1 Introduction

Surveying the Galactic plane for molecular gas has been pursued since the carbon monoxide 2.7-mm line was detected about three decades ago. There were several CO surveys toward the Galactic plane (Sanders et al. 1986; Dame et al. 1987; and see Combes 1991 for a review), and substantial information on the distribution and large-scale dynamics of molecular gas (and determination of the Galactic rotation curve) has been derived from these surveys. The ¹²CO (J=1-0) observations by the Columbia University group summarized in Dame et al. (1987) comprise one of the most extensive surveys for molecular clouds, encompassing 30% of the whole sky along the Galactic plane with an angular resolution of $0\hbox{$.\!\!^\circ$}5$. The Massachusetts-Stony Brook Galactic plane survey was conducted with a higher resolution of 3' - 6', toward the first Galactic quadrant, but was confined within the $\pm 1^{\circ}$ of the Galactic plane (Sanders et al. 1986). Since then, molecular clouds in the inner Galaxy region have frequently been studied, as molecular gas is largely confined to the Molecular Ring in our Galaxy (Scoville et al. 1987; Solomon et al. 1987).

  

Figure 1: The 100 $\mu$m ISSA image of a 20$^{\circ} \times 10^{\circ}$ region centered on the Galactic Anticenter. The location of our mapped area is marked with the solid box. The lowest contour level is 10 MJy sr-1, and the increment between levels is 5 MJy sr-1. The grey scale ranges from 1 to 200 MJy sr-1

In contrast to investigation inside the solar circle, the molecular gas in the outer Galaxy has been studied in less detail, although there is no confusion in distinguishing foreground from background clouds in the outer Galaxy. Excepting the Columbia survey which was made with a coarse resolution, the first systematic study of molecular clouds in the outer Galaxy was conducted by Mead & Kutner (1988). Recently, more systematic surveys have been reported on specific regions in the outer Galaxy (Carpenter et al. 1995; Heyer et al. 1998). May et al. (1993, 1997) have analyzed the large-scale physical properties of molecular gas in the outer Galaxy, especially in the third quadrant. A group at Nagoya University has been surveying the outer Galaxy in ¹³CO (J=1-0), focusing on the dense regions of molecular clouds and investigating star forming activity (Dobashi et al. 1994, 1996; Yonekura et al. 1997; Kawamura et al. 1998). In addition, there were several case studies toward the outer Galaxy molecular clouds; de Geus et al. (1993) reported that there are molecular clouds associated with star forming activity even in the far outer Galaxy (d=21 kpc and 28 kpc) beyond the known optical disk. Brand & Wouterloot (1994, 1995) focused on the luminously embedded star-forming regions of several molecular clouds in the far outer Galaxy. They showed that star forming clouds are distributed up to R=20 kpc. However, all these CO observations are biased towards the warmest material, and may underestimate, or even miss the colder molecular gas. In fact, Lequeux et al. (1993) found evidence for the existence of significant amounts of cold molecular gas in the outer Galaxy. Digel et al. (1994) also reported that there are several molecular clouds in the extreme outer Galaxy (18-28 kpc) which are much colder than the clouds in the solar neighborhood, and that the metallicity in the far outer Galaxy is so uncertain that the mass calibrating ratio could differ from the local value (see also Mead & Kutner 1988; Digel et al. 1990). Digel et al. (1996) showed that the clouds in the Perseus arm are systematically less luminous in CO by a factor of 3.5 than those in the local arm. In their CO(2-1)/CO(1-0) study, Usuda et al. (1998) also demonstrated that the intensity of CO emission per unit mass in the outer Galaxy is lower than that in the inner Galaxy. Along with the bright star-forming molecular clouds, the colder population is surely assumed to exist in the outer Galaxy, and more observations are required for investigating further analysis of physical properties of the outer Galaxy molecular clouds.

In contrast to other regions of the Galactic plane, the exact Galactic Anticenter is the least studied region as the conventional velocity-to-distance transformation is of no use because of lack of kinematic resolution. Moreover, the distribution of the molecular gas had been found to be relatively weak. The Columbia survey (Dame et al. 1987) covered the entire Galactic plane, but only samll part of the Galactic Anticenter region was discussed by Huang (1986), mainly focusing on supernovae, including Simeiz 147. In addition, the resolution of Columbia survey ($8\hbox{$.\mkern-4mu^\prime$}7$ beamwidth) was not adequate to determine the structure of the clouds, and no higher resolution observations in any molecular species have been published except Kawamura et al. (1998)'s recent ¹³CO survey with 8' grid spacing.

In this paper we present the first results of higher resolution ¹²CO observations of the Galactic Anticenter region, mainly focusing on the distribution of the molecular gas, as a first step in a statistical study of the clouds properties and star forming activity. Complete analysis of statistical properties will be presented in forthcoming paper. Observations, far-infrared (FIR) data and associated objects within the selected region are described in Sect. 2, while in Sect. 3 observational results and spatial-velocity maps are presented. The cloud identification method is briefly explained in Sect. 4. In Sect. 5 we delineate some highlights of the observational results, and discuss the correlation between CO emission and dust emission in Sect. 6. We summarize our results in the final section.

  

Figure 2: 12CO integrated intensity map. The grey scale range is 4 to 40 K km s-1. The lowest contour level is 5 K km s-1 and the increment between levels is 3 K km s-1. The dotted box encloses mapped area. The filled circles represents IRAS point sources with S60 > 1 Jy and S100 > S60