Download 2.3 Peculiar galaxies

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Figure 21: Examples of colliding galaxies.
Figure 22: The Milky Way and the Magellanic stream.
2.3 Peculiar galaxies
Lecture 4 : Cosmic Perspective 21.2, 21.3
In this section, we will talk about unnusual, but nevertheless very important objects : colliding galaxies, starburst galaxies, and active galactic nuclei.
Colliding galaxies. Earlier we stated that irregular galaxies are generally small objects that are
truly unstructured. However, many galaxies originally classified as “irregular” turned out to be large
galaxies in the process of colliding and possibly even merging. Unlike stars, galaxies naturally get
close together, so interaction is likely. In Fig. 21 you can see how “tails” of material are pulled
towards neighbouring galaxies. Over time, some galaxies can orbit around each other repeatedly and
gradually merge. If by chance one galaxy heads straight towards the middle of another, you can get
beautiful structures known as ring galaxies. (You will see more pictures in the lectures)
Mergers in the Milky Way . In an earlier section, we mentioned that the Magellanic clouds are
satellites of the Milky Way. The truth is a little more dramatic. Fig. 22 shows an all-sky picture
in which white-blue represents the distribution of stars, and pink traces the distribution of hydrogen
gas. You can see that the Magellanic clouds are actually being torn apart by the Milky Way, leaving
a stream of material. Over time the Magellanic clouds will be completely swallowed by the Milky
Way. Recent stellar surveys show several streams of stars in the Milky Way, which are the remains of
dwarf galaxies swallowed in the past.
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Figure 23: Three stages of star formation. In the Orion nebula (left) the stars are still completely buried. In
the Eagle nebula (middle) they are just starting to burn out. In the Pleiades (right) there is just a little gas and
dust left hanging around the new star cluster.
What Happens in a collision ? Individual stars almost never collide. The typical separation between
stars is many orders of magnitude larger than the size of a star, so a chance collision is extremely
unlikely. By contrast, the typical separation between galaxies (Mpc) is only about 20 times bigger
than the diameter of a large galaxy, so every so often they get quite close to each other. But galaxies
are made of stars, which as we have seen above, essentially never collide with each other. So when
galaxies collide don’t they just slide straight through each other ? Actually the effects of a collision are
quite dramatic. This is because what is going on is not a simple physical collision, but a gravitational
interaction.
First, think about a single star inside an isolated galaxy. It feels the summed-up gravitational force
due to the mass of all the other stars in the galaxy, and this force determines the motion of the star. In
other words, each star responds to the galaxy overall. Because isolated galaxies are nice symmetrical
things, that net force is usually towards the centre of the galaxy, and the star does a neat orbit around
the centre. Now imagine bringing another galaxy close. Our single star can then feel a force due to
both galaxies. The net result is quite complicated, and whats more keeps changing with time, as the
galaxies move closer. Rather than moving in a nice simple orbit, the stars do quite complex things.
Its actually quite hard to predict the detailed behaviour with pencil and paper theory, so astronomers
resort to simulating the behaviour on large computers. Sometimes these get made into very attractive
movies. Some beatiful examples are on John Dubinski’s galaxy dynamics web page.
What happens depends on the distance of closest approach (the impact parameter). If galaxies just
skirt by each other they will perhaps pull out tidal tails from each other and then carry on. If they head
close towards each other, the material is likely to orbit around in a complex manner until eventually
they completely merge.
Star formation in galaxies. We now move to an apparently separate subject which actually will
link up later... star formation. In the Milky Way and similar spirals, new stars are being formed at
an average rate of about one solar mass per year. Those new stars are formed with a large range of
masses. The massive stars are hot and blue and very luminous, but last a relatively short time - the
most massive ones only last for a few million years. Low mass stars are cooler and redder and much
less luminous, and last a long time - billions of years.
In elliptical galaxies, it seems that all the stars formed a long time ago, and there is no gas to make
new stars. The massive hot blue stars have all gone, and so the galaxy looks red.
In spirals, the disc has lots of gas, and new stars keep forming, so there are plenty of luminous hot
stars around, and the galaxy looks blue.
Hidden star formation. In the Milky Way, new stars are formed inside gas and dust clouds, like the
Orion nebula. At first the new stars are completely hidden. Their radiation heats up the dust cloud,
which then shines in the infrared. After a while, the stars start to “burn out” of their parent clouds, as
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Figure 25: The radiated energy distribution of a typical AGN over a large range of wavelengths.
seen in the famous HST Eagle nebula picture. In the Pleiades, the parent cloud is almost gone, but
there is still some gas and dust hanging around. So... the energy from the very newest star formation
comes out in the infrared; from young regions, in blue light; and from old regions in red light.
