THE SOMBRERO GALAXY .2. COLORS, KINEMATICS AND LINE STRENGTHS OF THE CENTRAL REGION

We present photometric (pre-COSTAR HST and ground-based) and new two-d imensional TIGER spectrographic data of the central region of the Somb rero galaxy (NGC 4594, M 104). This gives B - V and V - R(C) images, a s well as 2D line strengths and kinematical maps (LOSVDs). Analysis of these data lets us draw a clearer picture of the morphology of the So mbrero galaxy: it contains a massive metal-poor bulge whose outer part seems to be pressure supported, an outer stellar ring (the main disc) with a roughly solar [Fe/H], a rapidly rotating, metal-rich inner dis c ([Fe/H] similar to 0.5), and a point-like nucleus. Using the deconvo lved HST image, we have detected dust structures in the central arcsec onds of M 104. We also mapped the distribution of ionized gas from the [NI] doublet (TIGER spectra), which strongly peaks in the centre. The presence of the [NI] emission lines significantly affected the measur ements of the Mgb line strength in the central arcseconds. The derived stellar LOSVDs exhibit strong asymmetries close to the major-axis lin ked to the presence of the dynamically cold inner disc. The central ve locity and velocity dispersion gradients are consistent with the sugge sted presence of a central dark mass of similar to 2 10(9) M(circle do t)(Emsellem & Qian 1996: Paper 3). Alternatively, a nuclear end-on bar could conceivably produce these gradients. We then analysed various c orrelations between the colours, line strengths and kinematics of the central region. In particular, the local escape velocity is confirmed to be a significantly better indicator of the local line strength than the local velocity dispersion. We propose that the present morphology represents the end-product of the secular evolution of the galaxy due to a (now dissolved?) stellar bar. In this context, dynamical resonan ces are suggested to play an important role in the building of the pre sent structures. We thus associated the outer dust and HT ring to the OLR of the presumed bar. The double stellar disc structure is then dis cussed in the light of the bar hypothesis, the ILR corresponding to th e transition region between the two discs. This hints of the formation of the inner disc by infalling of pre-enriched gas. Different mechani sms can be responsible for the building of the bulge and halo: e.g. an early minor merger event which could have triggered the formation of a bar, or a dissipational collapse. These scenarios have to be further examined, particularly in the light of the recent observations of edg e-on S0 galaxies, which show structures resembling the ones of M 104.