Inclination-Dependent Extinction Effects in Disk Galaxies in the Sloan Digital Sky Survey

Abstract

We analyze the r-band absolute magnitude and u − r color of low-redshift (z < 0.06) galaxies in the Sloan Digital Sky Survey Data Release 6. Galaxies with nearly exponential profiles (Sloan parameter fracDeV < 0.1) fall on the blue sequence of the color-magnitude diagram; if, in addition, these exponential galaxies have M_r ~< − 19, they show a dependence of u − r color on apparent axis ratio q expected for a dusty disk galaxy. We create a subsample of bright exponential galaxies, with typical Hubble types Sbc and Sc; then, by comparing the normalized luminosity functions for galaxies with different values of q, we measure how the high-luminosity cutoff of the luminosity function depends on apparent axis ratio. In this way, we measure a dependence of dimming on apparent axis ratio well fit by the relation ΔM_r = 1.27(log q)^2, rather than the ΔM ⍺ log q law that is frequently assumed. When the absolute magnitudes of bright exponential galaxies are corrected to their "face-on" value, M^f_r = M_r − ΔM_r, the average u − r color is linearly dependent on M^f_r for a given value of q; the value of d(u − r)/dM fr ranges from ~0.10 for nearly face-on galaxies (q > 0.9) to ~0.26 for nearly edge-on galaxies (q < 0.3). When the dimming law ΔM_r ⍺ (log q)^2 is used to create an inclination-corrected sample of bright exponential galaxies, their apparent shapes are consistent with a distribution of mildly noncircular disks, with median short-to-long axis ratio γ approx 0.22 and median disk ellipticity ε approx 0.08.