Astron. Astrophys. 361, 500-506 (2000)

3. Results

Fig. 1 gives an overview over our mapping results. An optical image is shown on the left (as obtained from the Digitized Sky Survey, DSS). Our CO detections (as indicated by the box) are clearly located outside NGC 3077 (the elliptical object north-west of the centre). The area where we obtained an almost complete map in the 2 lowest CO transitions is shown in Fig. 1 (right). The CO emission is clearly extended over several kpc (11 kpc) and can be subdivided into at least two separate complexes. A third cloud has been detected towards the east of the two complexes. This area has only partly been mapped by us as yet and an average spectrum is presented in Fig. 3.

Fig. 1. Overview of the observed region. Left: optical image of NGC 3077 (elliptical object to the north-west) and its surrounding (taken from the digitized sky survey). For a complete optical picture of the M 81 triplet see Walter & Heithausen (1999). The box indicates the region which we mapped in CO; the open circle indicates the position towards which we searched for CO but couldn't detect any. Right: Blowup of our integrated CO map (J=10 transition). Contours are every 0.12 K () starting at 0.12 K . Observed positions are indicated as small circles. The beamsize is indicated in the lower right corner of the map; it corresponds to 330 pc at an adopted distance of 3.2 Mpc.

Fig. 2. Channel maps of complexes #1 and 2, averaged over 5 . Center velocities of each channel are indicated in the lower right corner. Contours are every 0.012 K () starting at 0.012 K. Offsets are relative to ;

Fig. 3. Average CO (top) and (bottom) spectra of complex #3.

Table 1 summarizes some observed and derived parameters for the three detected complexes. It also list a fourth position in the direction of a `Garland' object (Karachentsev et al. 1985a) where we could not detect CO (marked as a open circle in Fig. 1, left). In Table 1 the integrated CO line intensity, W(CO), the center velocity, , and the full width at half maximum of the lines, are derived from spectra averaged over the area with significant line emission of the single complexes. The radius is an equivalent radius of a circle surrounding the same area as where CO emission was detected ().

Table 1. Observed and derived parameters for molecular complexes

The velocity structure of complexes #1 and #2 is visible in the channel maps displayed in Fig. 2. Cloud #1 is extended from the south-west to north-east. Complex #2 is the southern more compact object. It shows evidence for further substructure. We expect these complexes to break up in more little clouds when observed at higher spatial resolution (see also the discussion in Sect. 5). Throughout the paper we discuss only properties of the whole complexes and do not subdivide them further, although especially for complex #1 there is evidence for substructure in both the channel maps (Fig. 2) and in the integrated map (Fig. 1).

In order to constrain the excitation conditions of the more extended, northern complex, we have observed it with the KOSMA 3 m radio telescope in the CO (32) transition. The spectrum is shown in Fig. 4 together with the (10) and (21) transitions convolved to the same angular resolution as that of the KOSMA spectrum (80", 1.3 kpc). The corresponding values derived from a gaussian analysis of the spectra are listed in Table 2. No CO (32) line emission was detected. The integrated line ratio for the lower two CO transitions is with no significant variation throughout complex #1. The upper limit for the ratio of the (32) line to the (21) line is ; implications on the excitation conditions are discussed in Sect. 4.1.

Fig. 4. CO spectra towards complex #1. The ) and (21) spectra have been convolved to an angular resolution of 80" (1.3 kpc), the size of the KOSMA beam at 345 GHz.

Table 2. Line parameter of CO spectra of complex #1

Towards complex #2 we find significant variation of the line ratio of the lower two CO transitions. Spectra towards the center of that complex are displayed in Fig. 5. The center position is easily detected in both the (10) and in the (21) transition, whereas in the surrounding positions the (21) transition is hardly detectable towards single positions. However, (21) emission is clearly present in the averaged spectrum. Values derived from a Gaussian analysis of the spectra for the center position and the average of the eight surrounding positions are listed in Table 3. The line ratio is for the center position and for the surrounding area. In Sect. 4.1 we discuss different beam filling and variation of the excitation conditions as possible causes for the variation of the line ratio.

Fig. 5. CO () (left) and (21) (right) spectra towards complex #2. The scale for each box is and mK.

Table 3. Line parameters of CO spectra of complex #2

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000
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