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4. VELOCITY PROFILES

One of the most standard technique for the determination of mass profiles relies on the abundant HI content of disk galaxies and its observed kinematics (see [1] and references therein). Some concerns were raised by the lack of spatial resolution in the central region which impairs the application of decomposition techniques to constrain the relative contribution of the stars, gas and dark matter. The use of Halpha mapping (via Fabry-Perot interferometers) seems to properly address this issue, although the complex dynamics expected in the central regions of spiral (and barred) galaxies enters then as an extra complication. A remarkable sample of 329 Halpha rotation curves of field spirals was obtained by Vogt et al. [15]. I should also mention the unique coverage of Virgo spirals conducted by Chemin et al. [3], which will permit a detailed study of the mass profiles as well as the impact of environment on the gas content, distribution and kinematics.

Two-dimensional maps are certainly a requirement if we wish to disentangle the global (circular?) motion from the effect of density waves. Via the combined use of high resolution ionised (Halpha) and molecular (CO) gas velocity fields, Simon et al. [16] constrained the mass variation with radius in the dwarf spiral NGC 2976. The addition of multi-colour optical and near-infrared images allowed the authors to suggest that the stellar mass fraction is relatively high in the central region, with the dark matter having then a profile shallower than rho propto r-0.17, with the caveat that the obtained central M / LK may be too low to be accounted by normal stellar populations. A similar study, but this time of a flocculent isolated spiral NGC 4414, had been conducted by Vallejo et al. [17], who combined high resolution CO data with extended HI rotation curve, to conclude that the mass-to-light ratio cannot be constant throughout the galaxy, but that dark matter has certainly a nearly negligible role in the central 7 kpc.

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