Galaxy properties

Looking at the different images of galaxies, it becomes immediately clear that there are two basic types, elliptical and spiral galaxies.

Ellipticals (also called early type galaxies, E galaxies for short)

• have smooth light distributions, where the isophotes (lines of constant SB) have (nearly) elliptical shapes. They are denoted as E$n$, where $n=10(1-b/a)$, and $a$ and $b$ are the semi-major and semi-minor axis of the ellipse. So, an E0 is round, and an E7 is flattened, $n=7$ is about how flattened E's get. A 3D ellipsoidal shape, with semi-axes $a$, $b$ and $c$ (ordered as $a\ge b\ge c$), is called oblate when $a=b>c$ (a pancake), prolate when $a>b=c$ (a cigar), and tri-axial when $a>b>c$ (a squashed American football). E's are probably tri-axial.
• have no current star formation
• have old stars
• have little gas and dust
• occur preferentially in groups and clusters of many Es
• have little evidence for rotation, so the stars must have large random motions to keep the system from collapsing (also called a hot stellar system)

Spirals (or late type galaxies, S galaxies for short)

• have most of the light coming from a thin disk. Seen face on, it is nearly circular, seen edge on, it is nearly a line. In their centre, they often have a nearly elliptical stellar system, similar in shape to a small elliptical, called a bulge. In addition, some have a bar as well.
• have light profiles with a characteristic spiral pattern
• have star formation going on, especially in their spiral arms
• have both old and young stars
• have gas and dust
• seem to avoid dense groups, and are very rare in clusters
• have large rotational velocities, enough to keep their stars on circular orbits.

We inferred the stellar properties from the fact that E galaxies tend to be redder than S. This suggested that they lack the young massive stars that produces most of the blue light. Hence our inference that the stellar population is likely to be older, with the massive stars that have short life times already dead. We will see later that the SB distribution, $SB(r)$, are quite different for these two types.

These are the archetypal properties of such systems, of course there are exceptions. For example, some ellipticals do have dust, as well as evidence for recent star formation. Some even have a small disk. Given that some spirals have a very large bulge, and some ellipticals have a disk, the distinction between the two types is not always as clear as you might think. Clearly it would be interesting to know what processes are responsible for forming an elliptical vs a spiral in the first place.

Both types are seen to have hundreds and sometimes thousands of globular clusters (GCs for short). These are very dense, spherical, gravitationally bound systems of typically $10^5$ to $10^6$ low mass stars, and they are usually found in a spherical distribution around their parent galaxy. The Milky Way has about 150 of them.

Figure 1.1: The Hubble sequence.

There are many schemes for classifying galaxies. A good scheme should be unambiguous, and ideally have some physics behind it. Classification along the Hubble sequence (Fig. 1.1) is still very popular. It divides galaxies in ellipticals and spirals. Spirals are divided in two strands, according to whether they are barred (SB), or not (S). The sequence has the shape of a tuning fork - the Hubble tuning fork. Along the handle of the tuning fork, the range goes from E0 to E7. The sequence then forks, toward the top are unbarred spirals (S), along the bottom are barred spirals (SB). Both strands are further divided along the sequence (away from the handle), as Sa toward Sc, and analogously SBa to SBc, depending on the relative strength of bulge to disk, and, correlated with this, how tightly/loosely wound the arms are. Sa's and SBa's have a large bulge and tightly wound arms, Sc's and SBc's have a much smaller bulge, and more loosely wound arms. Finally, any galaxy that does not fit into the sequence, is called irregular (Irr).

The physical scale of galaxies is huge: E's can have masses from as little as $10^7\hbox{$M_\odot$}$ to as much as $10^{13}\hbox{$M_\odot$}$, with linear sizes ranging from a few tenths of a kpc to hundreds of kpc. In contrast, spirals tend to be more homogeneous, with masses $10^9$-$10^{12}$$M_\odot$, and disk diameters from 5 to 100kpc or so.

Hubble thought (incorrectly) that this sequence was due to evolution, where galaxies evolve from being E0s, toward Sa or Sba, and later evolving in the more loosely wound Sc's or SBc's. Therefore these types are also called early type and late type. So E's are also called early type, and Ss are called late type. And an Sc is a late-type spiral, and Sa an early type spiral. We know now that this is not the way galaxies evolve, yet these names are still widely used.

There are other reasons why this is not a particularly good scheme. For example, galaxies may look quite different depending on the filter you use to observe them in.

Another type you'll often come across are cD galaxies, which stands for central Dominant: these are very large ellipticals, found in the centres of clusters, with a faint but very large outer halo of stars.

Yet another type are called S0 (or SB0 when barred) or lenticulars: they are the divide in Hubble's sequence between E and S. They have disks without gas or dust, and no recent star formation.

Galaxies can fail to fit in the sequence, for example because they recently had a tidal encounter with another galaxy. This typically leads to spectacular tidal arms, like the case of e.g. the Antennae.

Most smaller galaxies are of the Irregular type: no disk, no nice spherical distribution of stars either, but a loose aggregate of stars, like for example the Magellanic Clouds. Since there are far more small than large galaxies, most galaxies are in fact of type Irr. The aim of these lectures is to understand which types of observation can be used to describe these different types a bit better (the `biology bit of the lectures'), and also try to understand the physical processes behind them. May be you'll be surprised to learn that, for many of the most basic questions you might think of, such as - why are some galaxies of type E, and others of type S?, or, why do Ss have spiral arms? - we are only just beginning to understand the underlying physics, and we have by no means a well defined, generally accepted textbook answer. May be something for you to work on, when you start doing astronomy research yourself?