THE MARSEILLE OBSERVATORY H SURVEY OF THE SOUTHERN GALACTIC PLANE AND MAGELLANIC CLOUDS

The Survey of the Galactic plane

The aim of this survey is to give a more precise picture of the spiral structure traced by the star formation regions, especially the HII regions, and by the diffuse H emission. The method is to determine the ionized hydrogen spatial distribution by splitting the different H emissions met along the lines of sight from the velocity measurements.

The interpretation of the H data is helped by the use of other surveys at wavelengths not affected by absorption due to the presence of dust (e.g. continuum radio 5 GHz, H photographs, radio recombination lines, CO, IR). Indeed, HII regions are often grouped together at the edge of molecular clouds. It is then essential to use the Galactic plane surveys already performed in CO lines, radio recombination lines and radio continuum 5GHz emission in order to separate the different HII region-molecular cloud complexes met along the line of sight by itemizing the objects forming the same complex (e.g. molecular clouds, HII regions, OB stars) and to identify the dynamical motions inside HII regions (e.g. the champagne effect and bubble-like expansion) that we must take into account to determine their systemic velocities. When complexes are identified, we determine the kinematic distance from the systemic velocity (using a Galactic rotation model) and/or the stellar distance when the exciting stars are identified. In general, the stellar distance, based on spectrophotometric data, hence free from kinematic biases, will be favoured instead of the kinematic one. But often we have only the kinematical information. In such a case, when the H emission exhibits deviations from circular rotation, the distance determination is impossible

To determine the spiral structure of our Galaxy we must perform such a study in numerous directions. Consequently about 300 fields (38'X38') were observed, forming mosaics of 10 to 20 fields distributed along the galactic plane (234 l < 355 and -2 b < 2). An example of a mosaic is presented in Fig.2 with the corresponding velocities on Fig.3. The directions observed in H, which correspond to HII-region-rich zones and some strategic lines of sight in relation to the expected spiral structure (e.g. arm tangential directions), were selected on the basis of large scale photographic data (Georgelin and Georgelin, 1970) and the 5 GHz radio emission survey (Haynes and al., 1978).

In the present state of the Survey eight directions are fully observed (298 (Russeil, 1997), 301, 305, 308, 313, 320, 328(Georgelin et al., 1994) and 338(Georgelin et al., 1996)), and four zones remain to be completed (283, 290, 332, 350). Although each direction is kinematically particular, common features can be noted: 1) The diffuse emission is always detected, even in "empty" fields (see Fig.1, upper profile). Usually, it exhibits roughly the same velocities as the discrete HII regions, suggesting a physical link. Located in the arms the diffuse emission is sometimes the only tracer of the spiral structure, as it is evident in the 298 direction (Russeil, 1997). 2) We observe a systematic local diffuse emission ( 170pc) which may be linked to the near molecular clouds (e.g. Coalsack, dark clouds) and the OB Sco-Cen stellar association. 3) We have collected a large number of HII regions newly detected in H. Let us cite for example the H detection of a clump of distant HII regions located at 10kpc ( such as the HII region revealed by component 4 in the lower profile of Fig.1). 4) Several HII regions exhibit velocity departure from the circular rotation model making impossible the determination of their distance.

However, the four spiral arm structure, previously proposed by Georgelin and Georgelin (1976), is confirmed, but we have to wait for the complete survey to precise potential substructures. In parallel, we plan to compare the H observations with surveys carried out at other wavelengths to connect our different zones to larger scale observations, to quantify the absorption, to derive the physical conditions within the HII regions and to identify the nature of deeply embedded objects.

© Copyright Astronomical Society of Australia 1997