Question

Part of a foreground spiral galaxy (S) overlaps part of an elliptical galaxy (E). The background elliptical has a surface brightness of IE= IS= 21 mag arcsec^-2. What is the

combined observed surface brightness I obs in a patch where they overlap? Use Vega as a reference star since its mV= mB= 0.

Answer 1

The combined observed surface brightness in a patch where they overlap which will be given as -

I_obs = I_E + I_S

where, I_E = surface brightness of an elliptical galaxy = 21 mag arc/s²

I_S = surface brightness of a spiral galaxy = 21 mag arc/s²

then, we get

I_obs = [(21 mag arc/s²) + (21 mag arc/s²)]

I_obs = 42 mag arc/s²