An Investigation of Internal Extinction in Other Galaxies

Internal galactic extinction must be well understood in order to fully resolve many problems in extragalactic astronomy. Studies of the stellar content in galaxies, the modelling of stellar and dust content in spirals at large look-back times, and the diameter functions of galaxies require a good understanding of the effect of inclination on a galaxy's appearance.

A Review of the Basics

A simple plot (Figure 5, shown in highly reduced form below) of magnitude versus the log of the isophotal diameter shows a simple trend: brighter galaxies are larger. We expect this relationship to have a slope of five (see dotted line)... however there is a great deal of scatter. Color coding the galaxies by ellipticity shows that this scatter is in large part due to the inclination of the galaxies. The face-on galaxies tend to be brighter than the edge-on galaxies of the same diameter. However, the relationship between diameter, flux, and inclination is not quite clear.

Possible Scenarios:

• Galaxies are optically thin and move horizontally on the plot with increasing inclination.
• Galaxies are optically thick and move downward with increasing inclination.
• Galaxies are optically 'transluscent' as we might the path they trace with increasing inclination is somewhere in between.
  

The exact nature of the relationship between luminosity, diameter, and inclination CAN NOT be determined solely from fluxes and diameters, but requires knowledge of the distance to these galaxies, something first realized by Burstein and Lebofsky and analytically shown by Choloniewski.

Once we have distances...
Choloniewski worked out a set of estimators for , , and where

He solved for , , and using ZCAT data and diameters from the UGC. He found that spiral galaxies appeared to be optially thick. However, given the diameter-inclination effect known to be present in the UGC, any estimates for or should be considered suspect.

Go forward to New Results using the APS diameters