Spiral Galaxies

NGC2997 - courtesy of the Anglo-Australian Observatory

Introduction

For most people, the word "galaxy" brings to mind the shape of a catherine wheel, with twisting arms and a bright centre, suspended in a field full of stars. Galaxies of this kind are called the spiral galaxies, and there are several subdivisions within the group.

The majority of the bright galaxies in the neighbourhood of our own Milky Way are of the spiral type (although if we were to count all the galaxies, no matter how small and faint, in a given region, we would find irregular galaxies were most prevalent). Of the three basic types covered in the Educational Guide (Elliptical, Spiral and Irregular), spirals are intermediate in their range of masses and their size. Typically, these objects can contain between 10,000,000,000 and 400,000,000,000 times the mass of our Sun. Diameters can range from 16300 to 163000 light years (our own Milky Way is close to this upper value).

Classes of spiral galaxy

Each spiral galaxy is classified with a label which gives some indication of its appearance. All spiral galaxy labels begin with the prefix "S", and in general this is followed by a lower case letter which describes the "style" or overall appearance of the particular object. This letter may be either 0, a, b or c, and is determined by the arrangement of the spiral arms, and of the bright central condensation (the nucleus).

This image (left, courtesy of the SEDS archive) is of the galaxy M60, often listed as a type S0 (another name for which is 'lenticular', due to the characteristic lens shape seen in some examples) galaxy. Note that the fainter image to the right of the main object is the galaxy NGC4647. M60 is typical of this class of galaxy, showing relatively little in the way of surface features. In fact, S0 galaxies can be very similar in superficial appearance, to type E7 ellipticals, and are usually regarded as being objects intermediate between these ellipticals and type Sa spirals (covered below).

However, the S0 galaxies are different to ellipticals, although this is only apparent in objects which are seen edge-on. In such cases, an S0 galaxy shows signs of a disk, surrounded by a more spherical halo in which is embedded the nuclear bulge. This disk is not observed in ellipticals. It is also possible to distinguish S0's from true spirals, since, when seen face-on, they show no sign of the characteristic arms which grace the latter type.

This image (right, courtesy of the SEDS archive) is of the S0 / Lenticular galaxy M102, clearly showing the lens shape which is visible due to the orientation of the object as seen from Earth. It should be clear from the discussion above that the angle at which we view such a galaxy can affect the classification which it is given. There are, however, more fundamental differences which set S0 galaxies apart from the others; for instance, true spiral galaxies contain many young, population I stars, whereas S0's do not. In addition, many (although not all) such objects contain little or no gas and dust (like ellipticals, and in contrast to the gas and dust-rich spirals).

The next class of spiral we look at is the Sa. These systems have very tightly wound spiral arms, and large central nuclei. A good example of such a galaxy is Messier 65 (NGC3623), shown here courtesy of AAO. Although some of the spiral detail is lost because of the fact that the galaxy is not seen face-on, the difference between this and the S0 galaxies pictured above is clear, with the spiral shape of the disc being visible (note also the dense dust lane running through the centre of the disc).

In common with the remainder of the spiral class of galaxies, (and in contrast to the ellipticals), Sa's have both young population I, and older population II stars, although the fraction of young stars increases towards the Sc class. The young stars are to be found in the spiral arms, which mark recent (and ongoing) sites of star formation. These galaxies also contain much dust and gas (some of which is heated to form luminous nebulae, see the link to 'The Web Nebulae' on the links page). Once again, the amount of gas and dust increases, with Sa types having the least (about 2% of the total mass of the particular galaxy), and Sc's the most (around 10%).

The galaxy shown on the right is Messier 77 (NGC 1068), and is reproduced here courtesy of the SEDS archive. This is a member of the next class of spiral, the Sb. The bright central regions contain the majority of the galaxy's young stars, whilst regions further from the middle hold the older objects. As well as being an Sb type, M77 is also part of a class of galaxies known as Seyfert galaxies. Such objects have cores which emit strongly in the radio region of the electromagnetic spectrum, and observations using instruments such as the Hubble Space Telescope seem to confirm the belief that this galaxy, like other Seyferts, has a central region which is powered by material (such as stars and gas) falling into a black hole. It is interesting to note that the majority of spiral galaxies are of the "b" classification.

Of course, not all spiral galaxies can be classed as purely type a, b or c. Many galaxies have features which put them on the boundary between two types. Such objects are therefore given two letter designations. For example, the galaxy shown here on the left (courtesy of the SEDS archive), is Messier 94, and is designated type Sab; the spiral arms have the appearance of a "b" galaxy, but the central region is too bright, belonging to the "a" class.

