Observations and dynamical modelling of the E4 galaxy NGC 2974: evidence for an embedded stellar disc

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Abstract
It has recently been realized that some, and possibly very many, galaxies classified as elliptical might contain an embedded stellar disc. Here we present a case study: NGC 2974, classified as E4. Discs of dust, neutral gas (HI) and ionized gas (H alpha) have all been detected in this galaxy. The observed stellar rotation is much higher than in typical elliptical galaxies, (V/sigma)* approximate to 1.5, and the isophotes are slightly pointed. We present new long-slit spectroscopic data along the major, the minor and an intermediate axis of NGC 2974. From these data, we determine rotation velocities, velocity dispersions and deviations of the velocity profiles from a Gaussian, as quantified by the Gauss-Hermite moments h(3) and h(4). On the major axis, the velocity profiles are asymmetric, with the asymmetry changing sign upon going from one side of the nucleus to the other. We construct detailed dynamical models consisting of an axisymmetric bulge and a thin exponential disc. We study disc-bulge combinations that fit the observed surface brightness, ellipticity and a(4) profiles, and solve the equations of hydrostatic equilibrium for the disc and the bulge in the total potential of the system. The predicted kinematics and velocity profile shapes are compared with the data. To fit the observed ratio of major to minor axis rms motion without invoking the presence of a stellar disc requires the galaxy to have an intrinsic axial ratio q(t) less than or similar to 0.25. This is unrealistically flat: elliptical galaxies with apparent flattening q(a) less than or similar to 0.3 are never observed. An acceptable fit to all the available photometrical and kinematical data is obtained with a model inclined at similar to 57 degrees.5 that has an embedded stellar disc that contains similar to 7 per cent of the total light of the galaxy. The disc has V/sigma greater than or similar to 3 and is thus rotationally supported. At a larger inclination angle, NGC 2974 would presumably have been classified as an SO. We model the ionized gas kinematics of NGC 2974 presented by Amico et al. and Zeilinger et al. by assuming the gas to consist of individual clumps that orbit as test particles in the potential defined by the bulge and the embedded stellar disc. The model provides an excellent fit to the observed streaming velocities of the ionized gas, indicating that these are consistent with the potential derived from the stellar kinematics, and the asymmetric drift implied by the observed non-zero velocity dispersion of the gas. The kinematics of the stellar disc and the ionized gas disc are very similar, suggestive of a common evolutionary history. The total luminous mass of NGC 2974 is M(lum) = 2.4 x 10(11) h(50)(-1) M(circle dot) . The HI data presented by Kim et al. indicate that NGC 2974 has a dark halo with M(halo)(< 3.6 R(e)) = 3.6 x 10(11) h(50)(-1) M(circle dot) . In the region of our spectroscopic data (less than or similar to 0.5 R(e)) the dynamical influence of the dark halo is negligible.

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