What is the half-light radius of a galaxy?

[mathman]

In a recent paper by Peebles (arXiv:astro-ph/0309269), there was a discussion of galaxy merging. In it the term "half-light radius" was used to describe distances between galaxies. What is its definition?

 

[marcus]

Originally posted by mathman
In a recent paper by Peebles (arXiv:astro-ph/0309269), there was a discussion of galaxy merging. In it the term "half-light radius" was used to describe distances between galaxies. What is its definition?

You could just plug "half-light radius" into google.

when i did that the first thing that came out was
a definition, from some HST webpage'

"This is the radius within which half the light of the unconvolved model would be contained if it were radially symmetric (an axis ratio of unity). For axially symmetric galaxies, this definition is independent of the observed axis ratio of the galaxy, a parameter which depends on the intrinsic axis ratio and its inclination to the line-of-sight."


So in the simplest case where it is just a sperical blob, the halflight radius is the radius just big enough so that half the light is coming from within it

it is like the middle 50 percent of a bell-curve distribution
and helps to describe things that don't have sharp cutoff edges