Arecibo Observatory

Deriving the stellar mass of a galaxy from photometric observations

Before starting, you'll have to worry about correcting the observed apparent magnitude (in any filter) for (1) Galactic extinction; (2) internal extinction; and (3) K-correction. And if you are depending on us for a recipe to deal with those corrections, you will have to wait a bit for us to get around to doing it!

A popular prescription for deriving the stellar mass of a galaxy from observations of its colors is given in the paper by Bell et al 2005, ApJ 625, 23. Their equation 2 reads:
log10 M* = -0.4 (V - 4.82) + [ -0.628 + 1.305 (B-V) -0.15]
where the -0.15 dex term is employed to convert to stellar masses under the assumption of a Kroupa (2001) initial mass function. This conversion, which assumes a wide range in stellar ages within the population, is only an approximation and does not account for widely varying star formation histories. The reader should refer to the full article and references cited therein for full details.


Converting from SDSS magnitudes to UBVRI

Transformations from SDSS magnitudes and UBVRI are discussed on the SDSS website. As noted therein, there aren't explicit transformations for galaxies, but Lupton (2005) derived simple relations for stars:

B = u - 0.8116*(u - g) + 0.1313 sigma = 0.0095
B = g + 0.3130*(g - r) + 0.2271 sigma = 0.0107

V = g - 0.2906*(u - g) + 0.0885 sigma = 0.0129
V = g - 0.5784*(g - r) - 0.0038 sigma = 0.0054

R = r - 0.1837*(g - r) - 0.0971 sigma = 0.0106
R = r - 0.2936*(r - i) - 0.1439 sigma = 0.0072

I = r - 1.2444*(r - i) - 0.3820 sigma = 0.0078
I = i - 0.3780*(i - z) -0.3974 sigma = 0.0063


What is the "AB" magnitude system?

The AB magnitude system of Oke and Gunn (1983 is based on the presumption that a magnitude 0 object should have the same counts as a source of Fν. In other words, in the AB system, every filter has a zero-point flux density of 3631 Jy (1 Jy = 1 Jansky = 10-26 W Hz-1 m-2 = 10-23 erg s-1 Hz-1 cm-2).


Readers of this page are advised to check for more recent publications and to realize the limitations of the above relations as discussed on the SDSS site.


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Last updated Mon Oct 8 13:48:46 EDT 2007 by martha