Surface Photometry of Edge-On Galaxies - Part Four - Global Structure of Disk Galaxies

The surface brightness distributions of nine edge-on galaxies, NGC 3115 (S0^-), 4350 (S0), 4762 (S0⁰), 4710 (S0⁺), 5866 (S0⁺), 4594 (Sa), 7814 (Sab), 4565 (Sb), and 4244 (Scd) are decomposed with the help of a semi- empirical three-dimensional model, and the global structures of their two main components, the spheroid and the disk components, are investigated.

Our main results are the following. (1) The spheroid components of lenticular galaxies are basically the same as elliptical galaxies but have steeper luminosity gradients. The luminosity gradients of the spheroid components of spirals are steeper than those of lenticulars. (2) The ratio of the bulge to disk luminosities is considerably larger in lenticulars than in spirals. Results (1) and (2) support the hypothesis that lenticulars are transitional types between elliptical and spiral galaxies. (3) The shapes of the spheroid components of disk galaxies are usually flatter than those of typical ellipticals. (4) The disk components of lenticulars are ∼2 B mag arcsec^-2 fainter than those of spirals. (5) The B(0)c universality of the apparent disk components does not seem to be a property of the intrinsic disk components. Some disk galaxies have large disks with faint B(0)_c; some have small disks with bright B(0)_c.