5.1. The -2.6 km diffuse component
Detected all over the area studied, the -2.6 km component is the main diffuse H emission. Its intensity is, respectively, 3 and 8 times that of the H and OH night sky lines, and its velocity (V ) ranges from -0.6 to -5 km . A diffuse component with a V of - 4 km has already been detected at l = (le Coarer et al. 1992). Such a diffuse component may be the ionized counterpart of the nearby interstellar medium. An expansion motion of this interstellar medium has been shown (Crawford 1991) from the NaI and CaII absorption lines observed in the direction of 28 stars of the OB Sco-Cen association. Most of the components detected by Crawford exhibit negative velocities which are interpreted as matter out-flows due to the mass loss of the most massive stars of the association. Particularly, within the longitude range - , he shows several components with V between -4 and 0 km which can be associated with an envelope related to the Lower Centaurus Crux subgroup (LCC) (Blaauw 1964).
The -2.6 km diffuse H component is probably associated with this LCC subgroup at 130 pc (Degeus et al. 1989) and we can place it at the inner part of the local spiral arm.
5.2. The -25 km diffuse component
This emission is found all over the field with a -25 km mean velocity (velocity range: -21.5 to -29.5). The rotation model used (Brand & Blitz 1993), gives two likely kinematical distances for this emission: 5.2 kpc or 2.8 kpc.
We detected no discrete HII region with this velocity. However, some small regions, itemized by Brand (1986) exhibit CO velocity of the same nature: BBW 374 (V = -26.3 km , = 3.28 kpc), BBW 377 (V = -29.8 km , = 2.9 kpc) and BBW 384 (V = -25.7 km , =1.86 kpc). The two first regions are outside the area studied; the third one exhibits no H counterpart, suggesting that it is a reflection nebula.
In addition, a molecular cloud at l = , b = exhibiting a -25 km velocity (Cohen et al. 1985) and the large complex RCW 60-61-62 associated with the star group IC 2944 (Ardeberg & Maurice 1981) are at a distance of 2.5 kpc.
From the stellar distribution study (Sect. 3) we have found a maximum of early type stars at 2.5 kpc. We adopt this distance for the H diffuse component at -25 km . This diffuse emission allows us to link the Carina and Sagittarius spiral features. Humphreys (1972) already suggested that the presence of many supergiants between 2 and 4 kpc from the Sun might be an optical link between these spiral features.
Let us note that in this direction the diffuse component we detected is the unique velocity tracer of the nearest part of the Sagittarius-Carina arm.
5.3. RCW 64 (= G 299.363 - 0.257) and the -40 km diffuse component
This HII region, although rather bright at H wavelength, is barely detectable in radiocontinuum and on IRAS images. Its measured H velocity (V = -40 km ) is close to the H 109 (Caswell & Haynes 1987) and CO (Brand 1986) velocities (respectively -37 km and -40.4 km ). CO studies of Grabelsky (1988) show a maximum emission at l = and b = , with a -40 km velocity. This CO molecular cloud is probably associated with RCW 64. Besides, from Table 2, one can see that H2 CO and CO velocity components measured in the direction of some other HII regions are in good agreement with this molecular cloud presence. Let us note, also, that a diffuse emission is detected at -41 km at many places of the studied area.
Taking the Brand & Blitz (1993) Galactic rotation curve, with an orbital velocity of 220 km at the Sun radius (8.5 kpc), the RCW 64 velocity, exhibits a strong departure from the circular rotation, whatever the tracer used, which stops one from making any kinematical distance determination. Let us recall that the tangential point distance is 4.2 kpc in this direction, implying a minimum theoretical radial velocity of -30.3 km , which is already 10 km higher than the observed velocity. Such departure to pure circular rotation is found whatever the model of Galactic rotation used. It has been already mentioned by several authors (Humphreys 1971, Humphreys 1972, Alvarez et al. 1990, Brand & Blitz 1993). For example, Alvarez et al. (1990) exhibit, from CO terminal velocity measurements between l = and , velocities more negative by about 12 km than within the longitude domain - , a similar departure was also found by Brand & Blitz (1993) between l and (limit of their sample).
Fortunately, Brand (1986) identified some exciting stars of RCW 64 and derived a stellar distance of 5.37 kpc.
The galactic coordinates and distance of RCW 64 and its associated molecular cloud and stars place it between Carina and Centaurus arm. Then we cannot deduce to which arm it actually belongs. It may well be a spur between two arms (as one can observe in many other spiral galaxies). In order to confirm and delineate this possible spur, other H observations between galactic longitude and are needed.
5.4. The distant sources
Most of the sources detected at H wavelength
have positive velocities, which in this direction place them outside
the solar circle. It is the first time that so many distant HII
regions are detected. For all of these sources V
are in good agreement, and the H
emission coincides fairly well with the radio
continuum map. We discuss here some HII regions of special interest :
Except G 295.760 and G 298.187 all of the distant HII regions have velocities between +23 and + 31 km , suggesting that they belong to the same large complex. This complex may be related to the two molecular complexes presented by Grabelsky (1988) number 26 (l = 298.8o b = 0.2o) and 24 (l = 297.4o b = -0.5o) with respective velocities 25 km and 21.5 km . From the point of view of the galactic structure, these HII regions and molecular clouds can be grouped into a single complex, at a distance of 10 kpc using a flat rotation curve, = 8.5 kpc and = 220 km . This complex is fairly well located on a logarithmic spiral of 12o pitch angle, tracing the Carina arm.
G 295.76 and G 298.187 exhibit significantly different velocities (see Table 2). Their belonging to the same complex is far from proved. Moreover, H2 CO, OH and CO measurements given in Table 2, show some velocities with values significantly lower than 24 km , suggesting that there is another complex in the line of sight which may includes G 295.76 and G 298.187. However their individual kinematical distances lead to the values of distances 9.2 and 9.5 kpc, allowing them to still belong to the Carina arm. We conclude that all the HII regions of this study with positive velocities most probably belong to the farthest part of Carina arm.
© European Southern Observatory (ESO) 1997
Online publication: July 3, 1998