Lecture 19
Galaxies

Key Concepts:

  1. How were galaxies discovered?
  2. What types of galaxies ("Hubble types") are there? What characteristics distinguish them?
  3. Redshift: what is it? how is it determined?
  4. How do you measure distances to galaxies?
  5. Hubble Law: what is it? what does it tell us?
  6. How do you weigh a galaxy?

Discovery of Galaxies

Zoology of Galaxies (Hubble Types)

  • Ellipticals: galaxies of general elliptical shape and smooth featureless appearance (E, SO, cD)


  • Spirals: galaxies with two or more arms winding out of the center (barred vs. unbarred)


  • Irregulars: galaxies of irregular appearance with random bright patches


  • Also depends on the observed wavelength (e.g. NGC 253 in optical vs. near infrared)


Anglo-Australian Observatory (AAO) Galaxy Images

Hubble Heritage Images

The GEMS HST Images of Galaxies

Redshift and Velocity (using Doppler Effect)

Spectral line from a light source shifts in wavelength with motion:


Quiz 19a

Measuring Distances to Galaxies

Hubble's Law

In 1920, Edwin Hubble discovered that more distant galaxies have larger recession velocities, following the relation


The constant H that relates the recession speed V and distance D is refereed to as the Hubble constant.

==> UNIVERSE IS EXPANDING!

Turning this around, one can infer the distance of a galaxy with measured recession speed V as D = V/H. For the recession velocity in km/s, H = 72 gives D in megaparsecs (Mpc; million parsecs).

A crude age estimate of the universe can be derived as t = D/V = 1/H. Such a calculation suggests the age of universe between 12 and 15 billion years.

Quiz 19b

Weighing Galaxies: Dynamical Mass







  • constant rotation velocity means dark matter dominates the mass in galaxies

    •

Rotation curve data for an edge-on spiral in the Coma cluster (obtained by N. Vogt (NMSU) using the Palomar 5m telescope):



  • Review Questions

  • What is the electromagnetic spectrum?

  • What is a spectral line?

  • If light from the center of a star passes through the outer part of a star, the outer part of the star absorbs some light at sharp, well defined wavelengths - spectral lines

  • Light from the central part of a galaxy is absorbed by stuff in the outer parts of the same galaxy

  • Thus galaxy spectra (light from galaxies) contain dark lines superimposed on the normal spectrum at certain well defined wavelengths

  • If the galaxy is moving away from us, these lines are shifted to the red (longer wavelengths)

  • If the galaxy is moving toward us, these lines are shifted to the blue (shorter wavelengths)

  • It so happens that ALL galaxies exhibit red shifted spectral lines

  • Thus all galaxies are moving away from us. The Universe is expanding.

  • Hubble's law: A galaxys speed of recession is equal to its distance from us times Hubble's constant

  • Thus if galaxy A is twice as far from us as galaxy B, galaxy A is moving twices as fast away from us as galaxy B

  • By using a modified form of Kepler's law, we can "weigh" galaxies.

  • This way of weighing galaxies yields estimates that much greater than what we get if we just count points of visible light

  • There must be some hidden or dark matter which accounts for this

  • Put another way, stars in the outer parts of galaxies are moving around the galactic center at speeds that are too fast.

  • There must be another source of gravity or matter that holds these stars inside the galaxy.

  • This is "Dark Matter"

    •