Abstract

As part of a long-term project to revisit the kinematics and dynamics of the large disc galaxies of the Local Group, we present the first deep, wide-field (similar to 42 arcmin x 56 arcmin) 3D-spectroscopic survey of the ionized gas disc of Messier 33. Fabry-Perot interferometry has been used to map its H alpha distribution and kinematics at unprecedented angular resolution (less than or similar to 3 arcsec) and resolving power (similar to 12 600), with the 1.6 m telescope at the Observatoire du Mont Megantic. The ionized gas distribution follows a complex, large-scale spiral structure, unsurprisingly coincident with the already-known spiral structures of the neutral and molecular gas discs. The kinematical analysis of the velocity field shows that the rotation centre of the H alpha disc is distant from the photometric centre by similar to 168 pc (sky-projected distance) and that the kinematical major-axis position angle and disc inclination are in excellent agreement with photometric values. The H alpha rotation curve agrees very well with the HI rotation curves for 0 R 6.5 kpc, but the H alpha velocities are 10-20 km s(-1) higher for R > 6.5 kpc. The reason for this discrepancy is not well understood. The velocity dispersion profile is relatively flat around 16 km s(-1), which is at the low end of velocity dispersions of nearby star-forming galactic discs. A strong relation is also found between the H alpha velocity dispersion and the H alpha intensity. Mass models were obtained using the H alpha rotation curve but, as expected, the dark matter halo's parameters are not very well constrained since the optical rotation curve only extends out to 8 kpc.