Global Extinction in Spiral Galaxies

Magnitude-limited samples of spiral galaxies drawn from the Ursa Major andPisces clusters are used to determine their extinction properties as a functionof inclination. Imaging photometry is available for 87 spirals in B,R,I and K'bands. Extinction causes systematic scatter in color-magnitude plots. A strongluminosity dependence is found. Relative edge-on to face-on extinction of up to1.7 mag is found at B for the most luminous galaxies but is unmeasurably smallfor faint galaxies. At R the differential absorption with inclination reaches1.3 mag, at I it reaches 1.0 mag, and at K' the differential absorption can inthe extreme be as great as 0.3 mag. The luminosity dependence of reddening can be translated into a dependence onrotation rate which is a distance-independent observable. Hence, correctionscan be made that are useful for distance measurements. The strong dependence ofthe corrections on luminosity act to steepen luminosity-linewidth correlations.The effect is greatest toward the blue, with the consequence thatluminosity-linewidth slope dependencies are now only weakly a function ofcolor.Comment: 23 pages, 5 figure