Galaxy magnitudes

[obrazell]

Assuming GAIA does not do magnitudes for extended objects is there a transform to get visual magnitudes from the Sloan data or other survey that produces multi colour mags.?

Owen

[Steve Gottlieb]

Owen, I've been using the conversion magnitudes (designed for stars) on this page (http://www.sdss3.org/dr8/algorithms/sdssUBVRITransform.php#Lupton2005) for quite awhile and feel they give pretty good results, particularly for compact galaxies with well defined cores.

Specifically, the Lupton formula:
V = g - 0.5784*(g - r) - 0.0038; standard deviation = 0.0054

or equivalently, V = .4216*g + .5784*r - .0038 (round as you wish and the last term can probably be dropped)

For example with NGC 1, the SDSS gives:
g = 13.55
r = 12.70

Plugging into the formula results in V ≈ 13.05.
For comparison, RC3 lists V_T = 12.89.

Even if you just average the SDSS g and r, you'll be in the same ballpark.