Internal extinction in spiral galaxies in the near-infrared

In order to study the effects of internal extinction in spiral galaxies, we search for correlations of near-infrared (NIR) photometric parameters with inclination. We use data from the Two Micron All-Sky Survey (2MASS) Extended Source Catalog (XSC) on 15,224 spiral galaxies for which we also have redshifts. For 3035 of the galaxies, I-band photometry is available, which is included in the analysis. From the simple dependence of reddening on inclination we derive a lower limit to the difference in magnitude between the face-on and edge-on aspect of 0.9, 0.3, and 0.1 mag in I (0.81 mum), J (1.25 mum), and H (1.65 mum) bands. We find that the faintest isophotal radius reported in the XSC (at the 21st mag arcsec(-2) level) is closer to the centers of the galaxies than other common isophotal measures (e. g., the 23.5 mag arcsec(-2) radius in I band) and argue that it should not be assumed to represent an outer isophote at which galaxies are transparent at all viewing angles. A simple linear extinction law [i.e., DeltaM = gamma log (a/b)] is not adequate for the full range of disk inclinations, and we adopt both a bilinear and a quadratic law. A simple photometric model is used to explain the observed behavior. Internal extinction depends on galaxy luminosity. We show that for galaxies with a K-s total magnitude dimmer than -20, -20.7, and -20.9 the data indicate zero extinction in J, H, and K-s, respectively, while disk opacity increases monotonically with increasing disk luminosity.