1: The opening titles. The images of the solar system were
made by Nigel Metcalfe using a program called Celestia, which
was written by Chris Laurel.
2: Looking back at the disc and centre of the Milky Way.
(Sequence courtesy of Robert Patterson and Stuart Levy of the
National Centre for Supercomputing Applications at the
University of Illinois at Urbana-Champaign, USA)
3: These three images show sce
Cosmic cookery: growin
Carlton M Baugh describes
cosmological modelling – and
how to tell the world about it.
ne of the fundamental questions facing
cosmologists today is “How are galaxies made?”. Ever since astronomers
realized at the start of the last century that the
universe contained many billions of galaxies in
addition to our own Milky Way, the challenge
has been to explain where these vast agglomerations of stars came from. Why do galaxies come
in a range of distinct shapes or morphologies,
ranging from spheroidal elliptical galaxies to
disc-shaped spiral galaxies? Did galaxies form
Understanding how galaxies are made is one of the great unsolved problems in
cosmology. Last year, a team from the Institute for Computational Cosmology at Durham
University took part in the Royal Society’s Summer Science Exhibition, with a remit to
explain the latest research on galaxy formation to the public. The centrepiece of our
exhibit was a stereoscopic movie called Cosmic Cookery. Here I recount how the movie
takes the audience on a 3-D tour of the universe and back to the Big Bang, explaining our
current ideas about how galaxies are built.
in a spectacular flurry of star-forming activity
back in the early universe or is the process of
galaxy formation a more drawn out affair? Why
do we not see galaxies that are many orders of
magnitude brighter and more massive than the
Milky Way? Is there some process that effectively sets an upper limit on their size?
Tremendous advances have been made over
the past 20 years towards developing a theory
of galaxy formation. Through the use of increas-
1: A cosmic A-to-Z
Our view of the local universe has been
revolutionized over the past decade. The
breakthrough was made possible by the
development of multi-object spectrographs, that
could take spectra of hundreds of galaxies at the
same time within a single field of view. This gave
astronomers a tremendous multiplex gain over
the first redshift surveys carried out in the 1970s
(and indeed up to the early 1990s), in which the
redshift of only one galaxy could be measured
for each exposure and pointing of the telescope.
Two surveys have taken advantage of this
technological leap: an Anglo-Australian
collaboration (subsequently including
institutions in the USA), including the ICC,
called the two-degree field galaxy redshift survey
(2dFGRS); and the US-led Sloan Digitial Sky
Survey (SDSS). The 2dFGRS was completed in
2002, mapping the positions of more than
221 000 galaxies, 10 times bigger than the largest
previous complete survey. The SDSS is ongoing
and has now clocked over 600 000 galaxies.
A map as large as the 2dFGRS can be analysed
in several ways. Members of the ICC have led
analyses of the distribution of galaxies to
establish how the clustering signal depends upon
intrinsic properties such as luminosity and
colour, and also to ascertain whether or not
galaxies belong to groups or associations within
a common dark-matter halo. These analyses give
important new constraints on theoretical models
of galaxy formation, such as the efficiency with
which baryons are turned into stars in haloes of
different masses.
The distribution of galaxies on very large scales
can be readily interpreted in terms of the
clustering of the underlying dark matter. Shaun
Cole of the ICC led an investigation by the
2dFGRS team into the large-scale clustering of
galaxies, as quantified by the power spectrum
(Cole et al. 2005). The power spectrum is the
Fourier transform of the simplest measurement
of clustering, the auto correlation function. The
correlation function measures the excess
probability, compared with a random
A&G • April 2006 • Vol. 47
nes of flying through one of the largest maps of the universe, the 2dFGRS. (Images made by Nigel Metcalfe, Nick Holliman and the 2dFGRS team)
g galaxies in a computer
ingly sophisticated computer models, guided by
breathtaking observational breakthroughs, a
convincing paradigm that explains how galaxies
are built is beginning to emerge.
An equally daunting proposition is to explain
these ideas and results to a general audience.