Starburst galaxies. In a typical spiral galaxy, the rate of forming new stars is about one solar mass
per year. Ninety percent of the luminosity comes out in visible light, and ten percent in IR, partly
from those buried new stars, and partly from general heating of the interstellar medium. In rare
objects called starburst galaxies stars are being made at a rate of hundreds of solar masses per year.
They are a hundred times more luminous than normal galaxies - however, you wouldn’t spot this
from a normal visible light picture, because most of that energy comes out in the IR. Visually, these
objects often look like late-stage mergers, so somehow a galaxy collision has triggered the massive
burst of star formation.
[They are called starbursts because this phenomenon can’t last long; at a rate of hundreds of stars per year, it
will use up all the available gas in much less than the lifetime of the galaxy ]
Galactic Winds.
In starbursts, most of those short-lived new massive stars end
up as supernovae.
The outflows from all those supernovae
merge into a bubble of hot gas, which expands outwards, driving a wind. Fig. 24 shows the starburst galaxy M82. The
whitish light is the galaxy starlight; the red light shows disturbed gas being blown out of the galaxy. These galactic outflows are sometimes also seen in X-ray images, as the bubble of gas is very hot. Some astronomers argue that this process is how ellipticals are made. Two spirals collide, a starburst ensues, then the remaining gas is blown out, and a gasless elliptical is left behind. We will return to this issue later.
Figure 24: The starburst galaxy
M82.
Active Galactic Nuclei (AGN). A small fraction of galaxies show
a bright starlike nucleus, with strange properties which we will discuss below. This “activity” shows
itself fairly obviously in about 1% of local galaxies, known as Seyfert galaxies after Karl Seyfert who
first studied them in the 1940s. However, we now know that at a much lower level, something similar
is going on in perhaps ⇠ 20% of galaxies. At the other extreme, some of the rarest examples have
active nuclei which are so powerful that they can completely outshine their host galaxy. For distant
objects, where the host galaxy appears small on the sky, that host can be hard to see underneath the
powerful nucleus, and so what we see just looks like a point of light. This is known as a quasar.
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Figure 26: The visible light spectrum of a typical AGN, showing broad emission lines from fast moving gas.
[Historically, quasars caused much confusion. The name is short for “quasi-stellar radio source” because the
first objects found were radio sources but just looked like stars. Cutting a lot of history short, we now know
that quasars are just very luminous distant examples of AGN. However, the fact that there are so many of them
is of great interest, as we will see shortly.]
AGN properties : power. AGN, and especially quasars, can radiate a huge luminosity - up to a 100
times more than all the starlight from a normal galaxy. That power is radiated over a wide range of
wavelengths from radio waves to X-rays (see Fig. 25, which is very different from normal galaxies
which radiate most of their energy in visible light, with some IR. The brightest part of AGN radiation
however comes in the ultraviolet, indicating that the main power source has a temperature of around
100,00K.
AGN properties : fast gas. If we look at the visible and UV light spectrum of an AGN in more
detail, we see that as well as continuous light, we see bright emission lines at particular wavelengths.
(See Fig. 26.) This shows the presence of ionised gas. We also such emission lines in star formation
regions in the Milky Way - the hot stars ionise surrounding gas. In the AGN case, the bright UV
source must be ionising local gas. However, in AGN, unlike in star formation regions, the emission
lines are very broad. This is due to our old friend the Doppler effect - the gas must be moving around
as well as being ionised. If we measure the width
of a line at wavelength we can calculate the
typical gas velocity from v = c ·
/ . The result is that typical AGN gas is moving at ⇠ 10,000 km
s 1 - several percent of the speed of light !!
AGN properties : variability. The radiation from AGN changes on short timescales - in some
objects, days or even less. An object cannot change by a substantial amount faster than it can communicate across itself, which happens at a speed no faster than the speed of light. In other words,
from an objects of size R we shouldn’t see variations faster than
tmin = R/c
So t ⇠ 1 day means that R < 2.6 ⇥ 1010 km. You can compare this to the distance to Pluto, which
is ⇠ 1010 km. We find therefore that a typical AGN must be similar in size to the solar system - or
even smaller - despite being more powerful than a whole galaxy.
AGN properties : radio lobes. About 10% of galaxies containing an AGN also have giant radio
lobes - large regions of radio emitting plasma, a huge distance either side of the galaxy. Such objects
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Figure 27: Radio galaxies. On the left is a map of the radio emission from the source Cygnus A showing large
lobes separated by hundreds of kpc, with a bright central core, and thin jet-like features. The box indicates
where the host galaxy is. In the middle is the visible light image of the host galaxy for a similar radio source,
M87. On the right is the central region of M87, showing a jet-like feature continuing right down into the core.
are known as radio galaxies. Radio emission is caused by extremely high energy electrons spiralling
in magnetic fields, a process known as synchrotron radiation. Some very energetic process has to
cause those electrons to be accelerated up to such high energies. The nucleus is often linked to the
lobes by thin streaks of emission called jets, terminating in hot spots. (See Fig. 27). This strongly
suggests an outward ejection - so although the AGN phenomenon has its origin in a tiny spot in the
centre of the galaxy, it can have an influence on huge regions much bigger than the galaxy itself.
