets
can profoundly affect the gaseous
atmospheres around galaxies. The outer
diffuse emission (in blue in the colour
version) is Chandra X-Ray Observatory
data showing X-rays from the hot
atmosphere in the cluster, thus tracing the
distribution of the gas. Clearly, the radioemitting black hole jets (show in pink in the
colour version) are evacuating huge
bubbles in the gas atmosphere. The strong
interaction between the jet and the gas
transfers energy from the black hole to the
much larger atmosphere, keeping the gas
hot and stopping gas from cooling to form
new stars in the central galaxy Credits: Xray: NASA/CXC/U.Waterloo/C.Kirkpatrick et
al.;
Radio:
NSF/NRAO/VLA;
Optical:
Canada-France-Hawaii-Telescope/DSS.
Open questions and future horizons
These discoveries about the importance of black holes in galaxy evolution have generated huge excitement and interest
in the astronomical community. However, some key questions remain: What types of growing black holes are most
important for shutting off star formation? Do galaxy-scale winds from quasars even exist? What physically causes the
relationship between black holes and bulges? And finally, what comes first, the galaxy or the black hole? To answer
these questions, new generations of observational tools will allow an increasingly detailed picture of galaxies, their central
black holes, and their evolution over cosmic time. For example, the KMOS infrared spectrograph on the Very Large
Telescope will enable much more sensitive searches for evidence of galaxy-wide outflows in distant quasars. The
Atacama Large Millimetre Array, which comes on line in 2011, will be able to detect molecular clouds in galaxies from
which stars form, and so will test directly whether energy input from growing black holes is disrupting these clouds. In the
more distant future, the planned Wide Field X-ray Telescope satellite will identify the X-ray emission from millions of
growing black holes, and so enable precise statistical studies of how growing black holes are linked to their host galaxies
and large-scale structures. Along with ever more sophisticated theoretical simulations, these new observations promise to
yield a much deeper physical understanding of how the galaxies around us, and their central black holes, came to exist.
The fascinating story of black holes and galaxies has only just begun.
References and further reading
Bower, Richard G. et al. 2006, “Breaking the Hierarchy of Galaxy Formation”, Monthly Notices of the Royal Astronomical
Society, 370, 645-655
Gultekin, Kayhan et al. 2009, “The M- and M-L Relations in Galactic Bulges, and Determinations of Their Intrinsic
Scatter”, The Astrophysical Journal, 698, 198-221
Hickox, Ryan C. et al. 2009, “Host Galaxies, Clustering, Eddington Ratios, and Evolution of Radio, X-Ray, and Infrared-
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Selected AGNs”, The Astrophysical Journal, 696, 891-919
Hopkins, Philip F. et al. 2008, “A Cosmological Framework for the Co-Evolution of Quasars, Supermassive Black Holes,
and Elliptical Galaxies. I. Galaxy Mergers and Quasar Activity”, The Astrophysical Journal Supplement, 175, 356-389
Kitchin, Chris 2007, Galaxies in Turmoil: The Active and Starburst Galaxies and the Black Holes that Drive Them,
Springel
McNamara, Brian R. & Nulsen, Paul E. J. 2007, “Heating Hot Atmospheres with Active Galactic Nuclei”, Annual
Reviews of Astronomy and Astrophysics, 45, 117-175
Robertson, Brant E. et al. 2010, “Early Star-Forming Galaxies and the Reionization of the Universe”, Nature, 468, 49
Springel, Volker, Frenk, Carlos S. & White, Simon D. M. 2006, “The Large-Scale Structure of the Universe”, Nature,
440, 1137-1144
Internet: [email protected]
(Editor: Although the title was shown on p294 of last month’s paper issue, it was omitted from some
colour PDF files in error for which apologies)
______________________________________________________________________________
AURORAL NOTES
Edited by Tom McEwan
2011 March. All times UT
01-02
Ian Brantingham (Banff) - 01:00, quiet light, green, through cloud. Tom McEwan (Dalry)
23:02, quiet arc through cloud, faint, h 4°, 7°.
06-07
Ian Brantingham (Banff) - 00:30-00:50, faint horizon light.
10-11
Ian Brantingham (Banff) – 21:00-02:00, faint horizon light, 10°, with weak rays at 01:00.
11-12
Ian Brantingham (Banff) – 21:00-02:00, quiet homogeneous arc, faint, 10-15°.
METEOR NOTES
- 22:50-
Edited by Tony Markham
Eta Aquarids 2011
With New Moon occurring on May 3rd, the Eta Aquarid meteor shower is favourably timed in 2011.
Activity starts in late April and continues through to mid May with peak rates occurring during
May 4-5. The radiant is located at RA 22h20m, Dec –01.
The shower arises from the Earth’s post-perihelion encounter with the meteoroid stream of comet
1P/Halley ; the Orionids arising from the pre-perihelion encounter. However, whereas there have
been several occasions during the past 20 years on which enhanced Orionid rates have been
reported, there have been no corresponding reports of enhanced Eta Aquarid activity. This could be
due to less Eta Aquarid observations having been made. With the shower radiant being located near
the celestial equator and the Sun in early May being at a more northerly declination, observers at
northern latitudes can only see Eta Aquarid activity late in the night – and most meteor observers are
located at such latitudes.
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2011 April