Astron. Astrophys. 319, 788-795 (1997)

5. Discussion

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 :
G 295.144 - 0.628
This region exhibits intense and extended radio and infrared emission. Its color indexes are those of a classical HII region despite its relatively low luminosity ratio (Table 3). We find an H velocity of 30 km  , quite different from the H109 velocity (38 km  ). The velocity measured at H wavelength shows that this region is possibly associated with the complex at 10 kpc.
G 295.760 - 0.200
This small H emission is easily detected and coincides with very weak IRAS and radio emissions (it is the weakest radio source investigated by Caswell & Haynes 1987) and exhibits the highest / ratio (see Table 3). This source is surrounded by an extended H emission at the same velocity, without any radio continuum counterpart. It is impossible to know whether this emission is more extended, and forms for instance a shell around the source, because the line of sight almost crosses the dark cloud DC 295.8 - 0.3 seen to the Southwest of the region.
G 297.506 - 0.765 and 297.655 - 0.977
Both sources exhibit compact H emission. A nebula of 43' x 17' size (BBW 379), situated between these sources, has been itemized by Brand (1986), who linked it with the  x  H region centered around 297.7 - 0.4 (Georgelin & Georgelin 1970). He made a CO velocity measurement of 30.8 km  between these two radio sources (l = 297.58^o, b = -0.87^o) however we detect no H emission with positive velocity there. This CO emission is probably associated with the distant sources but not with the HII region detected by Brand which is certainly a local maximum of the H diffuse emission with V = -2.6 km  .
[297.93 - 1.75] (12 3   -63^o24')
Extended H emission, not mentioned in the literature, can be seen at 12 03 31   -63^o24' on Fig. 1a without any known counterpart in the radio continuum or infrared counterparts. It has a positive velocity and exhibits a small gradient: the brighest part being found at 30 km  and the faintest one at 23 km  .
G 298.187 - 0.782
This small, very bright, H source exhibits the smallest velocity and a slightly larger than average Doppler broadening. It also appears as a strong source on radio surveys (Caswell & Haynes 1987). Its true nature still remains unclear: planetary nebula (it is known as He 2 - 77 or PK 298 - 0.1) or normal HII region? In early observational studies it has been classified as a suspected planetary nebula (Sanduleak 1976). De Muizon (1988) finds the presence of far-infrared emission lines of ionisation stages commonly observed in planetary nebulae but not seen in HII regions, a total infrared luminosity between 1250 and 1800 and a mass of gas estimated to 6 10^-2 assuming a distance of 0.5 kpc. Then she suggests that this source is a planetary nebula rather than a compact HII region. However, the fact that it follows the infrared color criteria of Wood & Churchwell (1989) and Hughes & Mac Leod (1980) (see Sect. 4) and not those of Planetary nebulae (Emerson 1987) suggests it is an HII region. Moreover the kinematic studies leading to a 9.5 kpc distance, significantly modify the physical values. Adopting this last value of distance, the total infrared luminosity and the mass of gas values become respectively 5 10⁵ and 22 which are typical of normal HII regions. Caswell & Haynes (1987) from electron temperature and observed flux density at 5 GHz already argued that it is a normal HII region.
G 298.228 - 0.331
This region has strong infrared and continuum radio emission, but is barely visible in our H survey field. Its very small angular dimension, and the bright star found almost in the same line-of-sight, render the H profile extraction tricky. Nevertheless, it is the first HII region of this distant complex detected at H wavelength using a higher spatial resolving instrument (Georgelin et al. 1979). Two extended H emission regions are detected in the same field: around 298.43^o - 0.35^o (12 09 9 , -62^o34') and 298.36^o - 0.22^o (12 08 39.5 , -62^o26') which are almost certainly connected with the 298.228^o -0.331 radio continuum extension. These two H emissions exhibit respective velocities of 23 km  and 31.6 km  , comparable with the velocities of the other emissions.

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 = 298.8^o b = 0.2^o) and 24 (l = 297.4^o b = -0.5^o) 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 12^o 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
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