Other galaxies
8.2 Other galaxies
Galaxies, called originally ‘white nebulae’ have been observed for hundreds of years, but it was not until the early part of the last century that the debate as to whether they were within or beyond our Galaxy was settled – essentially when observations of Cepheid variables enabled their distances to be measured. They are, of course, objects outside our Galaxy and can now be observed throughout the universe. Galaxies can be divided into a number of types and then subdivided further to produce a classifi cation scheme fi rst devised by Edwin Hubble. As more and more galaxies were discovered, it became apparent that galaxies form groups (up to about ∼100 galaxies) or clusters (containing hundreds to thousands
of galaxies).
8.2.1 Elliptical galaxies
These, as their name implies, have an ellipsoidal form rather like a rugby ball (Figure 8.12). They range from those that are virtually circular in observed shape, called E0 by Hubble, to those, called E7, which are highly elongated (Figure 8.13).
Introduction to Astronomy and Cosmology
Figure 8.12 The giant elliptical galaxy, ESO 325-G004, in the Abell cluster S0740. Image: J. Blakeslee (Washington State University), NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
Figure 8.13 Classifi cation of elliptical galaxies.
This is a purely observational classifi cation and does not necessarily tell us how elongated a galaxy really is – if seen end on even a highly elongated galaxy or rugby ball will look circular! At the heart of large galaxy clusters one or more giant ellip- tical galaxies are often observed. They may contain up to 10 million million solar
Galaxies and the Large Scale Structure of the Universe
masses in a volume some nine times that of our own Galaxy ∼300 000 light years across. They are probably the result of the merger of many smaller galaxies. These are the most massive of all galaxies but are comparatively rare. Far more com- mon are elliptical galaxies containing perhaps a few million solar masses within
a volume a few thousand light years across. One interesting fact is that ellipsoidal galaxies do not appear to have any young stars within them so star formation appears to have ceased – all the gas having been used up to form stars in the past. Photographs of M31, the Andromeda Nebula, show its two companion elliptical galaxies. The nearer, more spherical in appearance, is of Type E2, whilst the more distant is of Type E5. Elliptical galaxies account for about one-third of all galaxies in the universe.
8.2.2 Spiral galaxies
Like our own Galaxy, these have a fl attened spiral structure. The fi rst observation of the spiral arms in a galaxy was made by the Third Earl of Rosse. During the 1840s he designed and had built the mirrors, tube and mountings for a 72 in. refl ecting telescope which for three-quarters of a century was the largest optical telescope in the world. With this instrument, situated at Birr Castle in Ireland, Lord Rosse made some beautiful drawings of astronomical objects. Perhaps the most notable was that, shown in Figure 8.14, of an object which was the 51st to
Figure 8.14 M51, the Whirlpool Galaxy, as drawn by the Third Earl of Rosse using the 72 in.
Introduction to Astronomy and Cosmology
Figure 8.15 Hubble Space Telescope image of M51. Image: NASA, ESA, S. Beckwith (STScI), and The Hubble Heritage Team (STScI/AURA).
be listed in Messier’s catalogue and known as the ‘Whirlpool Galaxy’. It was the fi rst drawing to show the spiral arms of a galaxy and bears excellent comparison with modern day photographic images (Figure 8.15). (The galaxy is interacting with a second galaxy, NGC 5195; shown in the lower left of Figure 8.14.)
Spiral galaxies make up the majority of the brighter galaxies. Hubble classifi ed them fi rst into four types: S0, Sa, Sb and Sc. S0 galaxies, often called ‘lenticular’ galaxies, have a very large nucleus with hardly visible, very tightly wound, spi- ral arms. As one moves towards Type Sc, the nucleus becomes relatively smaller and the arms more open. In many galaxies the spiral arms appear to extend from either end of a central bar. These are called ‘barred spirals’ and are denoted SBa, SBb and SBc. Our own Milky Way Galaxy was thought to be a Sb or Sc galaxy but there is now some evidence that it has a bar making it an SBb or SBc galaxy. Type S0 galaxies, like elliptical galaxies, do not appear to have star form- ing regions, but as one moves towards the Type Sc or SBc, which contain more gas, star forming regions and the resulting young O and B type stars are seen in abundance.
Colour images show a signifi cant colour contrast between the central core and the spiral arms. The core has a yellow-orange tint, whilst the spiral arms are blu- ish. The colour of the core indicates that the stars there are old, as yellow, orange and red stars live for long periods of time. In contrast, the spiral arms contain
Galaxies and the Large Scale Structure of the Universe
Figure 8.16 M81 in Ursa Major. Image: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).
many young blue stars. Though in percentage terms they are relatively rare they outshine stars like our Sun by thousands of times so their light tends to dominate the colour of the spiral arms. Though the centre of the Galaxy M81, shown in Figure 8.16, is somewhat overexposed, the colour differences can be seen in the image as the colours have been slightly enhanced.
