# Relation between isophotal radius and virial radius in spiral galaxies?

Is there any (proposed) relation between the $B$-band isophotal radius of a spiral galaxy and its virial radius ($R_{200}$)?

If you know of such a relation, please post a reference paper.

• Virial radius is based on a dynamic calculation, so it would include any alleged dark matter content. An isoPHOTal radius by definition would exclude dark matter. Therefore, either dark matter decouples them entirely, or there must some strict, predictable (assumed?) relation between the B band and the dark matter content, which I believe defeats the purpose. – Andrew Jan 29 '12 at 2:04
• @con-f-use: what is "B-band"? – Peter Mortensen Jan 29 '12 at 15:49
• en.wikipedia.org/wiki/Photometric_system – con-f-use Jan 30 '12 at 9:05

## 1 Answer

Not directly, because these two quantities are not known to correlate as strongly as other "intermediate" relations. But they do correlate, with bigger disks in bigger haloes; it's fairly intuitive.

You could construct a relation from e.g. The Luminosity-Size and Mass-Size Relations of Galaxies out to z ~ 3 (or more specific to B-band, but more simulation focused Interpreting the Evolution of the Size - Luminosity Relation for Disk Galaxies from Redshift 1 to the Present) and Constraints on the Relationship between Stellar Mass and Halo Mass at Low and High Redshift. The various forms of the Tully-Fisher relation may also be useful.