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Figure 8: Two typical galaxies. Both of these are quite like the Milky Way, but one happens to be oriented
face-on towards us and one edge-on. These pictures were taken by the amateur astronomer Russel Croman
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A universe of galaxies
In this section, we look at external galaxies and their properties.
2.1 The galaxy zoo
Lecture 2 : Cosmic Perspective 20.1
Island Universes. As we look around the sky, as well as stars, we see fuzzy patches known historically as “Nebulae” (latin for clouds). Cutting a lot of history short, we now know that some of
these fuzzy patches are indeed clouds of gas, and are inside the Milky Way; but others are “island
universes” - great collections of stars, like our own Milky Way Galaxy, but completely outside it.
Three of these are visible to the naked eye - the Andromeda Nebula, also known as M31, and the
Large and Small Magellanic Clouds. We now know that M31 is the nearest large galaxy to the Milky
Way, whereas the Magellanic clouds are dwarf galaxies which are satellites of the Milky Way. With
a small telescope it is easy to see many more such galaxies; with the sort of telescope an amateur
astronomer might have, you can see several thousand such galaxies around the sky. Two examples
are shown in Fig. 8.
Galaxies as far as the eye can see. With larger telescopes, we can see galaxies that are many times
fainter in every patch of sky that we look at. The deeper we look the more we see. The fainter galaxies
are mostly just further away. For example, the galaxies we see in the Hercules cluster (see lectures)
are about 105 times fainter than M31, and so are about 300 times time further away. (Apparent
brightness goes as the inverse square of distance...) Some galaxies however are simply intrinsically
less luminous than others, so they appear fainter at the same distance. In surveys covering the whole
sky, about a hundred million galaxies have been catalogued; in smaller patches of sky there are even
more galaxies, so many that we don’t bother cataloguing them.
The deepest picture ever taken is the Hubble Deep Field, made up of 10 days of exposures with the
Hubble Space Telescope. It covers only a tiny patch of sky - 200 times smaller than the full moon.
It shows many very very faint galaxies - 105 times fainter again than those in the Hercules cluster.
As we shall see later, the light from these galaxies left so long ago that they give us a glimpse of
galaxies soon after they must have formed - so galaxies much further away simply couldn’t be seen,
because there has not been time for the light from them to reach us. We are looking at the edge of the
observable universe !!
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Figure 9: Two contrasting edge-on spirals. M104 has a large bulge; NGC 5907 has a very small bulge; it
is almost pure disc. The pictures come respectively from the Anglo-Australian Observatory, and an amataur
astronomy called Ole Nielsen.
So how many galaxies are there? To answer that, we would need to know whether the Universe
is finite or infinite, which we will discuss later. But we can say roughly how many there in the
observable universe, by scaling up from the Hubble Deep Field. In 2 ⇥ 2 arcminutes there ⇠3000
galaxies. By extrapolating to the whole sky, we estimate that the total number of galaxies in the
observable universe is very roughly 1011 - a hundred billion. Coincidentally, this is about how many
stars there are in the Milky Way.
Types of galaxy. Galaxies can be divided into three main types.
• Spiral galaxies, such as the Milky Way, are disc like, with spiral arms within the disc, and with
a bulge at their centres
• Elliptical galaxies are smooth spheroidal clusters of stars
• Irregular galaxies are neither disc-like nor spheroidal, but of a more amorphous shape
More about spirals. Typical spiral galaxies are shown in Fig. 8. You can see that the discs of spiral
galaxies contain gas and dust particles as well as stars. The dust particles block light, producing a
patchy effect, just like we see in the Milky Way. When face-on, the spiral arms can be seen clearly.
When edge-on, the dust lanes and the central bulge become clearer. However, the relative size of the
bulge compared to the disc varies considerably - see Fig. 9. Note also that spiral galaxies are usually
blue-ish; this is because they are currently forming many new stars, which tend to be hot and so blue.
Intermediate between spirals and ellipticals we have “lenticular” galaxies. Although disc-like, and
with a bulge, they have no spiral arms and little gas.