8.2.3 Evidence for an unseen component in spiral galaxies – dark matter
In the 1970s a problem related to the dynamics of galaxies came to light. Vera Rubin observed the light from HII regions (ionized clouds of hydrogen such as the Orion Nebula) in a number of spiral galaxies. These HII regions move with the stars and other visible matter in the galaxies but, as they are very bright, are easier to observe than other visible matter. HII regions emit the deep red hydro- gen alpha (H-alpha) spectral line. By measuring the Doppler shift in this spectral line Rubin was able to plot their velocities around the galactic centre as a function of their distance from it. She had expected that clouds that were more distant from the centre of the galaxy (where much of its mass was expected to be concen- trated) would rotate at lower speeds – just as the outer planets travel more slowly around the Sun. This is known as Keplerian motion, with the rotational speed decreasing inversely as the square root of the distance from the centre. (This is enshrined in Kepler’s third law of planetary motion and can be derived from Newton’s law of gravity.)
Introduction to Astronomy and Cosmology
To her great surprise, Rubin found that the rotational speeds of the clouds did not decrease with increasing distance from the galactic centre and, in some cases, even increased somewhat. Not all the mass of the galaxy is located in the centre but the rotational speed would still be expected to decrease with increasing radius beyond the inner regions of the galaxy although the decrease would not be as rapid as if all the mass were located in the centre. To give an example; the rota- tion speed of our own Sun around the centre of the Milky Way Galaxy would be expected to be ∼160 km s . It is, in fact, ∼220 km s . The only way these results can be explained is that either the stars in the galaxy are embedded in a large halo of unseen matter – extending well beyond the visible galaxy – or that Newton’s law of gravity does not hold true for large distances. The unseen matter, whose gravitational effects Rubin’s observations had discovered, is called ‘dark matter’. Further evidence, some dating from the 1930s and some very recent, for the exis- tence of dark matter will be given below. A more detailed discussion will be given in Chapter 9.
A modifi ed form of Newton’s law called MOND (MOdifi ed Newtonian Dynamics) was proposed by Mordechai Milgrom in 1981, who pointed out that Newton’s Second Law (F ⫽ ma) when applied to gravitational forces has only been verifi ed when the gravitational acceleration is large and has never been verifi ed where the acceleration, a, is extremely small – as would be the case for stars towards the edge of a galaxy where the gravitational forces are very weak. With a suitable choice of parameters the observed rotation curves of galaxies can be accurately modelled by the MOND theory (Figure 8.17); however, it has
a much harder task explaining other observations that support the existence of dark matter, such as the dynamics of galaxy clusters and gravitational lensing, so MOND will not be considered further.
8.2.4 Weighing a galaxy
The observations of the hydrogen line described above can be used to calculate the mass of a galaxy. Figure 8.18 shows the hydrogen line spectrum of the nearby
galaxy M33 which lies at a distance of 2.36 ⫻ 10 22 m (∼2.9 million light years). The horizontal axis of Figure 8.18 has been converted from frequency to veloc- ity using the Doppler formula: ∆f/f ⫽ v/c. The hydrogen line spectrum of M33 has
a width in frequency due to the fact that it is rotating – one side of the galaxy is coming towards us whilst the other is moving away. The centre of the M33 . You might well deduce that the galaxy as a whole is moving away from us at this speed but, as all but a few galaxies are moving away from us, the sign convention that is used is that galaxies moving away from us are given positive velocities and those moving
Galaxies and the Large Scale Structure of the Universe
Figure 8.17 The galactic rotation curve for the galaxy NGC 1530.
towards us are given negative velocities. So this indicates that M33 is moving towards us at a speed of ∼180 km s . However, our Solar System is moving around
the centre of the galaxy at a speed of ∼220 km s and, having corrected for this, M33 is actually moving towards the Milky Way Galaxy at a speed of ∼24 km s .
The width of the spectral line is ∼200 km s so that the hydrogen at the edge of the galaxy is apparently moving around the centre at a speed of 100 km s . However, though the galaxy is presumably circular, its dimensions on a photo- graphic plate are ∼71 ⫻ 45 arcmin. This implies that it is inclined to our line of sight at an angle of arcsin (45/71) ⫽ ∼39°. As a result, the value we measure will be less than the true value due to the projection effect. (If the galaxy was per- pendicular to us, we would not observe any rotational width in the hydrogen line spectrum.) The true rotational velocity of the outer parts of the galaxy about its centre should thus be close to 100/sin(39) ⫽ 158 km s .
