Galaxy Morphology

Note that the classification of a given galaxy is not always universally agreed upon. The classifications below follow the morphological types quoted in SIMBAD:

You need a Java-capable browser to be able to use the applets. If they do not work with your Windows system, download the Java VM (Virtual Machine) for your version of Windows at the download section at

Note that many galaxies (for example, Centaurus A) when viewed in multiple wavelengths challenge our notions of galactic morphology gleaned from visible light observations.

The effective radius of a galaxy as measured by its interstellar gas is much larger than its visible radius; the low density in that region inhibits star formation. The 21 cm Hydrogen line is observable far beyond the stellar distribution. 21 cm discs are frequently warped.
Also, see NGC 2915: a radio spiral.
(View Cosmos DVD 6, episode 10, Milky Way and galaxy interaction simulations.)

Also see this site at the University of Hawaii.

Portfolio Exercise: Find an additional example of each morphological type (Sa, Sb, Sc, SBa, SBb, SBc, E, S0, SB0, I, Ring and Interacting) and verify the type using SIMBAD. Include an image and as much information as you can (including all URLs used) for each example.

The Sersic law is a useful model for the surface brightness (luminosity per unit area) of a galaxy:

Sersic law with re = 1 and I(re) = 1.

I(r) = I(re) e-bm ((r / re)1/m -1),
where re is the effective radius (containing half of the total luminosity), m is the Sersic index (6 for luminous ellipticals, 2 for dim ones, 1 for disc galaxies) and bm is an experimental fit, here taken to be 2m - 0.324.

Portfolio Exercise: Using the mass-luminosity relation, plot the mass as a function of radial distance for the parameters plotted above.
Note that this will be the total mass contained within a radius r.

Central Black Holes

Since 2001 it has been known that there is a correlation between the mass of supermassive central black holes in galaxies and the mass of the central bulge: the median black hole mass is .0013 times the bulge mass. This correlation hints that the events which lead to the formation of supermassive central black holes are the same as the events which lead to the formation of central bulges (in spiral or elliptical galaxies). (source)

More recently, it has been discovered that this ratio is 20 to 30 times larger for galaxies at high red shift (z > 6). (source) This seems to indicate that the black holes form before the central bulges. Interesting problem!

Note: massive black holes formed within 10 Kpc of a galactic center will spiral in to the center within the age of the universe.
In a "cold" disc, conservation of angular momentum keeps a central black hole from accreting matter; with nothing to speed up the matter, its distance from the center of rotation must stay constant. During mergers, however, both the black hole and the central bulge may grow.

Clusters of Galaxies

Here is a segment of the Coma Cluster (the foreground star in the upper right is HD 112887):

The Coma cluster in optical, with labels, and in xrays. (source)


Note that much of the x-rays emanate from a region which is visually devoid of matter; although this radiation comes from hot gases which are found in most clusters, it is not dark matter.

Galaxies orbit the center of mass of their clusters. In doing so, they can interact with the intracluster medium. A particularly clear example of this is ESO 137-001, which is moving at almost seven million km/hr. In this image, the dark blue trails are x-ray data from Chandra showing the material which has been stripped from the galaxy by ram pressure:

ESO 137-001 in optical, and in x-rays. (source)

©2014, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

Please send comments or suggestions to the author.