2 Data taking

2.1 Infrared data and associated objects

To examine the dust emission distribution of the region, we have acquired a wide area (30$^{\circ} \times 20^{\circ}$) IRAS Sky Survey Atlas (ISSA) image of the IRAS 100 $\mu$m band centered on the exact Galactic Anticenter position of (l, b) = ($180^{\circ}, 0^{\circ}$)from the Infrared Processing and Analysis Center (IPAC) using Skyview Virtual Observatory (http://skyview.gsfc.nasa.gov/skyview.html). In Fig. 1 we present a 20$^{\circ} \times 10^{\circ}$ subregion of the original area obtained. In this figure the strong dust emission around $l = 170^{\circ}$ to $174^{\circ}$ traces several HII regions, including Sh 229, Sh 235, and Sh 236, and many dark clouds. Another strong dust emission region located at $l = 187^{\circ}$ to $190^{\circ}$ and beyond is the Gem OB1 giant molecular cloud complex, which was studied extensively by Carpenter et al. (1995). We selected the area indicated with a solid-line box in Fig. 1 as our first target region of the Galactic Anticenter CO Survey (GACCOS) since its dust emission feature is fairly isolated from other brighter emission regions. In addition, we have acquired IRAS point sources from the IRAS Point Source Catalog. Only those IRAS point sources with conspicuous flux (with S₆₀ > 1 Jy and S₁₀₀ > S₆₀) were collected.

Several molecular clouds are located within the selected region, including 4 Lynds dark clouds (Lynds 1550, Lynds 1555, Lynds 1557, and Lynds 1560). Three Lynds bright nebulae (LBN 823, LBN 824, and LBN 825) and only one HII region Sh 241 (Sharpless 1959) are present. Two reflection nebulae, DG85, DG86 (Dorschner & Gürtler 1964) are also present, and DG86 (also known as vdB65; van den Bergh 1966) is located close to Sh 241. Overall, compared to the inner Galaxy region, the selected region seems to be quiet, except for the only HII region Sh 241.

2.2 CO observations

We have mapped a 17 deg² section (l, b) = ($178\hbox{$.\!\!^\circ$}0 \sim 186\hbox{$.\!\!^\circ$}0,\ 3\hbox{$.\!\!^\circ$}5 \sim 6\hbox{$.\!\!^\circ$}0$) in ¹²CO J=1-0 using the 3-mm SIS receiver on the Taeduk Radio Astronomy Observatory (TRAO) 14-m telescope. The SIS receiver was developed at TRAO and has been operating since summer 1995 (Han et al. 1995). The beam size (FWHM) is 47'' and the grid spacing of the map is 3'. We mainly used a 250-kHz filterbank, which covers a velocity range of 170 km s^-1 with a resolution of 0.65 km s^-1. In addition, we also used 1 MHz filterbank, which covers a velocity coverage of 650 km s^-1 with a resolution of 2.6 km s^-1, providing a large velocity range in order to search for any detectable emission at other velocities. The filterbanks were centered at $v_{\rm LSR}=0$ km s^-1 for all observations.

  

Figure 3: 12CO velocity maps of the Galactic Anticenter. Two velocity channels are binned together in each map; thus, each map has a velocity range of 1.3 km s-1. The grey scale range is 0.4 to 8 K km s-1. The lowest four contours are 1, 2.5, 4, 6 K km s-1, and the increment between levels above 6 K km s-1 is 2 K km s-1. The dotted box represents the actual mapped area

 

Figure 3: continued

 

Figure 3: continued

  

Figure 4: Position-velocity maps (Galactic latitude-velocity). The grey scale range is 2.5 to 40 K. The lowest contour level and the increment between levels are 2.5 K

 

Figure 4: continued

 

Figure 4: continued

 

Figure 4: continued

All observations were made by position switching between observed positions and reference positions which were carefully selected to be free of CO emission. Each reference observation was shared among 2 to 6 map positions, depending on the sky stability. Calibration was accomplished by frequently observing an ambient temperature load. All antenna temperatures quoted are corrected for atmospheric extinction, and for the forward spillover and scattering losses of the antenna and radome ($\eta_{\rm fss}$ = 0.63 at 115 GHz), and are therefore on the $T_{\rm R}^*$ temperature scale defined by Kutner & Ulich (1981). The system temperature varied between 700 K and 1000 K depending on the elevation and the weather. All spectra were examined visually both before and after fitting the baseline, linear or polynomial. The collected spectra were transformed into IRAF data format using a modified FCRAO task package developed at the Five College Radio Astronomy Observatory. Most of the subsequent analysis was done within IRAF using this package. The average rms noise level of the data is estimated to be $\Delta T_{\rm rms} \sim$ 0.25 K in $T_{\rm R}^*$ at a velocity resolution of 0.65 km s^-1.