Knowing the distance of the galaxy and its angular size we can calculate its radius.
Introduction to Astronomy and Cosmology
Figure 8.18 The hydrogen line spectrum of M33 in Triangulum. Image: Christine Jordan, University of Manchester.
M33 is ∼71 arcmin across which is 71/(60 ⫻ 57.3) ⫽ 0.020 rad. It lies at a
distance of 2.36 ⫻ 10 22 m.
The radius of M33 is thus ∼0.5 ⫻ 0.020 ⫻ 2.36 ⫻ 10 22 m 20 ⫽ ∼2.4 ⫻ 10 m.
If the mass distribution of the galaxy is symmetrical then the gravitational effect on the hydrogen gas at the edge of the galaxy is the same as if all of the galaxy’s mass was concentrated at its centre. One can thus use an identical method to that used to calculate the mass of the Sun.
The gravitational force on a small mass at this distance must equal the centripetal acceleration:
GMm /r 2 ⫽mv 2 /r
Galaxies and the Large Scale Structure of the Universe
where M is the mass of the galaxy, m is the mass of a small volume of hydrogen, r is the distance of the hydrogen from the centre of the galaxy and v is the velocity of hydrogen around the centre.
This gives:
⫽ 9 ⫻ 10 40 kg ⫽ 9 ⫻ 10 40 /2 ⫻ 10 30 solar masses ⫽ ∼45 thousand million solar masses.
We have another method of estimating the mass using what is termed the ‘mass to light ratio’ of stars. This is simply the ratio between the mass of a star or star cluster divided by its luminosity – our Sun has, by defi nition, a mass of 1 and a luminosity of 1 so its mass to light ratio is 1. One could assume that all the stars in M33 were similar to our Sun in terms of their mass to light ratio. If we then calculate the luminosity of M33 compared with that of our Sun we will directly get an estimate of the mass of M33 in solar masses.
ference of 24.3 magnitudes. This corresponds to a difference in luminosity of 2.512 24.3 ⫽ ∼5.2 ⫻ 10 9 , which gives a mass estimate of ∼5 thousand million solar masses. This is a factor of 10 less than the value derived above. M33 obviously has mass which does not emit light, such as dust and gas, and not all stars will have the same mass to light ratio as our Sun—hot stars are very luminous for their mass compared with our Sun and cooler stars (of which there are many more) less luminous. The average mass to light ratio for stars, gas and dust in our own Galaxy is ∼1.5 so, assuming a similar mix, this would give M33 a mass of
∼8 thousand million solar masses. The fact that this is still a factor of ∼6 less than that derived dynamically is further evidence of the presence of dark matter in the galaxy – there appears to be ∼5 times as much dark matter than normal matter in the makeup of the galaxy!
8.2.5 Irregular galaxies
A small percentage of galaxies show no obvious form and are classifi ed as Irreg- ulars. One nearby example is the Small Magellanic Cloud (SMC). Its companion, the Large Magellanic Cloud (LMC), is usually classifi ed as one too, though it shows some features of a small barred spiral. Such small galaxies are not very bright so we cannot see too many but they may, in fact, be the most common
Introduction to Astronomy and Cosmology
than found in our Galaxy. In 30 Doradus, often called the Tarantula Nebula because of its spidery appearance, the LMC has one of the largest star formation regions known. It contains many young stars; one of these gave rise to a super- nova, called 1987A, observed in February 1987 – the nearest to us for several hundred years.
8.2.6 The Hubble classifi cation of galaxies
A schematic, called ‘Hubble’s Tuning Fork’ is used to illustrate Edwin Hubble’s classifi cation scheme (Figure 8.19). Hubble believed that, as they aged, galaxies evolved from the left end of the tuning fork toward the right. He therefore called elliptical galaxies ‘early galaxies’ and spiral galaxies ‘late galaxies’. This idea was not correct. Spiral galaxies rotate, and so have angular momentum, whilst ellipti- cal galaxies do not. There is no way that elliptical galaxies could suddenly gain angular momentum so they could not turn into spiral galaxies. It is, however, the case that the merging of spiral galaxies probably results in the formation of giant elliptical galaxies.
Figure 8.19 Diagram summarizing the Hubble galaxy classifi cation scheme.
Galaxies and the Large Scale Structure of the Universe