Astron. Astrophys. 327, 325-332 (1997)

3. Distribution in the galactic disk

Once the heliocentric distance for each cloud in our sample is determined (see Sect. 3.1.1), then the corresponding galactocentric distance R can be computed. Column 8 in Table  2 displays this parameter. The galactocentric distance distribution of the 177 outer Galaxy clouds is shown in Fig. 7. The sample appears complete for distances up to 15.5 kpc from the galactic center.

Fig. 7. Galactocentric distance distribution of the 177 outer Galaxy clouds for which physical properties have been derived. The sample appears to be incomplete for distances beyond 15.5 kpc from the galactic center

Fig. 8 shows the distance from the plane, z, as a function of the galactic longitude for all our clouds. To visualize the effect of warping and flaring of the molecular disk the ensemble of clouds has been divided in three groups depending on their galactocentric radius: a) between 9 and 11 kpc; b) between 11 and 13 kpc; and c) between 13 and 19 kpc. For our sample of clouds the maximum distance from the plane, z of about 700 pc below the plane, is found at around in galactic longitude. The thickness of the molecular disk increases with galactocentric radius from 200 pc for the nearest group to 800 pc for the farthest.

Fig. 8. Distance from the galactic plane as a function of the galactic longitude for our sample of clouds. To visualize the warping and flaring of the molecular disk the ensemble of clouds has been divided in three different galactocentric distances: a between 9 and 11 kpc, b between 11 and 13 kpc, and c between 13 and 19 kpc. The straight line indicates roughly the inclination of the plane at each range of distances. The flaring increases from 200 pc for the nearest group up to 800 pc for the farthest

The distribution of the clouds in the third quadrant projected on the galactic disk is shown in Fig. 9, where we have represented, arbitrarily, by larger filled circles the clouds which are more massive than . No grand design spiral pattern can be readily visualized from the projection of the clouds on the galactic disk. A similar result has been obtained by Wouterloot et al. (1990) using CO emission associated with IRAS sources in the second and third galactic quadrants.

Fig. 9. Distribution of the outer Galaxy clouds from our sample projected on the galactic disk. The Sun is at , . The circle segments correspond to galactocentric distances of 10, 15, and 20 kpc, respectively. The small and large filled circles represent clouds less and more massive than , respectively. No grand design spiral structure can be readily visualized from this figure

From the study of the physical properties of the 177 molecular clouds located in the third galactic quadrant, beyond 2 kpc from the Sun, we can conclude the following:

1. The clouds follow the size-line width and size-mean number density power-law relations found by other authors.
2. Under the assumptions made the clouds are in approximate virial equilibrium because they come relatively close () to fulfill the requirement of for clouds in virial equilibrium.
3. Adopting molecules cm^-2 (K km s , twice the value used for inner Galaxy clouds, the derived statistically agree with the computed .
4. The clouds are less massive, smaller and with narrower line widths than those in the inner Galaxy.
5. The mass spectrum for the clouds with has a slope -1.45, which is similar to that found for the inner Galaxy clouds (-1.50).
6. The warping and flaring of the outer molecular disk is clearly delineated.

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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