Astronomy 12 - Spring 1999 (S.T. Myers)

Extragalactic Distance Indicators

There are a number of extragalactic distance indicators used when dealing with galaxies outside our local group. In particular, there are eight main methods used to find the distance to the Virgo cluster.

Method Virgo Distance (Mpc)
1. Cepheids 14.9 1.2
2. Novae 21.1 3.9
3. Planetary Nebula Luminosity Function 15.4 1.1
4. Globular Cluster Luminosity Function 18.8 3.8
5. Surface Brightness Fluctuations 15.9 0.9
6. Tully-Fisher relation (spirals) 15.8 1.5
7. Faber-Jackson/D-sigma relation (ellipticals) 16.8 2.4
8. Type Ia Supernovae 19.4 5.0

This table was adapted from Jacoby etal 1992, PASP, 104, 599.

A summary of these methods:

  1. Cepheid variable stars and their cousins the RR Lyrae stars are the primary distance indicators in the galaxy and the local group. They pulsate radially (basically at the sound speed in the envelope of the star) with a period inversely proportional to the square root of the density. They have a period-luminosity relationship that when calibrated gives accurate distance moduli (0.16 mag or better). With M ~ -20, the brightest Cepheids can be see in the Virgo clusters galaxies by HST.

  2. Novae on white-dwarf stars can be used as indicators by measuring velocities of expanding photospheric gases and the light curves, plus the usual assumptions about blackbodies (L ~ R^2 T^4). Just usable out to Virgo distances.
  3. Planetary nebula luminosity function uses the statistical distribution of planetary nebula brightnesses as a standard candle. It appears that there might be a cutoff to the brightest PNs at M ~ -4.48. The basis of this method is being debated.
  4. Globular cluster luminosity function assumes that the luminosity function of globular clusters around giant elliptical galaxies is "universal", and has a characteristic turnover magnitude (around M ~ -6.5).
  5. Surface brightness fluctuations uses the mottling in the profile of elliptical galaxy surface brightnesses due to the brightest stars (which are unresolved). The galaxy light becomes "smoother" with distance as there are more stars averaged in each square arcsecond, and this can be quantified as a distance indicator given suitable assumptions.
  6. Tully-Fisher relation relates the velocity width and luminosity of spiral galaxies.

  7. Faber-Jackson relation or the preferred version D- relation work for elliptical galaxies. In the latter, the isophotal diameter of the galaxy is obtained, and thus the method provides a standard rod.
  8. Type Ia supernovae are the brightest supernovae, and thus can be seen over large distances. They have absolute magnitudes brighter than -20 and thus are easily identified. Recent programs to monitor these in a large sample of galaxies have yielded interesting results. A relation between thier luminosity and decay time are at the heart of this method.

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