Brightness distributions in compact and normal galaxies. III - Decomposition of observed profiles into spheroid and disk components

Abstract

view Abstract Citations (351) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Brightness distributions in compact and normal galaxies. III. Decomposition of observed profiles in spheroid and disk components. Kormendy, J. Abstract Exponential disk parameters are studied for seven SO compacts and one normal galaxy. Freeman's central disk brightness B(O)c is usually normal, i.e., 21.65 B mag (hereafter B , but is faint in I Zw 21(22.78 B and VII Zw 793 (22.68 B . This confirms a general suspicion that there exist galaxies with faint disks. The scale lengths are 7 kpc (H0 = 50 km s - Mpc 1), at the long end of the normal distribution. This quantifies Zwicky's description of galaxies with "large nearly uniform halos." Spheroids contribute significant light in regions dominated by the disk. This fact was neglected in previous studies. We therefore decompose the observed profiles into spheroid and disk components. The transition between the components is so sharp that one of them must have a cutoff. Arguments are presented which suggest an inner cutoff in the disk, and in fact a completely cutoff profile 1(r) = lo exp [- (ar + fl3r -3)] fits the underlying disks very well. This diminishes the distinction between Freeman's Type I and Type II profiles-Type II behavior may require only a spheroid small and faint enough to uncover a cutoff which exists in many disks anyway. The disks studied become l fainter when the spheroid contribution is removed. Motivated by this, we use model galaxies to investigate the effect of the spheroid on the B(0)c value derived for a composite profile. The models are sums of exponentials and de Vaucouleurs laws, each with a wide range of parameters. They reproduce very well the distribution of B(O)c values observed by Freeman. Apparently the near universality of B(0)c = 21.65 + 0.30 B is due largely to the spheroid contribution. Real disks can be much fainter. Measurements of galaxies with no spheroid are found to be consistent with this conclusion. Subject headings: galaxies: photometry - galaxies: structure Publication: The Astrophysical Journal Pub Date: October 1977 DOI: 10.1086/155589 Bibcode: 1977ApJ...217..406K Keywords: Astronomical Photometry; Brightness; Compact Galaxies; Galactic Structure; Light Curve; Astronomical Catalogs; Astronomical Models; Curve Fitting; Disks (Shapes); Iterative Solution; Spheroids; Astrophysics full text sources ADS | data products SIMBAD (8) NED (8)