Using a samples of 61506 spiral galaxies selected from the SDSS DR2 , we examine the luminosity function ( LF ) of spiral galaxies with different inclination angles .
We find that the characteristic luminosity of the LF , L ^ { * } , decreases with increasing inclination , while the faint-end slope , \alpha , depends only weakly on it .
The inclination-dependence of the LF is consistent with that expected from a simple model where the optical depth is proportional to the cosine of the inclination angle , and we use a likelihood method to recover both the coefficient in front of the cosine , \gamma , and the LF for galaxies viewed face-on .
The value of \gamma is quite independent of galaxy luminosity in a given band , and the values of \gamma obtained in this way for the 5 SDSS bands give an extinction curve which is a power law of wavelength ( \tau \propto \lambda ^ { - n } ) , with a power index n = 0.96 \pm 0.04 .
Using the dust extinction for galaxies obtained by Kauffmann et al .
( 2003 ) , we derive an ‘ extinction-corrected ’ luminosity function for spiral galaxies .
Dust extinction makes M ^ { * } dimmer by about 0.5 magnitudes in the z -band , and about 1.2 magnitudes in the u - band .
Since our analysis is based on a sample where selection effects are well under control , the dimming of edge-on galaxies relative to face-on galaxies is best explained by assuming that galaxy disks are optically thick in dust absorptions .