Last year, the Institute for Computational Cosmology (ICC), part of the Department of
Physics at Durham University, made a successful
application to participate in the Summer Science
Exhibition 2005. This event, run by the Royal
Society, showcases exciting research from all
fields of science for members of the public. Summer Science has its roots in the soirees run by
the Society in the late 1800s. The modern event
is much more inclusive and is attended annually
distribution of galaxies, of finding two galaxies
at a given separation. In the gravitational
instability paradigm, the power spectrum grows
in amplitude as the distribution of matter gets
lumpier. On small scales, the shape of the power
spectrum can also change with time. Worse still,
this distortion in shape could be different for
galaxies than for the dark matter. On large
scales, however, these complications do not
occur. Hence, an accurate measurement of the
shape of the power spectrum of galaxies on large
scales gives the shape of the power spectrum of
the dark matter. Moreover, this spectrum can be
related simply to the spectrum of density
fluctuations in the early universe. The shape of
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by around 4000 members of the general public,
school pupils and dignitaries such as Members
of Parliament.
Cosmologist for a day
The ICC display was called “Cosmic Cookery:
Growing Galaxies in a Computer”. Our exhibit
had a number of elements, giving visitors the
chance to become cosmologists for the day and
putting them in control of a computer simulation of a cosmic collision between two spiral
galaxies to make a new galaxy, thereby testing
ideas about how elliptical galaxies are made.
The focus of the exhibit was a stereoscopic
movie which, as we shall see below, started with
a tour of the universe before explaining how we
the spectrum at this epoch depends on basic
cosmological parameters, such as the fraction of
matter that is in the form of baryons and the
density of matter in units of the critical density
(i.e. the density required to give the universe a
flat geometry). In particular, the presence of
baryons in the matter budget of the universe
leaves a characteristic oscillatory signature in the
power spectrum which was evident in the
2dFGRS measurement.
Recently, we combined the 2dFGRS
measurement of the power spectrum of galaxies
with a compilation of the latest measurements of
the temperature anisotropies in the microwave
background radiation (Sanchez et al. 2006).
think galaxies are made. The movie took around
six months to compile, using a combination of
images generated from observational data and
from computer simulations.
Providing the viewer with a stereoscopic or
3-D view of the universe was an essential feature of the movie; the stereoscopic animations
were generated by the the e-Science Research
Laboratory at Durham University, led by Nick
Holliman. Holliman’s group attained 3-D
effects by producing slightly offset images in
linearly polarized light. The images were projected onto a screen and viewed with glasses fitted with filters. These glasses looked as if they
came from the movie Top Gun and were a big
hit with the younger members of the audience.
These two observables depend upon the basic
cosmological parameters in different ways, so
using them in combination can significantly
tighten the constraints on some parameters. The
best fitting cold dark matter model is shown by
the solid line in figure 9. Here we have plotted
the power spectrum of galaxies (blue symbols)
and the spectrum of temperature fluctuations in
the microwave background on the same axes. It
is a remarkable vindication of the cold dark
matter model that these two observations,
measuring the properties of the universe
separated in time by nearly 14 billion years, are
in such spectacular agreement with the
theoretical prediction.
4: The Millennium Simulation. Brighter colours indicate higher densities of dark matter. (Images by Volker Springel, John Helly
and the Virgo Consortium)
The collaboration with the e-Science group is
continuing and is becoming increasingly valuable for research, as well as for making animations for outreach activities. Modern
simulations and astronomical observations produce huge datasets that are difficult to manipulate by traditional methods. The visualization
of these huge datasets literally gives the
researcher a new view of the subject and will be
invaluable in aiding the development of new
analysis techniques and methodologies. The
production of the stereoscopic animations for
the Cosmic Cookery movie also presented new
challenges for Holliman and his team; their
experiences have been written up in a research
paper that was presented at a meeting on
Stereoscopic Displays and Applications in the
US in January this year (Holliman et al. 2006).
Whistle-stop tour of the solar system
Here we take the reader on a scene-by-scene run
through of Cosmic Cookery. We give a brief storyline and state the origin of the images used in
each segment. Certain sequences show results of
research that was either carried out at the ICC
or in which members of the ICC have played a
major role; these results are dealt with in more
detail in the boxes.
The movie starts with an invitation to the
audience to take a flight through the universe in
an “impossibly fast” spaceship. The voyage
begins, appropriately, at the Earth (figure 1).
The camera then moves swiftly through the
solar system, pausing to marvel at the giant red
spot of Jupiter and the rings and moons of Saturn. The solar system sequence was created by
Nigel Metcalfe using software called Celestia,
which was written by Chris Laurel. This software is used by planetariums and can be downloaded free of charge from http://www.shatters
Celestia can be used to produce an animation
by specifying a flight path and outputting a
sequence of still images along the path. However, Celestia does not produce stereo output,
and so left and right viewpoints had to be captured separately and combined to produce a
stereoscopic view of the solar system.