More about Ellipticals. Elliptical galaxies are smooth and spheroidal, with no sign of disc or spiral
arms or gas or dust. Some are more or less spherical, and some are ellipsoidal with all three axes
different - somewhere in between doughnut shaped and cigar-shaped. They vary greatly in size, as
can be seen in Fig. 10. They are usually reddish, and seem to contain no newly formed stars, only
very old ones.
More about Irregulars. Some galaxies have no symmetrical structure - no disc or spiral structure
and no spheroidal bulge or nice elliptical shape. They often have new (blue) stars, and gas and dust,
but occasionally seem to be just large star clusters, like the Sagittarius dwarf shown in Fig. 11.
The Hubble Tuning Fork. The main galaxy types can be arranged in a sequence, as shown in Fig.
12. For spirals, the bulge/disc ratio and the spiral arm tightness correlate, making a sequence, labelled
Sa!Sb!Sc. There is a parallel sequence of barred spirals. Eliptical galaxies are placed in order of
ellipticity, from round (E0) to the most elongated (E9). Lenticular galaxies are in between.
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Figure 10: Two contrasting ellipticals. The one on the left is a giant elliptical, with ⇠ 1013 stars. The one on
the right is a dwarf elliptical, with only ⇠ten million stars.
Figure 11: Two irregular galaxies, both taken from the Hubble Space Telescope archive. NGC 1427A is has
lots of gas, dust, and new blue stars. The Sagittarius dwarf, which is a satellite of the Milky Way, seems to be
just a loose collection of old stars.
Figure 12: The Hubble Tuning Fork diagram.
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Figure 13: The large scale distribution of galaxies. Both pictures are basically slices taken through a three
dimensional map made from galaxy surveys. The left hand plot is from the CfA redshift survey, and covers a
distance range of about 600 Mly. It clearly shows the honeycomb-like structure that galaxies follow. This image
is known as the “CfA stick man”. The right hand plot is from the 2dF redshift survey, led by John Peacock
at ROE, and carried out with the Anglo-Australian telescope. It covers a distance range of about 4 Gly. The
honeycomb structure is repeated many times, but on the largest scales the pattern is smoother on average.
The distribution of galaxies. Galaxies are not smoothly distributed. They clump together. Lets take
a tour, moving out from the Milky Way. Pictures to go with this tour will be seen in the lectures.
The Milky Way is part of a small group of galaxies known as the Local Group, a few Mpc across. It
contains two big galaxies (the Milky Way and M31), two middle sized galaxies (M33 and the LMC)
and a large number of dwarf galaxies - about 50 known at the last count. Most of these dwarfs are so
small and feeble, that if similar dwarfs were to exist much further away than the Local Group, they
would be too faint to be seen. So the working assumption is that there are indeed very many such
dwarfs. Some of these small galaxies are satellites of the bigger ones - the small galaxies are actually
in orbit around the big ones.
As we move outwards, we see other such small groups of galaxies - for example the M81 group and
the Leo triplet (see lecture slides). Then as we move even further out, over distances of tens of Mpc,
we find every so often there is a large cluster of galaxies containing thousands of galaxies. In some
cases the cluster is symmetrical, and concentrated towards the middle. In other cases, the clustering
is relatively loose and irregular. The space between the galaxies in these clusters is not empty, but
seems to be smoothly filled with low density but very hot gas. How do we know this ? Because we
see this hot gas via its X-ray emission. The spectral shape of the X-ray emission allows us to measure
its temperature, which turns out to be T ⇠ 108 K. - a hundred million degrees. At this temperature,
gas will be highly ionised : protons and electrons move separately.
As we move out towards even larger scales, over hundreds of Mpc the galaxy distribution takes on a
kind of frothy or honeycomb-like appearance, characterised by filaments and voids - relatively empty
regions. This becomes clearer when we make a 3D map, as shown in Fig. 13. (Estimating the distance
of galaxies is explained in the next lecture.) Explaining the existence of the honeycomb structure, as
well as the occurrence of rich clusters, is a major challenge for cosmological theories.