A STUDY OF INCLINATION EFFECTS ON GALAXY SURFACE BRIGHTNESS

The inclination effects due to the internal absorption on the surface brightness of spiral galaxies at the center (mu-0) and at the equivalent radius (mu(e)) are investigated using the V-band data obtained with the KISO Schmidt telescope. The results confirm the general trend of the inclination effect from the ESO Schmidt data in the B band, in particular, the small inclination effect on mu(e) (C(e) congruent-to 0) for middle-type spirals (T = 3-5), upon which the current argument of highly opaque nature of galaxy disks by Valentijin is based. Those empirical results are compared with predictions from simple models of disks and bulges obscured by an exponential absorbing layer with various optical thickness (0 less-than-or-equal-to tau-0 less-than-or-equal-to infinity) and relative scale height (0 less-than-or-equal-to xi = beta(d)/beta(s) less-than-or-equal-to infinity). The general trends of variation in mu(e) and mu-0 over various types of spiral galaxies are interpreted in terms of increasing bulge/disk ratio, and of decreasing xi with decreasing type index, without assuming highly opaque disks. When the height of absorbing layer is restricted to xi less-than-or-equal-to 1, the layer must be highly opaque for a pure-disk galaxy of C congruent-to 0. However, the small inclination effect (C(e) congruent-to 0) can also be reconciled with an extended absorbing disk (xi > 1) of moderate or small optical thickness (tau-0 < 1). One needs to investigate more in details of bulge/disk ratio and of scale height xi in order to determine the optical thickness tau-0 from this kind of study.