Leaving the Milky Way
A key aim of the opening sequences was to convey to the non-specialist the relative scale of different astronomical objects and to illustrate
what a galaxy is and how they are distributed in
5: The pattern of temperature ri
background radiation.(Image co
Microwave Anisotropy Probe [W
the universe. After leaving the solar system
behind, we soon emerge from the disc of the
Milky Way, glimpsing a spectacular view of our
galaxy as we look back towards the galactic centre (figure 2). We then pass the Magellanic
Clouds, before moving towards the Virgo Cluster with M87 at its centre. The images for this
sequence were provided by Robert Patterson
and Stuart Levy from the National Centre for
Supercomputing Applications at the University
of Illinois at Urbana-Champaign, USA. The
software used to produce the images is a 3-D
visualization tool called Partiview, developed by
Stuart Levy. This software is being used by the
Hayden Planetarium, part of the American
Museum of Natural History, in their Digital
Universe exhibit and also featured in a recent
television documentary.
Our cosmic back yard
We continue our journey out into the local universe, flying through a map of our cosmic
neighbourhood, the two-degree Field Galaxy
Redshift Survey (2dFGRS, see box 1). This survey looks out in two directions in the sky and so
has two parts shaped like pie wedges. We fly
through one wedge, starting at the apex, before
2: The Millennium Simulation: a computer model of the universe
Numerical simulations have been instrumental in
driving many of the advances in theoretical
cosmology over the past 20 years. The cold dark
matter model is particularly amenable to
computer modelling. First, the initial conditions
for the calculation are encoded in the cosmic
microwave background radiation. Secondly, the
dark-matter particles only interact through
gravity; fast algorithms exist for computing the
long-range gravitational forces between
particles. Finally, the dark matter is cold, which
means that the small scale, thermal motions of
the particles can be ignored, unlike in the case of
massive neutrinos, a one-time candidate for the
dark matter, which was referred to as hot dark
As computing power has ramped up according
to Moore’s Law, the number of particles used to
represent the cold dark matter in simulations has
also increased, with the biggest simulation of the
day following a relation similar to the curve
quantifying the increase in CPU speed. Last year,
the Virgo Consortium for Cosmological
Simulations, an international collaboration
between institutions in Germany, the UK
(including the ICC, the UK base of the Virgo
Consortium), Canada, the USA and Japan,
reported on a calculation called the Millennium
Simulation, which was ahead of its time in that it
actually jumped off the simulation version of
Moore’s Law. The first results from the
simulation were published in Nature, with an
image of the dark matter in the simulation
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pples in the cosmic microwave
urtesy of the Wilkinson
MAP] science team and NASA)
6: A high-resolution simulation of the formation of a dark matter halo. (a): A snapshot at an early time in the simulation.
(b): The halo at the present day. (Images courtesy of Adrian Jenkins and Nick Holliman)
panning out to look back at the filamentary
structure traced out by the galaxies, which is
sometimes referred to as the “cosmic web” (figures 3a,b,c). This sequence was produced in
Durham by Nigel Metcalfe and Nick Holliman.
The galaxy images are high-quality snapshots of
a small sample of nearby galaxies, collated from
the archives of various telescopes. An image is
then assigned to a 2dFGRS galaxy on the basis
of its colour, i.e. redder galaxies in the 2dFGRS
are matched to an image of an elliptical galaxy.
The size of the image depends upon the absolute
luminosity of the galaxy.
A computer-generated universe
Now that we have seen the lumpy distribution
of galaxies in the 2dFGRS, we can ask if computer-generated universes look anything like the
real thing. Figures 4a and 4b show stills from a
fly-through of the Millennium Simulation, an
enormous calculation carried out by the Virgo
Consortium for Cosmological Simulations
(described in more detail in box 2). The simulation tracks the growth of structure in the dark
matter, as gravity acts so that tiny ripples in the
density of matter in the early universe grow in
size. The sequence starts with a global view of
appearing on the front cover (Springel et al.
