Overview of Galactic Morphology

Astronomy 626: Spring 1995

Galactic classification is harder than stellar classification because the spectral class of a star depends mostly on mass, while the state of a galaxy depends on events throughout its life.

Classification Schemes

The Hubble System (Sandage 1961)

The classic `Tuning-fork' diagram: E galaxies are graded by flattening; S galaxies are classified by (i) bulge/disk ratio, (ii) pitch angle of spiral arms, and (iii) `resolution' of arms; SB galaxies have strong central bars; S0 galaxies are disk galaxies without spiral structure.

NB: The classification of E galaxies is objective and unambiguous, while that of S galaxies is subjective and uncertain; this can cause problems.

The de Vaucouleurs System

Defines two-parameter continuum at each point along S0-Sd sequence.

The Morgan System (Morgan 1958)

Three classification dimensions:

cD galaxies are a special case, with very extended outer envelopes not apparent in short exposures. Here the `c' prefix indicates a supergiant galaxy.

The DDO System

Distinguishes disk galaxies by central concentration and evidence of gas content. Defines luminosity classes (I -> V) on basis of `quality and length of spiral arms.'

Quantitative Morphology

Many galaxies can be characterized photometrically as superpositions of disk and spheroid components.

Spheroid profiles

E galaxies and bulges of S galaxies are remarkably well-fit by de Vaucouleurs' (1948) law: the log of the surface brightness is a linear function of the 4th root of the radius. This is an empirical rule without theoretical justification. Stellar systems subjected to rapidly changing gravitational fields often develop profiles which resemble this law.

If the isophotes are not circular it is natural to use the ellipsoidal radius, since this fits the shapes of elliptical galaxies to 1% or better in most cases.

Disk profiles

The disks of S and S0 galaxies are often fit by an exponential law: the log of the surface brightness is a linear function of the radius. Once again, this is an empirical rule. Large S galaxy disks seem to have a rather small range of central surface brightnesses, roughly 21.65 B-mag/arcsec^2 (Freeman 1970).

In the vertical direction, disk luminosity-density profiles are often fit to the sech^2 distribution predicted by the self-gravitating isothermal sheet model (van der Kruit & Searle 1981).

Reading & References


Due date: 1/19/95

1. Open the Hubble Atlas to four different pages and classify one galaxy on each page according to the Morgan (1958) system; list each galaxy and your classification, and briefly note the features which led to your answer.

2. How does the surface brightness (defined as luminosity per unit solid angle) of a transparent, infinitely thin disk galaxy depend on the angle of inclination?

Joshua E. Barnes (barnes@zeno.ifa.hawaii.edu)

Last modified: January 12, 1995