D. Russeil , Q. A. Parker ,, PASA, 18 (1), in press.
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- Identification of a new large structure at l=298
- Morphology and nature of extended emission features
Some preliminary results
Identification of a new large structure at l=298
The galactic direction l=298 is known to point between two spiral arms, implying weaker interstellar absorption towards this region. In radio emission ten sources (assumed HII regions) are observed with positive velocity in this direction (Caswell and Haynes, 1987). Using the mean rotation curve of Brand and Blitz (1993) implies that these HII regions are located outside the solar circle at about 9.5 kpc from the Sun and hence are localised in the far part of the Carina arm.The MHS has surveyed the region and detected seven of these sources and several emission patches at the same velocity (Russeil, 1997). These HII regions, being quite remote, exhibit small projected angular sizes (typically <4 arcminutes). The MHS is unable to provide much 2-D structural information for most of them. However with the equivalent AAO/UKST H survey material the detailed morphology of these regions can be clearly seen. For example we can see (fig.1) that the known HII region G297.655-0.977 at 12h01m30.3s, -6305'04" (Caswell & Haynes, 1987) and a similar but newly discovered H/HII region at 11h59m45s, -6222'07" exhibit sharp, bright edges from where a more diffuse emission extends. This suggests they are both located on the edge of a parental molecular cloud located to their east. Such detail is completely lost in the equivalent MHS images. It is the first time that the precise morphology of these HII regions has been seen. In addition, the associated AAO/UKST survey image (field HA135h, exposure HA17941) reveals a large, faint, diffuse emission region (fig.2) which traces a semi-circular structure of about 2 diameter centered at 12h09m24s, -6235' (298.5-0.3). This coincides in its South-West limb with the HII regions G297.506-0.765 (12h00m33.5s, -6235'), G297.655-0.977 (12h01m30.3s, -6305'04") Caswell and Haynes (1987), the region at 11h59m45s, -6222'07" and several emission patches detected at the same positive velocity by the MHS. The MHS data yields a velocity of 28 km for G297.655-0.977 (fig.3) and 26 km for G297.506-0.765 as well as a mean velocity of 28 km for several H patches which are in good agreement with the radio data (Caswell and Haynes, 1987).
These combined observations strongly suggest a link between these bright regions and the large semi-circular structure with the giant molecular cloud n26 (center 298.8-0.2, Reff=124 pc with d=9.5 kpc, Vlsr=25 km ) listed by Grabelsky et al. (1988). Moreover the radius of the large structure (R166 pc with d=9.5 kpc) is close that of this molecular cloud n26. Planned follow-up multi-spectral observations using the MHS instrument in other sections of this large circular structure would allow a more precise determination of this intriguing feature's nature and kinematics.
Morphology and nature of extended emission features
From MHS observations, diffuse emissions often appear as uniform layers across the field or as discrete patches of H emission at different velocities. The high resolution of the AAO/UKST survey allows clarification of the nature of these patches as either filaments, sharp edges of bright clouds or as more diffuse structures. Such structural information can help us to understand the different velocity components and intensity variations observed in these sources with the MHS. One good example is again given by objects detected in the AAO/UKST field HA135h (exposure HA17941) and the associated MHS H emission observed around 11h50 -6215'. The MHS data in this region reveals strong localised diffuse H emission extending across the entire 40 arcminute field of view (see fig.1a in Russeil, 1997) and with a mean velocity Vlsr=-34 km . The drastic high intensity of this H emission is difficult to link with other fainter and smoother detected diffuse emissions at negative velocity seen in the vicinity. Such emission can however be attributed to the large SNR G296.05-0.5 (Caswell and Barnes, 1983). Many thin filaments around 11h49 -6220' evident in the equivalent AAO/UKST H survey film (fig.4) can clearly be linked to SNR G296.1-0.7 (Longmore et al., 1977; Whiteoak & Green, 1996) which is a portion of SNR G296.05-0.5.
The detailed morphological study of H emission permitted by the high resolution AAO/UKST survey images can also help in understanding the physical connection of different emission features when kinematic information is not alone sufficient. Consider the case of RCW48 (Rodgers et al., 1960). This region is located towards the galactic longitude 283 which is known to be tangential to a spiral arm. Hence, in this special circumstance, features can have a large range in distance without exhibiting a corresponding difference in velocity. RCW48 is a large ionized region appearing as a collection of thin filaments and exhibiting a center-edge velocity gradient (Deharveng and Maucherat, 1974; Chu, 1982). The MHS also detected this velocity gradient with the outer part having a mean velocity of about -8.6 km . There is also more extended patchy diffuse emission evident in the MHS data which exhibits a similar mean velocity of -6 km . Are these structures linked ? On the AAO/UKST film we can precisely delineate the structures belonging to RCW48 and clearly see a superposition with the more patchy background emission, with no obvious change of morphology or interaction evident between the two (fig.5). This strongly suggests that this general patchy emission and that from RCW48 are not linked. We can conclude that they are at two different distances tracing two different star-forming complexes.
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Title/Abstract Page: First results from the
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