Models of elliptical galaxies: NGC 3379, 4261, 4278 and 4472

Abstract

Summary We investigate whether the observed kinematics and surface photometry for the elliptical E1/E2 galaxies NGC 3379, 4261, 4278 and 4472 can be modelled under the assumptions of (i) axisyrnmetry, (ii) a distribution function of the form f = f (E, Lz,) and (iii) constant mass-to-light ratio. The methods used are an extension of the work of Binney, Davies & Illingworth (Paper I). Models satisfying the above assumptions fit the observations of all four galaxies remarkably well. For all galaxies, the rotation curves rule out isotropic velocity dispersion tensors. However, an excellent fit to the data can usually be obtained by including a measure velocity of anisotropy. Only in the case of NGC 4472 do the velocity dispersion profiles suggest that the radial and vertical dispersions are decoupled and thus the distribution function depends upon a third integral. In NGC 4278 the rotation velocity changes sign at R ⋍ 30 arcsec. Such behaviour is allowed by our models, but since it is associated with a large isophote twist, it is likely that it indicates a triaxial figure. We have developed a prolate model for NGC 4261 which accurately predicts the kinematics, although the comparison is limited to the rms line-of-sight velocities. We investigate how the predicted kinematics vary as a galaxy’s isophotes are varied from ‘boxy’ to ‘discy’ in shape merely by deforming the bulge rather than introducing a thin disc. A discy model rotates faster on the major axis than the equivalent boxy system, but the effect is too small to account for Bender’s suggested correlation of (v/σ)* with disciness (which exhibits a large scatter). In boxy galaxies the rotation velocity falls more slowly away from the equatorial plane than in the equivalent discy galaxies.