1 Introduction

The study of the large scale spiral structure of our Galaxy requires us to identify and find the distances of the large star-forming complexes defining the arms. In order to do this we use the main tracers of star formation, particularly HII regions and their exciting stars and the molecular clouds where they were born.

The H$\alpha$ Survey of the Milky Way, being undertaken at La Silla (ESO) by the Marseille Observatory, allows us to probe the discrete HII regions, and the more diffuse H$\alpha$ emission, in the direction of the southern Galactic plane. Such a survey is necessary in order to resolve the distance ambiguity problem in the radio domain for lines of sight at l > 270$^\circ$ (two distances are then possible for a given velocity). Also, it allows us to observe the diffuse interstellar medium widely described by Reynolds (1979, 1984, 1987). Closely linked to HII regions from the kinematics point of view, the diffuse H$\alpha$ emission is sometimes the unique tracer of the spiral structure (Russeil 1997).

The H$\alpha$ emission of HII regions is the link between molecular clouds and the young hot stars, so it allows the determination of the stellar distance to the molecular clouds. Unfortunately, because of the interstellar absorption, the distances of stars can be estimated only for the nearest ones. Moreover, spectrophotometric data of southern early type stars are not numerous, so often kinematic information alone is available. H$\alpha$ detection is then essential to help in chosing between the two allowed kinematic distances.

Since the complexes are active star forming zones, they frequently exhibit small scale gas velocity variations from place to place (``champagne'' flows, internal motions, etc). In order to determine their systemic velocities we have to quantify these variations gathering the multiwavelength kinematic information of the different sources forming a complex. Then we determine the distance using a rotation model.

The present study covers the Galactic longitude and latitude ranges l = 301$^\circ$ to 324$^\circ$ and b = -2$^\circ$ to 2$^\circ$. Here we present only the data, further interpretation in terms of spiral structure being required to put this zone into relation with the adjacent ones. Georgelin et al. (1987, 1988) have already examined the HII regions in this area, but the new observations from our H$\alpha$ survey of the southern Galactic plane clearly show the necessity of this more detailed study.

The radio continuum at 5 GHz (Haynes et al. 1978) in this longitude range exhibits either radio sources in close groups (l $\sim$ 306.5$^\circ$  and l $\sim$ 317$^\circ$) or larger groups (l $\sim$ 311$^\circ$  - 313$^\circ$) or over an extended emission area (l $\sim$ 307.6$^\circ$, l $\sim$ 300$^\circ$, 304$^\circ$ and l $\sim$322$^\circ$  - 324$^\circ$). In any one group, it is not rare that sources exhibit different velocities. Indeed, for such a line of sight several arms are going to be intercepted and a superposition of emissions located at different distances is usually expected, in which case it is important to distinguish the different velocity groups and to place them in a coherent spiral structure. In Sect. 2 of this paper we will present observations and data reduction. Section 3 is devoted to the study of the kinematical informations between l = 301$^\circ$ and 324$^\circ$. The stellar information and distance estimations will be also discussed in this part of the paper. The conclusion is given in Sect. 4.

  

Figure 1: a) H$\alpha$ mosaic of the observed fields in the 301$^\circ$ zone. Each image is obtained by adding $\lambda$ maps over the whole free spectral range, flat fielding and then correcting for distortion. This is equivalent to a photograph obtained through a filter with a 10 Å bandwidth. The positions of radio sources and HII regions are shown with the corresponding H$\alpha$ velocity values, when observed

 

Figure 1: b) Spatial distribution of velocity components in the 301$^\circ$ area. Only the non-local components are indicated. Note that the velocities of the diffuse H$\alpha$ emission, as well as those of the discrete HII regions, are shown at the points of measurements. Bold letters correspond to H$\alpha$ emission patches in the mosaic. Very closely placed numbers represent multiple components at one point of measurement

  

Figure 2: Same as Fig. 1 for the 305$^\circ$ zone

 

Figure 2: continued

  

Figure 3: H$\alpha$ mosaic of the observed fields in the 308$^\circ$ zone. Each image is obtained by adding $\lambda$ maps over the whole free spectral range, flat fielding and then correcting for distortion. This is equivalent to a photograph obtained through a filter with a 10 Å bandwidth. The positions of radio sources and HII regions are shown with the corresponding H$\alpha$ velocity values, when observed. Velocities indicated in parentheses are for diffuse components present all over the field. The other velocities are for the points indicated. Only the non-local components are indicated. Bold letters correspond to H$\alpha$ emission patches in the mosaic

  

Figure 4: Same as Fig. 3 for the 313$^\circ$ zone

  

Figure 5: Same as Fig. 3 for the 320$^\circ$ zone