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Astron. Astrophys. 342, 671-686 (1999)

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5. Discussion and conclusions

We have presented the ionized-gas and stellar kinematics, measured along the major axis, for a sample of six early-type spiral galaxies. (Due to the high values of [FORMULA] in their inner regions, the gas RCs can not be used as circular-velocity curves.)

For NGC 2179 and NGC 2775, we have modeled both the stellar and the gaseous kinematics to derive the mass contribution of the luminous and dark matter to the total potential, improving on the efforts by Kent (1988) from gas kinematics alone.

We have found that the innermost kinematics ([FORMULA]) is very well and uniquely reproduced by taking into account the two luminous components. In the (very luminous) early-type spirals considered here, there is a large inner region in which (essentially) light traces the mass and the DM is a minor mass component. This is agreement with the `weak' maximum disk paradigm proposed by Persic & Salucci (1990), but in disagreement with the claim by Courteau & Rix (1998) according to which in the most luminous spirals DM is a protagonist at essentially any radii.

More in detail we have found that in NGC 2179 the combined stellar and gaseous rotation data (measured out to about [FORMULA]) require the presence of a massive dark halo. In NGC 2775, more luminous and massive than NGC 2179, we can rule out a significant halo contribution out to [FORMULA]. This result complies with the general trend of mass distribution known for later spirals (Persic et al. 1996).

Salucci & Persic (1997), considering a large number of galaxies of mixed morphologies (ellipticals, late spirals, dwarfs, and LSBs), have suggested that the halo structural parameters and the connection between the dark and the luminous matter show a strong continuity when passing from one Hubble Type to another. Ellipticals, considered as luminous spheroids, and spirals, considered as luminous disks, are evidently very different systems, markedly discontinuous in terms of the distribution and global properties of the luminous matter. However, in the structural parameter space, ellipticals and spirals are contiguous, the main difference being that the former are more concentrated in both the dark and luminous components, probably due to the baryons' dissipational infall being deeper in ellipticals than in spirals (e.g., Bertola et al. 1993). If so, it is hardly surprising that Sa galaxies, being in some sense intermediate systems consisting of a luminous spheroid embedded in a luminous disk, fit in the regularity pattern of the dark-to-visible mass connection shared by ellipticals and spirals. In Fig. 15 we plot our derived dark-to-visible mass ratios at the farthest measured radii for NGC 2179 and NGC 2775 (filled circles) onto the distribution, derived by Salucci & Persic (1997) for galaxies with the same visible mass. The agreement is good. Even if the present result on early-type spirals is preliminary and without pretending to draw general conclusions from one particular case, it nevertheless seems to agree with the idea that, for galaxies of all morphological types, the dark-to-luminous mass ratio at any given radius depends only on the (luminous) mass of the galaxy.

[FIGURE] Fig. 15. The dark-to-luminous mass ratio as a function of radius (normalized to [FORMULA]) for late spirals of same stellar mass as NGC 2179 and NGC 2775 (solid lines ), compared with our derived values for NGC 2179 and NGC 2775 (filled circles )

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© European Southern Observatory (ESO) 1999

Online publication: February 23, 1999