4 Cloud identification

The major portion of the molecular gas is within a velocity range of -10 to 10 km s^-1, and there are several pieces of small isolated clouds scattered on the small scale, especially in the region of (l, b) = ($178^{\circ} \sim 180^{\circ}, 5^{\circ} \sim 6^{\circ}$) (see Figs. 3 and 4). However, it is also found that some portion of the CO emission shows more than one peak; many spectra show evidence that two or more clouds are blended together in some directions, mostly from $l=179^{\circ}$ to $182^{\circ}$ (see Sect. 5).

To effectively identify these clouds from the 3-dimensional data cube set (l, b, v), we used a code developed by Lee et al. (1997), which is working as a user task within IRAF. Using the ¹²CO cube data, they define a cloud to be an object composed of all pixels in longitude, latitude, and velocity that are simply connected and that lie above some threshold temperature. This is the same method employed by Scoville et al. (1987) and Lee et al. (1990). Ideally, one would like to define clouds with a 0 K threshold temperature. However, low threshold temperatures are impractical in view of the noise level in the spectra and more importantly because of the blending of adjacent clouds which occurs in the observed region. On the other hand, with too high a threshold temperature, regions are severely truncated, and it is impossible to obtain a reliable estimate of the sizes and thus the masses of the clouds. To define clouds we should choose a reasonable threshold temperature. Above the arbitrary threshold temperature, only those clouds with 2 or more pixels in each l, b, and v are retained. In this way, we identify 30 clouds with a threshold temperature of 1.25 K ($= 5\sigma$). One of those is found to be very large, and seems to be blended with several adjacent clouds with similar velocities. Using a higher threshold temperature of 2.5 K ($= 10\sigma$) we have separated the object into several subclouds. The number of subclouds determined in this way is 12, and the identified clouds and subclouds are listed in Table 1. Intriguingly, the subclouds appear in two distinct mean velocity groups, one with $v_{\rm LSR} \sim$ 1 km s^-1, and the other with $v_{\rm LSR} \sim 4$ km s^-1, which also implies that the largest cloud (No. 18) is a blended one. The first column is the cloud number; centroid positions and their dispersions are given Cols. 2-5. Column 6 is the centroid velocity and Col. 7 is the linewidth at half maximum. Peak temperature and the number of pixels retained within the cloud follow in Cols. 8 and 9. In this table we do not include detailed physical parameters, as distances are yet to be determined, though the distance of cloud No. 8 was found to be 4.7 kpc, which is associated with the HII region Sh 241 (Moffat et al. 1979).

Recently, Kawamura et al. (1998) listed the clouds in this region using their ¹³CO survey conducted with the 4-m telescope in Nagoya University. The number of clouds identified in their survey was 7, much smaller than the number of clouds identified in this work. The Kawamura et al. (1998)'s distance determination toward these clouds (d=1.1 kpc) was based on Racine (1968)'s photometry of a star BD $+30\hbox{$.\!\!^\circ$}1096$. However, the coordinate of the star (l,b) = ($180\hbox{$.\!\!^\circ$}7, 4\hbox{$.\!\!^\circ$}3$) is very close to the HII region Sh 241, the distance of which is known to be 4.7 kpc (Moffat et al. 1979), which is by far more distant than those of the presumably local clouds. Thus, the distances of the clouds with noticeable opaqueness are likely ill-determined and should be redetermined. In fact, we are imaging these regions with deep CCD observations to estimate the distances, and for further analysis. The physical properties of the identified clouds will be presented in a separate paper with determination of distances of the clouds (Lee et al. in preparation).

  

Figure 5: Average of all 7 000 spectra. While there is sharp cut in the positive-velocity emission regime at 7 km s-1, there is more extended emission in a negative-velocity regime up to -25 km s-1

  

Figure 6: Global integrated position-velocity map. The lowest contour is 4.5, 9, and 15 K, and the increment between the levels above 15 K is 6 K. The grey scale range is 2 to 60 K

  

Table 1: Identified clouds and subclouds