Finally in our investigation, we come to the Sc class of spirals, which are often referred to as the grand design. Such galaxies, like NGC 2997 (below, courtesy of AAO) have very open, "untidy" spiral arms and relatively small nuclei.

These spirals have the highest proportion of gas and dust of any of the normal spiral types. This image shows the yellow light of the central regions, where old stars are found, and the blue glow of the hotter, younger stars which are formed in the spiral arms. Tracing along the path of these arms, we can also see small red patches; these are glowing clouds of gas and dust, heated by nearby stars to form nebulae, and in some of these nebulae, stars are being formed, just as they are in our own galaxy, the Milky-Way.

Rings and Bars

The diversity of spiral galaxies does not end with the simple a,b,c classification. There exists a sub-branch of spirals known as barred spirals, in which the spiral arms emerge from a bar passing through the galactic centre, rather than them coming directly from the centre themselves; such galaxies are given the prefix SB instead of the simple S, and astronomers believe that our Milky-Way belongs to this class.

Once again, however, these objects are labelled according to the appearance of the arms and the central bulge, and so we see labels such as SBa, SBb, and SBc, as well as intermediate types such as SBbc, and so on. A particularly beautiful example of a barred spiral is Messier 83 (above, courtesy of AAO). This galaxy is one of our nearest neighbours in space, lying at a distance of around 12 million light years (remember that this means the light we see from the galaxy tonight started out on its journey to us over 12 million years ago !). There are many more examples of barred spirals, such as the SBb object NGC 1365 shown left (courtesy of AAO), in which dark dust lanes can be seen running through the arms.

In addition to bars, some galaxies exhibit rings around the central regions. One such galaxy is NGC 2523 (left, courtesy of the Digital Sky Survey). Here we can see the ring around the nucleus, and the bar which passes through the galactic centre, touching the ring on opposite sides. Spiral arms are then observed to emerge from the point where ring and bar touch.

What are the spiral arms ?

The spiral arms are regions where stars are being formed. Here we find the hottest, youngest and brightest stars, and it is for this reason that the arms can be so visible. Along with fully formed stars, we find sites of stellar formation, with hot glowing clouds of gas and dust forming the "stellar nurseries" which we see as nebulae in our own galaxy.

If spiral arms are the areas where stars are formed, what mechanism forms the spiral arms ? A quick "thought experiment" shows that it isn't simply due to the rotation of the galaxy. If this was the case, the arms would "wind up" as the galaxy rotated, becoming tighter and tighter with time. Galaxy rotation periods are far shorter than the age of the universe (for example, our own Sun completes one "orbit" of the galatic centre every 240 million years), so we would expect to see the great majority of galaxies in this "wound-up" state. Yet, when we look out into the universe, we see galaxies with a whole range of spiral arm designs - some tight, and some loose. So it can't just be a simple case of a galaxy's rotation which causes the spiral shape. There must be some other cause. But what is it ?

This is a question which has occupied astronomers for many years. The most favoured theory is called the density wave theory. A good example of a density wave is very frequently observed on motorway journeys, where a traffic jam forms. We slow down and join the queue, moving forwards painfully slowly, and expecting to pass the obstruction on the road which has caused the hold-up. But instead, we see nothing unusual, and in a (hopefully !) short time, we've moved through the queue of vehicles and begin to speed up again. In reality, these hold-ups can be caused by very minor events - for instance, a car slowing down to turn off the motorway.

Now imagine what this same scene must look like from someone in a helicopter high above the road. Briefly looking down the length of the motorway, the observer would see cars travelling at similar speeds, and with some space inbetween each other. But at one point, the observer notices that the cars are moving very slowly and are close together, spending some time in this group before leaving it, speeding up and continuing on. So, whilst this "bunch" of cars - the queue - exists for some time, at any particular time it is made up of a different set of cars. This is a density wave.

In a spiral galaxy, it is believed that the density wave rotates slower than the material in the galactic disc, so that stars and gas are able to "overtake" the wave. In effect, the spiral pattern is not "frozen into" the stars, but instead it moves through them. As gas in the interstellar medium passes into the density wave, it becomes more dense, and (as discussed in the section on stellar evolution), this can lead to the formation of new stars. Now, the very hottest, (and brightest) stars have short lifetimes, so that they are born, live their lives, and die very close to the density wave. This is why the spiral arms are traced by the brightest stars. There are many faint stars, but few very bright ones, in the gaps between the arms, because these are areas which the density wave passed through long ago, and the bright "beacons" have had time to die. But in time to come, the wave will again revisit these regions, and star formation will occur there again.