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

The instrument

The instrument, located at the Southern European Observatory in La Silla, uses a 36 cm telescope equipped with a focal reducer (final F/D=3.3), a photon counting camera and a scanning Fabry-Perot interferometer, which provide the ideal characteristics for a systematic survey: large field (38'X38'), high luminosity, high spectral resolution and limited spectral range centered on the H line. In practice we use two different Fabry-Perot interferometers depending on the expected velocity range of the object to observe. They offer free spectral ranges of 376 km and 115 km with respective spectrum samplings of 16 km and 5 km . For these interferometers the finesse is about 12 or 10, explaining why we scan through 24 scanning steps according to common sampling criterion. The recent determination of the flux detection limit is about 0.2 Rayleigh (1 Rayleigh = 2.4 erg at H) for a typical observation time of two hours. This allows us to observe not only the individual HII regions but also the diffuse emission of the ionized gas.

The interpretation of the data requires splitting of the observed profiles into elementary components as is illustrated in Fig.1. The night-sky lines (geocoronal H and OH lines) and the nebular ones are respectively modeled by a pure instrumental profile, and an instrumental profile convolved with a gaussian ( details are given in Georgelin et al., 1994).

 
Figure 1: H profiles resulting from the addition of several pixel profiles covering an "empty zone" (upper profile) and the HII region G296.593-0.975 (lower profile) in the direction l=298. The first profile is decomposed into two nightsky lines (components 2 and 3) and two nebular components ( components 0 and 1 ) associated to diffuse emissions. In the second profile, one more emission ( component 4) line appears at positive velocity, corresponding to the HII region.

 
Figure: H mosaic of the observed fields in the 305 area. This image is obtained by adding 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 2.4 Å bandwidth. The positions of radio sources and HII regions are shown with the corresponding H velocity values, when observed.

 
Figure 3: Spatial distribution of velocity components in the 305 area. Only the non-local components are indicated. Note that the velocities of the diffuse H emission, as well as those of the discrete HII regions, are shown at the points of measurements. Bold letters correspond to H emission patches in the mosaic. Very closely placed numbers represent multiple components at one point of measurements.

© Copyright Astronomical Society of Australia 1997