Half-Light Radius: Definition & Galaxy Merging

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In summary, the term "half-light radius" is used to describe distances between galaxies in a recent paper by Peebles (arXiv:astro-ph/0309269). It refers to the radius within which half the light of a galaxy would be contained if it were radially symmetric. This definition is independent of the observed axis ratio of the galaxy and is useful for describing objects without sharp cutoff edges.
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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?
 
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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
 

1. What is the definition of half-light radius?

The half-light radius is a measure of the size of a galaxy. It is the radius at which half of the galaxy's total light is emitted. This means that half of the galaxy's stars and other light-emitting objects are contained within this radius.

2. How is the half-light radius calculated?

The half-light radius is typically calculated by fitting a model to the galaxy's light profile. This model is often a Gaussian or Sersic profile, which describes how the light is distributed across the galaxy. The radius at which the model reaches half of its maximum value is then determined as the half-light radius.

3. What is the significance of the half-light radius in galaxy merging?

The half-light radius is important in understanding galaxy merging because it can provide clues about the history and evolution of galaxies. When two galaxies merge, their half-light radii may change due to the redistribution of stars and other objects. By studying the changes in half-light radius, scientists can learn more about the process of galaxy merging.

4. How do galaxies with different half-light radii merge?

Galaxies with different half-light radii can merge in various ways. In some cases, the smaller galaxy may be completely absorbed by the larger one, resulting in a larger half-light radius. In other cases, the two galaxies may merge to form a new galaxy with a different half-light radius, depending on the distribution of stars and other objects in each galaxy.

5. Can the half-light radius be used to determine the age of a galaxy?

No, the half-light radius cannot be used to determine the age of a galaxy. It is simply a measure of the size of a galaxy and does not provide information about its age or formation. Other methods, such as studying the chemical composition of stars or the rate of star formation, can be used to estimate the age of a galaxy.

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