Astron. Astrophys. 320, 957-971 (1997)

8. Morphology of the emission

The different positions of the emission maxima for the different rotational transitions of can be explained by the combination of the spatial distribution of the hydrogen density, column density of , and the overlap of two molecular clouds, with distinct radial velocities and physical conditions, along the line of sight. The absorption seen in the J=5-4 transition does not have an important influence in the spatial shift of the maxima since the intensity of the J=5-4, J=8-7 and J=12-11 lines only decreases by 20% due to the hot diffuse envelope in front of the continuum source.

In the lower panels in Fig. 8, we show the derived physical conditions along a right ascension cut at the Sgr B2M declination. The hydrogen density, the column density and the kinetic temperature have a maximum towards Sgr B2M, decrease abruptly towards the east but smoothly towards the west. Under these conditions the intensity maxima for the lines should peak at different positions for the different rotational lines. The higher J lines will peak where the column density and hydrogen density are highest. The second panel of Fig. 8 shows the expected line intensity for the three lines along the cut, while the upper panel shows the observed .

Fig. 8. Physical conditions, and predicted and observed line intensities along a right ascension strip at the declination of Sgr B2M for the 55-65 component. Panel a observed of the transitions J=12-11, J=8-7 and J=5-4. Panel b Predicted for the transitions J=12-11, J=8-7 and J=5-4 K=0,1 as a function of the offset in right ascension along a constant declination strip. Panels c -e values of , column density and n(), along the strip.

One also must consider that the integrated intensity maps (Fig. 2) include the emission from other molecular clouds with different radial velocities. The lower density gas at 67-78 in the northwest of Sgr B2M excites the J=5-4 transitions but not the J=8-7 and J=12-11 lines. This makes the integrated intensity emission to shift to the northwest of Sgr B2M. In Fig 8. we also find a high hydrogen density at the interface between the hot cores and the ambient cloud towards positive right ascentions. This should be further investigated with higher resolution.

© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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