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 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.