INTERACTIONS OF GALAXIES

With the exquisite vision of the heavens provided by modern ground-based and space-based observatories, it has become increasingly apparent that galaxies are not “island universes”; they do not evolve in isolation.

Evidence of Interactions

Nearly all galaxies belong to clusters, and the galaxies take up a larger fraction of the cluster's volume than do the stars in a stellar cluster.We also know that the spacing between galaxies is typically only 100 times larger than the size of the galaxies themselves (see Section 27.3). Densely populated clusters, such as the Coma cluster (Figs. 26.1 and 26.2), have a higher proportion of early-type galaxies (ellipticals) in their centers than they do in their outer, less dense regions. As was already mentioned in Section 25.4, the central regions of these rich, regularly shaped clusters also have a higher proportion of early-type galaxies than the centers of less populated, amorphous-shaped irregular clusters, such as the Hercules cluster (Fig. 26.3). These observations seem to correlate with the increased probability of interactions and/or mergers between galaxies in regions of higher galaxy number density. Interactions tend to increase the velocity dispersions of stars in the galaxies involved, possibly destroying disk structures in late-type galaxies and causing the galaxies to relax to early-type r1/4 profiles (Eq. 25.2).

AVLA radio survey of the H I layer of galactic disks found that at least 50% of all disk galaxies display warped disks. Also, more than half of all elliptical galaxies harbor discrete shells of stars. Some disk warping may be due to tidal interactions with smaller satellite galaxies, and, as we shall see, shells in ellipticals are signatures of mergers.

Observations also suggest that hot, X-ray-emitting gas occupies much of the space between the galaxies in rich clusters and has a mass equal to or exceeding the mass of all of the cluster's stars (Section 27.3). It seems that the gravitational influence of interacting galaxies is largely responsible for removing the gas from the individual galaxies that make up the cluster, while still leaving the gas trapped in the cluster's overall gravitational well.

For many astrophotographers, the spiral galaxy M 31 was their first extragalactic subject – the first image remains in memory. This is a good point of introduction to the astrophysics of these spiral systems.

THE CLASSIFICATION OF SPIRAL GALAXIES

Spiral galaxies with their spiral arms are prototypical galaxies. These impressive forms allow even those with little knowledge of astronomy to instantly identify spirals as “galaxies” in their images. Spiral arms have thus played a deciding role in the various classification schemes for this type of galaxy. Note that all classification schemes are based on the view onto the main plane of the galaxy. In other words, the “face-on” view is what matters for the classification. Galaxies seen from an oblique inclination are rotated in order to simulate a face-on view. The classification of edge-on galaxies is difficult.

An edge-on spiral galaxy allows a view into its galactic plane with large amounts of gas and dust. However, this makes it more difficult to say anything about the spiral arms. This dilemma also applies to the classification of our Milky Way, since Earth is located within the main plane which thus doesn't afford a view onto this plane. Further conclusions require complex measurements in wavelengths which are able to penetrate the thick clouds of gas and dust. Today, the consensus among astronomers is an SB classification with four spiral arms. However, the Milky Way did not always have this four-arm structure; it is suspected that tidal interactions induced disturbances in the plane of the disc which led to the splitting of two dominant spiral arms.

The Hubble classification, which was mentioned in the Introduction, has proved useful over many years and distinguishes spiral types based on the compactness of the pattern of the arms. The nomenclature uses, in addition to the letter S, the suffixes a, b, and c, for example describing as Sa a galaxy the arms of which appear very tight and as a rule can be followed over one or two full revolutions. Sb denotes types the arms of which are not as tightly wound and more coarsely structured compared to Sa.