Galaxies

Galaxy Morphology

	M 104, a spiral galaxy of type Sa.	source
	M 64, a spiral galaxy of type Sb.	source
	NGC 4622, a spiral galaxy of type Sc.	source
	M 51, a spiral galaxy of type Sc.	source
	ESO 510-G13, a twisted spiral galaxy.	source
	NGC 4314, a barred spiral galaxy of type SBa.	source
	NGC 1300, a barred spiral galaxy of type SBb.	source
	NGC 1097, a barred spiral galaxy of type SBbc, in optical.	source
	NGC 1097 in infrared.	source
	M 83, a barred spiral galaxy of type SBc.	source
	M 83 in ultraviolet.	source
	NGC 1132, an elliptical galaxy.	source
	NGC 5866, a lenticular galaxy.	source
	NGC 2787, a barred lenticular galaxy.	source
	NGC 4449, an irregular galaxy of type I.	source
	M 82, an irregular galaxy of type II.	source
	Hoag's Object, a ring galaxy.	source
	NGC 4038, 4039, interacting galaxies.	source
	The interacting galaxies of Arp 147.	source
	The interacting galaxies of Arp 148.	source
	Stephan's Quintet, x-ray gasses glowing in blue.	source
	Dwarf irregulars like I Zwicky 18 are thought to combine into larger galaxies over time.	source

Spiral Galaxies

• Considering spiral types a → c, the central bulge is smaller, arms are more separated and the age is greater in type c.
• Most disc stars in our galaxy have completed over 40 orbits around the galactic center.
• Galaxies with thick discs are typically older than those with thin discs.
• Stars in the bulge date from the formation of the galaxy; star formation in the disc is ongoing. The stellar halo (about 1% of the galaxy mass) is the oldest part.
• Spirals are primary sites for star formation; short-lived stars can't migrate far, so the birth rate is greater.
• Near IR (1-5 μm) is due mostly to red giants, so it tracks mass; blue light is mostly from young stars and so tracks star formation.
• Interstellar gas is essential for persistent spiral structure, but spirals are probably not permanent; they continually dissolve and re-form. Gas can lose energy but not angular momentum; spiral arms form since they allow transfer of angular momentum outward and mass inward.
• Most spirals have two arms, and most trail along the direction of galaxy rotation, so whatever creates spirals must not be time-reversible.

Barred Spiral Galaxies

• Approximately 30 to 50% of all spirals are barred.
• Spirals commonly start at the ends of the bars; there is usually rapid star formation there.

Elliptical Galaxies

• Ellipticals are often subclassed from E0 (nearly spherical) to E7 (very elongated); the eccentricity of an En galaxy is n/10.
• Elliptical galaxies have almost no interstellar gas or dust; therefore there is no ongoing star formation. The stellar populations of most ellipticals are very old, on the order of the age of the universe.

Lenticular Galaxies

• Lenticular galaxies have type "S-zero".
• Lenticular galaxies have a disc but no gas or spiral.

Irregular Galaxies

• Some references (including SIMBAD) do not distinguish between Irregular types, or use distinctions other than I and II.
• Irregulars are very common, and rich in gas.
• They are often formed from galaxy interactions (see below).

Ring Galaxies

• Ring galaxies are very rare (about 1 in 10000); they are formed by head-on collisions with a disc galaxy.

Interacting Galaxies

• NGC 4013 has a tidal stream.
• The Hercules Cluster indicates that galaxy interactions may be common.
• Interactions and mergers probably contribute to differences in morphology.
• When galaxies collide, their gasses do but their stars do not; stellar orbits will, however, change. The colliding gasses feed star formation.

  This simulation of galaxy interactions illustrates how unusual morphologies might form over cosmological time scales: it spans over a billion years, and produces a result comparable to NGC 7600. (source)

Dwarf Galaxies

• Some references also consider dwarf galaxies as a separate morphological type.

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.

(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:

I(r) = I(r_e) e^{-b_m ((r / r_e)1/m -1)},

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

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.

Clusters of Galaxies

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.

• The distribution of galaxies is homogeneous on scales ≥ 100 Mpc.
• Clusters of galaxies in the early universe seem to have a larger percentage of spirals than nearer clusters.
• Dense clusters have a larger percentage of ellipticals.
• Lenticular galaxies are more common in the centers of clusters.
• Galaxy clusters are young; most members have made very few orbits about the center of mass.
• Collisions between galaxies in a cluster are much more frequent than those between stars in a stellar cluster.
• Most of the baryonic matter in clusters is in the hot gasses between galaxies.

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:

Galaxy clusters can also interact gravitationally. Here we see the results of such an interaction:

• Table of Contents
• References
• Index

©2017, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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