The Millennium Simulation used more than
10 billion particles to model the dark matter in a
volume bigger than that sampled by the 2dFGRS
or SDSS maps. The calculation ran for 28 days
on a 512-processor supercomputer belonging to
the Max Planck Society in Germany, clocking up
343 000 processor hours. The output from the
simulation takes up more than 25 Tb of disc
space. Despite the large volume covered by the
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the simulation at the present day, which shows
that the dark matter traces out a filamentary
pattern bearing a striking resemblance to the
2dFGRS galaxy map. The camera then zooms
into one of the biggest dark-matter haloes in the
Millennium Simulation, which is equivalent in
size to a cluster of galaxies bigger than the Virgo
Cluster. The images for the Millennium Simulation fly-through were generated by Volker
Springel of the Max Planck Institut für Astrophysik at Garching, Germany, and by John
Helly of the ICC.
The origin of cosmic structure
Where did the structures we see in the Millennium Simulation come from? In the current paradigm, the structures have a quantum origin;
fluctuations in the density of the universe on
microscopic quantum scales are thought to
have been boosted to classical scales during a
period of extremely rapid expansion of the universe called inflation. In the Big Bang cosmology, the early universe was much denser and
hotter than it is today. Matter was therefore in
the form of an ionized plasma and the free electrons coupled strongly with the photons. Variations in the strength of the gravitational
calculation, the huge number of particles
employed means that dark-matter haloes as
small as those thought to host the Large and
Small Magellanic Clouds, small neighbours of
the Milky Way, can be resolved in this
simulation. This unprecedented resolution in a
cosmologically representative volume makes the
simulation an ideal test-bed for models of galaxy
formation. The simulation contains around
20 million dark-matter haloes and so can resolve
between 50 and 100 million galaxies. Running
potential due to density fluctuations were
reflected by changes in the temperature of the
radiation. As the universe expanded, the pressure and temperature fell and the plasma
became a neutral gas of atoms leaving few electrons to interact with the radiation. The density
fluctuations present in the universe at this time,
the epoch of recombination, are fossilized in
temperature anisotropies in the cosmic background radiation. The cooling of the radiation
caused by the subsequent expansion of the universe means that this radiation is seen today as
feeble microwaves. The cosmic microwave
background radiation is dappled with hot and
cold spots, which reflect the fluctuations in the
density of matter in the universe around
400 000 years after the Big Bang (figure 5).
The cradles of galaxy formation
The pattern of hot and cold spots seen in the
microwave background provides the initial
conditions for computer simulations of structure formation in the universe. The next
sequence shows an ultra-high-resolution simulation concentrating on the formation of a single dark-matter halo. At an early time, the
matter that ends up in the halo is split into less
models of galaxy formation on such a scale
presents a new set of computational challenges.
Nevertheless, the first calculations of the
properties of galaxies expected in the
Millennium Simulation universe have recently
appeared (Croton et al. 2006, de Lucia et al.
2006, Bower et al. 2006). This new generation of
galaxy-formation models suggests that the
growth of supermassive black holes found in
galactic spheroids is intimately linked to the
growth of the galaxy hosting the black hole.
7: A synthetic galaxy generated using a computer model that tracks the
evolution of the baryons and the dark matter. (Image by Takashi Okamoto,
Richard Bower, Adrian Jenkins, Nigel Metcalfe and Nick Holliman)
massive progenitors, as shown in figure 6a, the
first still. The distribution of dark matter is
quite stringy and filamentary. The force of gravity, acting over billions of years, brings these
fragments together and the halo builds up as
smaller haloes merge together. The second still,
figure 6b, shows the halo at the present day.
Yellow indicates a higher density of matter. The
final structure is more spherical. A few highdensity lumps can be seen within the larger
halo. These are called substructures and correspond to the cores of the ancestors of the final
halo. The more diffuse outer regions of these
progenitor haloes have been stripped off during
the merger process. Only the high-density central regions of the progenitors retain their identity after merging with a more massive
8: A real galaxy and a computer model side by side. Can you tell which one
is which? (Image by Takashi Okamoto, Richard Bower, Adrian Jenkins,
Nigel Metcalfe and Nick Holliman)
structure. Cosmologists believe that galaxies
grow within dark-matter haloes.
