In external spiral galaxies the star formation complexes can be detected optically far from the galactic center. However in our Galaxy, optical star formation tracers situated at large distances from the Sun are not easily observable because of interstellar extinction. As a result, a large part of the Galaxy is optically hidden, principally toward the galactic center. Fortunately, in some particular directions, distant HII regions can be detected. This is the case toward galactic longitude l = , where the line of sight is found between two spirals arms, implying weak absorption effects. Eight distant HII regions in the Carina arm have been detected at this longitude using a 36 cm telescope in La Silla devoted to an H Survey of the Milky Way. Never before have so many HII regions been optically observed at such a distance (10 kpc) in the same field. Well known in the radio continuum (Haynes et al. 1978), only two of these had already been optically detected: 298.2 - 0.3 (Georgelin et al. 1979) and 298.19 - 0.77 (Henize 1967).
Some HII regions have been detected in H at large distances. For example W 58G at a distance of 12.4 kpc (Georgelin et al. 1988) and the farthest one detected by Degeus et al. (1993) at a distance of 21 kpc. These very distant regions allow us to probe the galactic gravitational potential but such HII regions are rare and isolated. The HII regions detected near l = are spatially grouped which suggests a common origin and behaviour.
The comparison of H maps with maps at other wavelengths allow a detailed analysis of each source. In particular, the infrared observations help to form an understanding of the H detections and reveal interesting object which have no H nor a radio emission counterpart.
After the description of the observations and results presented in Sect. 2, Sect. 3 is devoted to the study of star and star cluster distributions. In Sect. 4 we introduce a discussion about the infrared observations of the studied field. Finally, in Sect. 5, we discuss each HII region and in Sect. 6 are presented the conclusions.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998