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

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3. Results

3.1. Morphology of the region

Fig. 1 shows our collection of profiles of [FORMULA] lines towards Sgr B2M and Fig. 2 shows the integrated line intensity maps of the [FORMULA] J=11-10 line, [FORMULA] J=5-4, J=8-7, J=12-11 lines and [FORMULA] J=5-4 line. In these maps the positions of the ionized gas (HII regions) have been obtained from the recombination line emission and are shown by filled triangles. The angular resolution of our data only allows us to separate the two main strong complexes of HII regions: Sgr B2M and Sgr B2N. The offsets in the maps are measured relative to the Sgr B2M position ([FORMULA] =17:44:10.6, [FORMULA] =-28:22:05).

[FIGURE] Fig. 1. [FORMULA] J=5-4, J=6-5, J=8-7 and J=12-11 transitions towards Sgr B2M. The vertical lines indicate the position of the main isotope K components. The K=0 [FORMULA] component is also marked. The J=5-4 K=4 and J=6-5 K=5 lines are seen in absorption. Radial velocities are computed from the main isotope K=0 components.
[FIGURE] Fig. 2. Integrated line intensity maps towards Sgr B2 of a [FORMULA] J=11-10, b [FORMULA] J=5-4, c [FORMULA] J=5-4, d [FORMULA] J=8-7 e [FORMULA] J=12-11. a Velocity interval: 45 [FORMULA]. Contour levels of 5.5 K [FORMULA]. Lowest level: 2 K [FORMULA]. b Velocity interval: 93 [FORMULA] comprising transitions K=0,1,2,3. Contour levels of 10 K [FORMULA]. Lowest level: 7 K [FORMULA]. c Velocity interval: 93 [FORMULA] comprising transitions K=0,1,2,3. Contour levels of 14 K [FORMULA]. Lowest level: 5 K [FORMULA]. d Velocity interval: 80 [FORMULA]. Contour levels of 33 K [FORMULA]. Lowest level: 43 K [FORMULA]. e Velocity interval: 100 [FORMULA]. Contour levels of 47 K [FORMULA]. Lowest level: 20 K [FORMULA]. The lowest contours correspond approximatedly to 5 [FORMULA]. The filled triangles show the position of the Sgr B2M and Sgr B2N HII regions derived from the [FORMULA] line maps. The HPBW of the telescope is shown in the lower right corner.

The map of emission from the [FORMULA] J=11-10 line is almost identical to that from the [FORMULA] J=5-4. Both arise from a ridge which has a total extent of [FORMULA] in the North-South direction and [FORMULA] wide in the East-West direction. This ridge is centered on Sgr B2M. The most intense emission arises from a 4' long ridge. Toward the north, the ridge bends to the east. This structure is similar to that seen in the dust emission at 1.3 mm by Gordon et al. (1993). Although the J=8-7 [FORMULA] line map is less complete, the bow-like shape is less pronounced in the J=8-7 than in the J=5-4 line map. The maximum of the molecular emission for the J=5-4 [FORMULA] and J=11-10 [FORMULA] transitions is [FORMULA] west of the peak of Sgr B2M and [FORMULA] west of the peak of Sgr B2N. These offsets between the molecular and ionized material cannot be due to pointing errors because both, molecular and line emission were observed simultaneously. The offset between the HII regions and the molecular emission changes with the observed molecular line. The molecular emission of the J=8-7 line peaks closer to Sgr B2M and extends from [FORMULA] to [FORMULA] west of the HII regions. The maximum emission from more highly excited [FORMULA] peaks towards the HII regions: the [FORMULA] J=12-11 transition only shows two maxima which coincide with the position of the HII regions. As it will be discussed in section 8, the different locations of the molecular maxima for each transition are caused by a combination of optical depth and temperature effects.

Because of sensitivity, the [FORMULA] J=5-4 line is less extended than the main isotope but shows a spatial distribution similar to the high J lines of [FORMULA] ; this emission peaks very close to Sgr B2N.

3.2. The kinematics in the Sgr B2 molecular cloud

It is well known that Sgr B2 shows a very complex kinematic structure with several molecular clouds at different radial velocities along the line of sight (see for example, Martín-Pintado et al. 1990). In fact, most of the line profiles do not show a gaussian shape, indicating the presence of several velocity components along the line of sight. This is illustrated in Fig. 3 where we show the spatial distribution of the [FORMULA] J=11-10 line for different radial velocity intervals. The kinematic structure of the molecular gas has been derived from [FORMULA]. The multiple K component structure for [FORMULA], which overlaps in velocity in Sgr B2, makes the spatial structure derived from this molecule very uncertain. Low radial velocities are mainly found towards the south, while high velocities (67-78 [FORMULA]) are towards the north. This structure is confirmed by gaussian fits to the profiles of the [FORMULA] 11-10 lines measured towards Sgr B2. We find that the peak velocities obtained from the gaussian fits mostly fall into four intervals: 44-54 [FORMULA], 55-66 [FORMULA], 67-78 [FORMULA] and 90-120 [FORMULA], which correspond to four different clouds (de Vicente, 1994). These velocity groups overlap along the line of sight, but can be separated in position.

[FIGURE] Fig. 3. Integrated intensity maps of [FORMULA] J=11-10 for the radial velocity intervals given in the lower left corner of the panels. Contour levels correspond to an integrated intensity of 1.3 K [FORMULA] being the lowest level 1 K [FORMULA].

This velocity structure is consistent with that described by Martín-Pintado et al. (1990) who proposed the existence of 3 molecular clouds with radial velocities of 55 [FORMULA], 65 [FORMULA] and 80 [FORMULA], in front of the HII regions. The lowest radial velocity material (44-54 [FORMULA]) is located to the south of Sgr B2M. This shows a velocity gradient from 45 [FORMULA] to the south, to 50 [FORMULA] to the north of this cloud. The bulk of the emission (55-66 [FORMULA]) has an elongated structure that overlaps in the south with the cloud at 44-54 [FORMULA] and in the north with the 67-78 [FORMULA] cloud. The 67-78 [FORMULA] material is located in a cloud with a nearly triangular shape, with its lower vertex on Sgr B2M and the upper vertices offset ([FORMULA]) and ([FORMULA]) from Sgr B2M. For velocities higher than 90 [FORMULA] the molecular gas is concentrated in a cloud of [FORMULA] diameter placed to the southwest of Sgr B2M.

3.3. The absorption line in [FORMULA]

Our data also show absorption lines for [FORMULA] towards Sgr B2M. Surprisingly, the absorption lines are only observed for the J=5-4 and J=6-5 lines of [FORMULA] in the J=K component (see Fig. 1). In agreement with the absorption lines observed in other species like [FORMULA] (Wilson et al. 1982; Vogel et al. 1987; Hüttemeister et al. 1993) and [FORMULA] (Rogstad et al. 1974, Henkel et al. 1983; Martín-Pintado et al. 1990, Mehringer et al. 1993), the absorption in [FORMULA] towards Sgr B2M also occurs at a radial velocity of 65 [FORMULA]. The absorption lines in [FORMULA] were checked using different reference positions and shifting the central frequency of the receiver. These tests eliminate the possibility that the absorption line comes from emission in the reference position or from the image band. Therefore the absorptions are due to [FORMULA] and arise from gas located in front of the continuum sources in Sgr B2M.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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