5. Discussion and conclusion
We have modelled the gravitational potential of 4 elliptical galaxies using a triaxial mass distribution. The models are able to reproduce the observed velocity fields in a satisfactory way. In particular, in the case of NGC 1453 the model reproduces the non-circular motion along the minor axis of the gaseous disk (PA ) at least in a qualitative way. Also in the case of NGC 2974 there is evidence of non zero velocities along the minor axis (PA ) but in this case our model is not able to reproduce this effect.
For NGC 5077 we find good agreement between the model and the observations. NGC 7097 is the only galaxy for which the model was not been able to reproduce the observed gas kinematics. The discrepancy occurs mostly at a single position angle. We remark that the improvement given by the triaxial modeling is not only in reproducing the velocity field, but also the two dimensional shape and the viewing angles of the galaxy. The present method gives also a result that is not strongly model dependent. The only important assumption is that the gas has settled in the principal plane and that it is moving in ordered orbits.
In 2 out of 4 objects (NGC 2974 and NGC 7097), the gas is moving in the plane to the short axis while in the other two cases (NGC 1453 and NGC 5077) the gas is on a plane to the major axis as expected if the gas has an external origin. The resulting profiles are nearly constant with radius. It has to be noted that, even if the model has been convolved for the seeing, both the mass and the light density distribution (and profile) are not reliable in the inner arcsecs. From simple numerical simulation we estimate the limit of to be the limit inside which the derived profile can be disturbed by the seeing. Moreover the ionized gas velocity fields of the sample galaxies have a central peak in the velocity dispersion. In the inner the velocity dispersion that for NGC 2974 reaches the value of . This high value can be attributed only partially to an effect induced by the seeing. Indeed, when the central velocity gradient is high, the seeing blurs the spectra lowering the central velocity gradient and increasing the value of the velocity dispersion. By means of numerical simulations we found that, for NGC 2974, the seeing could produce a value of the velocity dispersion not higher than and hence cannot account for the observed value of . It is possible that in this central region the ionized gas can be supported by pressure (Bertola et al. 1995, van der Bosch & van der Marel 1995).
The largest radial variation of our derived profiles is an increase by a factor of 2 from the center to the outer regions of NGC 5077. For the other 3 galaxies we observe a constant value (in NGC 7097) and a decrease by a factor of 1.6 and 1.3 (in NGC 2974 and NGC 1453 respectively). The fact that when all galaxies are averaged we do not observe an overall trend (i.e. a systematic increase of as we expect if dark matter is present) indicates that luminous matter dominates the mass distribution inside . seems to be higher for objects of higher luminosity and the mean value of is about 5 . This value matches well the mean value of found by van der Marel (1991) for galaxies of the same mean luminosity. Previous dynamical studies of the ESO Key Programme (Saglia et al. 1993; Bertin et al. 1994) also are consistent with the present indication that is approximatively constant out to at a value of about 5 . We found the presence of dark matter in 3 out of the 9 objects of the sample whose kinematics was measured typically out to 1- . We can consider this as an indication that the central regions of elliptical galaxies are dominated by the luminous matter while dark matter begins to be dynamically important at 2- . Only beyond this limit the presence of dark matter is compelling (Bertola et al. 1993; Carollo et al. 1995).
The use of ionized gas kinematics described in the present work provides a good determination of the mass-to-light ratio in the inner regions of elliptical galaxies (inside ), where it turns out to show only little variations. It is encouraging to note that our analysis, based on a method which is completely independent from the use of the stellar kinematics, leads to results that are in good agreement with those obtained with the most recent stellar dynamical models.
To detect the presence of dark matter it is therefore necessary to determine at distances well beyond from the center. In the framework of the present Key Programme this means to use more extended gaseous disks, such as the HI disks which sometimes are observed in elliptical galaxies. As far as the use of the stellar dynamics is concerned, an improvement can be obtained by observing the line profiles at the farthest distances allowed by the state of the art detectors. The determination of the velocity dispersion together with that of the shape of the lines allows one to obtain results which are not model dependent. These results could be compared with those, which are now in a way of rapid accumulation, based on the X-ray halo emission and on the dynamics of planetary nebula.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998