## 2. Mass modelsB91 used the family of separable triaxial models with constant density cores described by ZF to reproduce the ionized gas velocity field of NGC 5077 with a constant model. Hubble Space Telescope observations indicate that elliptical galaxies generally have cusped central surface brightness profiles (Crane et al. 1993; Jaffe et al. 1994). Accordingly, we adopt the same approach as in B91, but we use a family of non-rotating triaxial mass models with a central density cusp. They are triaxial generalization of the so-called -models (e.g., Carollo 1993; Dehnen 1993; Tremaine et al. 1994) introduced and described in detail by de Zeeuw & Carollo (1996, hereafter ZC). These models allow for variations of ellipticity and position angle of the major axis of the isophotes with radius. The mass models introduced by ZC are based on the spherical -models with density profile: This spherical density, and its associated gravitational potential, are functions of three parameters: . The first of these, (), measures the "cuspiness" of the central density profile which diverges as . For high values of we have a steep central profile while the case corresponds to the model with finite central density and a central density gradient intermediate between the King and Hubble profiles, similar to the separable models described by ZF, and applied by B91. is the total model-mass of the galaxy, and is a scale-length, which is related to (Dehnen 1993). The models are made triaxial by adding two low order spherical harmonics terms to the potential of the spherical models, with appropriately chosen radial functions, given in Eq. (2.5) of ZC. This introduces four free parameters in addition to , and , which can be chosen as the intrinsic axis ratios , , , of the triaxial surfaces of constant density at small and large radii, respectively. A stable configuration of cold gas in a nonrotating triaxial galaxy
can only be a ring or a disk in one of two principal planes, either
perpendicular to the long axis of the figure or perpendicular to the
short axis (Tohline, Simonson & Caldwell, 1982; Merritt & de
Zeeuw, 1983). We use the convention that the © European Southern Observatory (ESO) 1997 Online publication: June 5, 1998 |