Growing a galaxy in a computer
Next we rewind the computer simulation back
to the initial conditions and repeat the calculation again, but this time we include baryons as
well as dark matter. This new calculation is
much more difficult to carry out. In addition to
the force of gravity, the baryons are subject to
other forces and phenomena such as star formation that in many cases are poorly understood (see box 3 for more details). One of the
long-standing problems in numerical galaxy
formation has been to produce discs that resemble observed spiral galaxies. It turns out, as
explained in box 3, that the morphology of a
galaxy depends sensitively on the construction
history of the dark matter halo and the way in
which the astrophysics of galaxy formation are
modelled. A model disc galaxy like that shown
in figure 7 took a lot of tinkering with the recipe
to get the mix of ingredients just right.
Is the computed galaxy realistic?
The final sequence of Cosmic Cookery starts by
comparing the galaxy grown in the computer
with a real galaxy (figure 8). The real image has
been processed in the same way as the computer
galaxy, and shows stars and cold gas. The simulated galaxy disc has a very similar size and
structure to the real one. This gives theoreticians
some encouragement that they are on the right
path to understanding how galaxies are made.
3: Growing a galaxy in a computer
The focus in computational cosmology is shifting
firmly towards understanding the formation of
galaxies. This is a much more challenging
problem than following the growth of structure
in the dark matter, because a much wider range
of phenomena have to be considered. Quite
often, these processes are nonlinear and the
underlying physics is poorly understood. The
spatial resolution required to follow the
formation of galaxies is more demanding than
that needed for the dark matter alone; typically,
the scale-size of a galaxy is 10 times smaller than
the radius of its dark-matter halo.
The current paradigm for galaxy formation is
that dark-matter haloes act as the cradles in
which galaxies grow. The dark haloes are built up
as density fluctuations grow through
gravitational instability, with overdense regions
accreting matter. The baryons initially trace the
distribution of the dark matter and get heated up
when the dark matter collapses to form a self-
gravitating lump or halo. Unlike the dark matter,
the hot baryons can lose energy by radiation. The
baryons cool down when they lose energy on
radiating away photons, and the corresponding
drop in pressure causes the baryons to shrink
towards the central parts of the halo, forming a
disc that retains a non-zero size because it is
rotating. The exact size of the cold gas disc that
results depends upon the conservation of the
angular momentum of the gas, as we shall see.
Once there is a supply of cold gas, the gas is
turned into stars. In the absence of a theory for
star formation, modellers resort to using a recipe
or rule to describe the rate at which cold gas is
transformed into stars. Such a recipe contains
parameters whose values are set so that the
model reproduces a particular observational
result. Once stars have formed, those above eight
solar masses undergo a supernova explosion,
depositing energy and metals in the interstellar
medium. This can have an impact on the
subsequent rate of star formation, if the energy
from the supernova heats up some of the cold
gas and moves it from the disc.
The class of models developed to follow the
processes thought to be important for galaxy
formation are called semi-analytical galaxy
formation models. These are ab initio calculations
that start from primordial density fluctuations
and follow the growth of dark-matter haloes, the
heating and cooling of gas, star formation and
events such as supernova explosions, which are
called feedback processes. In addition, the models
can trace mergers between galaxies and track the
stellar populations of the model galaxies to
generate synthetic spectra.
One particularly challenging problem that the
models have faced recently was to explain the
discovery of a population of galaxies at high
redshift seen only by emission from the dust they
contain, which is detected at sub-millimetre
wavelengths (Smail et al. 1997, Blain et al. 2002).
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Preparing the Cosmic Cookery exhibit took
around six months, involving staff from the
ICC, the Durham e-Science Research Laboratory and the Electrical and Mechanical Workshops of the Department of Physics at Durham.
Phil Brown of Media 55 Ltd carried out the production of the movie, with Nick Holliman,
delivering the final copy the weekend that we
left for London! Many organizations contributed directly and indirectly towards the cost
of mounting the exhibit, including PPARC, the
Royal Society, the de Laszlo Foundation, Inition
Ltd, the RAS and the National Centre for Supercomputing Applications in the USA. After a hectic day setting up the Cosmic Cookery stand,
under the command of the ever-resourceful and
indispensable Lydia Heck, we were ready for
the public. At this point the demonstrators, a
mixture of postgraduates and staff, actually
began to enjoy themselves. They found that
explaining their research work to an enthusiastic and genuinely interested public was a
rewarding experience.
