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 2 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 magnitudes in I (0.81 um), J (1.25 um) and H (1.65 um) bands. We find that the faintest isophotal radius reported in the XSC (at the 21st mag/arc sq level) is closer to the centers of the galaxies than other common isophotal measures (e.g. the 23.5 mag/arc sq 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. Delta M = gamma log(a/b)) is not adequate for the full range of disk inclinations and we adopt both a bi-linear 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 total magnitude dimmer than -20, -20.7 and -20.9 the data indicates zero extinction in J, H and K respectively, while disk opacity increases monotonically with increasing disk luminosity.