AGN properties : nuclear jets. Measurements in the central regions of radio galaxies show that such
jets can continue to be seen down to very tiny scales. Repeated measurements on the very smallest
scales show that structure within those jets is indeed moving outwards, and that the measured velocity
is close to the speed of light.
Black holes and gravity power. So somehow we need to do the following :
• generate huge amounts of power
• do this in a tiny region
• eject high-speed jets
The only way we know how to do this is to have a supermassive black hole at the centre of the galaxy,
and to have material fall onto it. Matter falling onto any gravitating object will gain energy, but a
black hole is so compact it can generate much more energy per unit accreted mass than anything else.
The energy you can gain from a lump of matter m starting far away and then falling onto a mass
M and radius R is
GM m
E=
R
So a for a given M the trick is to make R as small as possible. The smallest possible size is that of a
black hole, the size of which can be taken as the radius of the event horizon or Schwarzschild radius,
which is given by RSchw = 2GM/c2 . (We won’t prove that in this course !) Nothing can escape from
the event horizon, including radiation. So even if our mass M were to shrink below this size, any
energy gained by matter falling past this point won’t get out. So the absolute maximum amount of
energy our mass m could liberate would be given by formula above if we substitute R = 2GM/c2
E = GM m ·
c2
1
=
mc2
2GM
2
So it looks like we could extract half the Einstein rest mass energy of
practice ?
m. Can we get that much in
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Figure 28: Evidence for nearby supermassive black holes. The left hand picture shows HST data from M87
showing the the H↵ emission line from hydrogen gas. At locations either side of the nucleus the emission is
displaced in wavelength, indicating rotation. The right hand picture shows the orbits of stars in the nucleus of
the Milky Way, measured by groups in Germany and California. An animated version can be seen here
Black hole accretion discs. If the available gas simply fell radially downwards towards the black
hole, the energy it would gain would be kinetic energy, and it wouldn’t give much radiation; it would
just disappear down the black hole. However, if, as is very likely, the gas is rotating around the black
hole, it will actually spiral in slowly. This is called an accretion disc.
The energy is gained more gradually. As each layer of the disc slips downwards, half of the gained
gravitational energy goes towards speeding up the rotation; the other half goes into heating up the
gas, so finally we get radiation. The temperature we expect is about 105 K, just right for explaining
the ultraviolet radiation we see. Detailed calculations show that at three times the event horizon, gas
orbits become unstable and after that the gas plunges into the black hole. So the true inner radius for
generating energy is ⇠ 6GM/c2 , and the maximum output we can get is ⇠ mc2 /12.
The rotation can also cause twisted magnetic field lines. Charged particles then get flung out along
these field lines to form the jet.
The broad picture of accretion onto black holes explaining AGN seems quite convincing, but the
details are still controversial - this is a very active area of research (pardon the pun).
Evidence for massive black holes. Are there really massive dark objects at the centres of galaxies?
Most nearby galaxies that seem to be “passive” i.e. not showing obvious current signs of activity,
nonetheless show stellar motions that seem to be faster than you can explain with those stars alone,
i.e. there are signs of a dark central mass in essentially all galaxies. There is particularly strong
evidence in two very nearby galactic nuclei - M87 in the Virgo cluster, and in the centre of our very
own Milky Way.
In M87 we can see gas that on one side of the nucleus is receding from us, and on the other side is
approaching us, indicating rotation at a speed of ⇠ 800 km s 1 . Because we know the distance, we
can calculate the central mass from Newton’s laws, just like we use the orbital velocity of the Earth
to calculate the mass of the Sun. The result is a mass of ⇠200 billion solar masses. However, the
starlight we can see in this central region represents much less than this - so its not the stars causing
this - its a dark mass.
At the centre of our own Galaxy, there are some luminous stars which we can spot individually. Over
a period of years these stars move, so we can track their orbits. These orbits indicate an unseen object
with a mass of about two million solar masses. The size of this object must be well inside all those
stellar orbits, which makes it quite small, and hard to be anything other than a black hole.
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Evolution of AGN. Luminous quasars are seen at very large distances, with large redshifts. As we
shall discuss more carefully later, objects at large distances are seen as they were in the past. The
number of quasars we see at such distances, compared to the AGN we see locally, indicates that such
luminous AGN we much more common in the past. This is a topic we will return to in a later section.