5 Some highlights

Below we present some of the survey results and some discussion.

1. Most of the CO emission is confined within the velocity range from -10 to 10 km s^-1. This can also be confirmed from Fig. 5, which is the average of the 7 000 spectra comprising the entire region mapped. In addition, a couple of small clouds with anomalous velocities ($v_{\rm LSR}=-20$ km s^-1) toward $l = 180^{\circ}$ and $b = 5^{\circ}$ to $6^{\circ}$ are identified. Their velocities of -20 km s^-1 are exceptional for this direction. Brand & Blitz (1993) reported a streaming motion in the outer Galaxy in their study of its velocity field. Considering their estimate of a streaming motion of 3.8 km s^-1 and possible local random motion, the velocity of -20 km s^-1 at the exact Antigalactic Center region is therefore quite exceptional. Five HII regions (Sh 231 - Sh 235) with abnormal velocities ($v_{\rm LSR} = -13$ to -23 km s^-1) are found (Blitz et al. 1982; Brand & Blitz 1993) around $l = 173^{\circ}$. Heyer et al. (1996) observed these objects in ¹²CO and ¹³CO, but no specific discussion on the velocities on these objects was included. More detailed study on this abnormal velocity field may clarify the physical and dynamical status of the clouds within it. In fact, we are observing these objects at higher resolution in several molecular transitions to analyze the physical status of the clouds. Figure 6 shows a spatial-velocity map integrated along the latitude of $3\hbox{$.\!\!^\circ$}5$ to $6\hbox{$.\!\!^\circ$}0$. This figure provides a longitudinal view of the velocity distribution, and also confirms that the CO emission is concentrated into the velocity range mentioned above. Some of the regions within this velocity range are likely to be nearby dark clouds, as they are clearly associated with noticeable opaque areas on the Palomar Observatory Sky Survey (POSS) plates. As mentioned previously, there are several Lynds clouds, which are generally local clouds.

  

Figure 7: The peak 12CO temperature distribution of three different clouds; Lynds 694 (dotted line), Sh 287 (solid line), and the mapped region in this study (thick solid line)

2. The highest antenna temperature ($T^*_{\rm R}=13.2$ K) is found at the position of (l,b) = ($180\hbox{$.\!\!^\circ$}8, 4\hbox{$.\!\!^\circ$}0$). This is close to the direction of the HII region Sh 241, an active star forming region. The peak temperature map is not shown in this paper as it is similar to Fig. 2. Most of the clouds have $T^*_{\rm R} <$ 6 K and the average $T_{\rm R}^*$ is about 3.5 K. This is illustrated in the histogram (Fig. 7). In this figure the CO peak temperature distributions of three molecular gas regions are presented; a typical GMC associated with the HII region Sh 287 (Lee 1994), a typical dark cloud, Lynds 694 (Lee & Lee in preparation) without star forming activity, and the region mapped in this work. Only those pixels above the $5\sigma$ level are entered for the target region (1 800 spectra), and the numbers of pixels of Sh 287 and Lynds 694 are normalized to that of our target region mapped. Clearly, CO emission of the Galactic Anticenter molecular gas is similar to that of Lynds 694, a dark cloud in the solar neighborhood, and is substantially weaker than that of the GMCs with some exceptional portions directly associated with Sh 241. Thus, most of the region may have similar properties to those of dark clouds in the solar neighborhood, though this remains to be determined in future work.

  

Figure 8: Example of spectra at the position of (l,b) = ($180\hbox{$.\!\!^\circ$}85, 4\hbox{$.\!\!^\circ$}10$) shows a possible bipolar outflow feature

  

Figure 9: HI integrated intensity map. The lowest contour is 1 700 K km s-1, and the increment between the contours is 100 K km s-1. The grey scale range is 1 600 to 2 000 K km s-1

3. Several CO emission peaks arise around IRAS point sources (see Fig. 2), some of which are associated with HII regions and possible bipolar outflows, implying that star forming activity is clearly going on. At the position (l, b) = ($180\hbox{$.\!\!^\circ$}85, 4\hbox{$.\!\!^\circ$}10$) there is a feature similar to bipolar outflow (Fig. 8), though this should be confirmed with higher resolution observations. Only those IRAS point sources with conspicuous flux (S₆₀ > 1 Jy) are shown in Fig. 2; the weaker point sources which are prevalent all over the CO emission boundary are excluded. It is notable that IRAS point sources with conspicuous flux are all confined to the region within $l < 182^{\circ}$, which has stronger CO emission than the rest of the region. In fact, the average temperature of the remaining portion ($l\gt 182^{\circ}$) is at least 2 K lower, which is strongly implying that there exists a substantial amount of colder gas features without internal heating sources, as well as brighter and warmer components associated with internal heating sources. However, the CO emission peaks are not as bright as those of local GMCs containing HII regions. This may be caused in part by beam dilution effects. In fact, the distance of the HII region Sh 241 was estimated as 4.7 kpc (Moffat et al. 1979). The case for beam dilution is also supported by the fact that the brightest lines in each peak are very localized. According to Moffat et al. (1979), the central star of Sh 241 is an O9 star, however, on the wisp of nebulosity there is another OB star, the distance of which is over 10 kpc from the sun, though this needs to be confirmed. If their estimate was correct, then it would confirm star forming activity at one of the most distant regions in the outer Galaxy.

4. The linewidths of the CO emission of the clouds ($\Delta v$ = 1 to 4 km s^-1; see Table 1) are generally narrower than those of local GMCs ($\Delta v \geq$ 5 km s^-1; Blitz 1987), and more or less similar to those of dark clouds in the solar neighborhood. The peak temperature ranges from 2 to 8 K except for one cloud directly associated with Sh 241. This implies that the most of the CO emission comes from cold dark clouds, which normally do not have large bulk motions.