First results from the combination of the AAO/UKST and Marseille H surveys

Introduction

It is well established that H emission is the best tracer of star-forming complexes and hence of the spiral structure of galaxies (e.g. Hodge and Kennicut, 1983; Considère and Athanassoula, 1982,1988). For external galaxies, the distribution of star-forming regions along the spiral arms is generally evident from direct imaging. For our Galaxy the spiral arms are strung out along the line of sight which leads to the superposition and mixing of information from different spiral arm complexes making it difficult to distinguish them. Hence, to study the detailed large scale structure of our Galaxy, we need to identify, discriminate and collect star-forming complexes and determine their distances. Distances are based on the correct identification of HII region exciting stars within the complexes and/or on the determination of the systemic velocity. In practice, identification of HII region exciting stars is difficult beyond 6 kpc from the sun due to interstellar absorption (e.g. Russeil, 1998; Georgelin and Georgelin, 1971; Forbes, 1988). For the majority of HII regions, the only alternative is to obtain velocity information on the gaseous component. Although radio observations are useful in this regard they do not enable resolution of the distance ambiguity problem for regions inside the Galactic solar orbit. Here H information is essential to discriminate between the far and near distances proposed by the Galactic rotation model. Indeed, the detection of an H counterpart of a radio source or the detection of diffuse H emission at the same velocity as a star-forming complex will favour the choice of the near distance.

Determination of the spiral structure of our Galaxy is thus based on two important points: (i) the detection of H from HII regions and (ii) accurate determination of their systemic velocity. The first point requires deep H imaging to detect faint HII regions from different spiral arms. The second point requires HII region velocities (including information on internal motions and close environment) and a large field of view to permit detection and grouping of sources with velocity departures from circular rotation and large scale distribution on the plane of the sky. Observed velocity departures can be caused by energetic processes evolving in the galactic disk such as stellar winds from massive stars, supernovae explosions and impacts of high velocity clouds on the galactic disk. All these processes are sources of compression and acceleration of interstellar matter which can trigger star formation. Objects formed under these conditions may retain components of these accelerations (e.g. Rizzo & Arnal, 1998) which then contribute to velocity departures from the Galactic rotation model. Bubbles, shells and cavities in the interstellar medium (ISM) observable at different wavelengths are the remnants of such process induced star formation. It is important to identify these large structures and the HII regions they triggered for the best estimation of their systemic velocity.

In this paper we describe some initial combined results from two very different but highly complementary Galactic Plane (GP) H surveys which can help address these issues. These are the AAO/UKST and Marseille H surveys. In section 2 we summarize the characteristics of the two surveys and in section 3 present some preliminary results which illustrate the important additional insights that the combination of the two surveys offer for the study of Galactic structure.

© Copyright Astronomical Society of Australia 1997