The Cosmic Cookery exhibit is on permanent
display in the Ogden Centre for Fundamental
Physics at Durham. It is used on university open
days and in the annual Durham University Science Festival. The Odgen Centre (which comprises the ICC and the Institute for Particle
Physics Phenomenology) has a vigorous outreach programme, part-funded by PPARC and
run by Dr Pete Edwards. As part of this work,
there are numerous visits to the Ogden Centre
each year by school parties, who get to see Cosmic Cookery. To date, the programme has
reached more than 6000 school children of all
ages in the Northeast. Elements of the Cosmic
An exhaustive search through the parameter
space of the models did not yield a solution that
could explain both the galaxies seen at high
redshift and the properties of galaxies in the local
universe. In an act of desperation, we tried a
model in which the initial mass function (IMF)
with which stars are produced in bursts is flat.
This means that the same proportion of stars of
all masses are made in bursts, rather than the case
with quiescent or steady star formation in discs,
for which we use a standard choice of IMF, which
is biased in favour of low-mass stars. These
bursts of star formation are triggered in the
models by certain types of mergers between
galaxies. By favouring high-mass stars in bursts,
more metals and more ultraviolet light are
generated. Shorter wavelengths are better
absorbed by dust, so more energy goes into
heating the dust. However, the higher yield of
metals means there is also more dust. These two
effects combined boost the flux from the model
A&G • April 2006 • Vol. 47
9: The
fluctuations in
the cosmic
as measured
by WMAP,
plotted in the
same units as
the power
spectrum of
galaxies in the
2dFGRS. The
solid line
shows the
cold dark
matter model.
(Image by Ariel
P(k)/(h–1 Mpc)3
What next for Cosmic Cookery?
k/(h Mpc–1)
Cookery movie will be seen this year by around
10 000 school children across the UK, as Dr
Edwards is delivering the Institute of Physics
Schools and Colleges Lecture “Gravity, Gas and
Stardust” in 2006. We also hope to take the
Cosmic Cookery display to different venues
across the Northeast.
Initiatives like the Ogden Centre outreach
programme and Cosmic Cookery should prove
to be invaluable resources in efforts to reverse
the trend of declining numbers of pupils taking
A-level physics. ●
galaxy at submillimetre wavelengths. This
controversial change to the IMF resulted in a
model that could reproduce the distribution of
galaxies in the universe at both high and low
redshifts (Baugh et al. 2005).
Support for a non-universal IMF also came
from a numerical simulation of the formation of
an individual galaxy (Okamoto et al. 2005). A
long standing problem in numerical simulations
has been to form a disc-like galaxy that has the
same size as observed spiral galaxies. The
simulations tend to make galaxies that are an
order of magnitude too small and look more like
ellipticals than spirals. Part of the problem is that
gas tends to cool early on in the simulations,
forming small clumps that merge together to
make larger galaxies. During the merging
process, the clumps of cold gas and stars tend to
give up their angular momentum, transferring
the spin to the outer parts of the dark halo. Thus
the resulting disc contains too little angular
C M Baugh, Institute for Computational Cosmology,
Dept of Physics, Durham University, UK.
Baugh C M et al. 2005 MNRAS 356 1191–1200.
Blain A W et al. 2002 Physics Reports 369 111–176.
Bower R G et al. 2006 astro-ph/0511338.
Cole S et al. 2005 MNRAS 362 505–534.
Croton D et al. 2006 MNRAS 365 11–28.
de Lucia G et al. 2005 astro-ph/0509725.
Holliman N et al. 2006 SPIE 6055A in press.
Okamoto T et al. 2005 MNRAS 363 1299–1314.
Sanchez A G et al. 2006 MNRAS in press.
Smail I et al. 1997 ApJ 490 L5.
Springel V et al. 2005 Nature 435 629–636.
momentum and is much smaller than real spiral
discs. Okamoto et al. found that if they also
adopted a top-heavy stellar IMF in starbursts,
the higher number of supernova explosions that
result from forming more high-mass stars means
that much less gas cools down in small clumps at
early times. The baryons tend to stay in the hot
phase and cool at a later stage in the evolution of
the galaxy. By cooling later on, more of the
angular momentum of the gas is retained and a
beautiful disc galaxy results (see figures 7 and 8).
There is one note of caution. A spiral galaxy is
rather like a soufflé: the astrophysical conditions
have to be just right or this experiment in cosmic
cookery turns out all wrong. The morphology of
the galaxy is sensitive to the way in which the
star formation and the associated feedback from
supernova explosions are modelled.
Nevertheless, this calculation represents an
encouraging step towards understanding how
galaxies like our Milky Way were made.