Brightness distributions in compact and normal galaxies. II - Structure parameters of the spheroidal component

Abstract

Systematic properties of galaxy spheroids are studied by using simple fitting functions to derive the brightness, size, and shape parameters of their brightness profiles. Fitting the de Vaucouleurs r114-law to the profiles of 16 red compact and 19 normal galaxies yields the following results. 1. Ellipticals with close, massive neighbors have bright outer halos which are not possessed by more isolated objects. Possible tidal explanations are considered. 2. The brightness and radius parameters are related by Bov = 3.02 log r0 + 19.74 B mag arcsec -2 The half-light radius r0 varies from 1.0 to 14.1 kpc, and the brightness Bov at r0 varies from 19.8 to 23.3 B mag arcsec-2. The parameter along the B0 (log r0) relation is absolute magnitude. With much scatter, more luminous galaxies are more diffuse (i.e., have larger r0 and fainter Bov). 3. Most red compacts are not abnormally compact. 4. The B0 (log r0) relation can be used to measure relative distances. We use it to examine the consistency of velocity distances to galaxies in the Virgo cluster versus galaxies 1.6-3.5 times farther away. This suggests that Virgo is closer than its redshift distance by (m - M) < 0.07 + 0.23 mag, a null result supporting the uniform Hubble flow advocated by Sandage and Tammann. In the Appendix, detailed comparisons are made of how well the fitting functions of de Vaucouleurs, Hubble, and King describe galaxy profiles. The Hubble and King functions fully confirm results obtained above with the de Vaucouleurs law. Relations among the parameters of different models show that they all measure the same physical quantities. To varying degrees, the Hubble and King models fit tidal halos, while the r 114-law does not. However, the de Vaucouleurs law fits unperturbed galaxies best, and is generally the most convenient fitting function. Subject headings: galaxies: photometry - galaxies: structure