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

1. Introduction

In recent years the outer Galaxy (the region beyond the solar circle) has gained attention despite the fact that a large fraction of molecular gas and regions of star formation are found in the inner Galaxy. The study of molecular clouds in the outer Galaxy is interesting for the following reasons: a) there is no distance ambiguity, that is, for a given rotation curve a unique distance can be calculated from a measured radial velocity; b) the boundaries of the molecular clouds can be determined directly from the observations because there is no source confusion nor a need of subtracting a diffuse background emission; c) the warping and flaring of the molecular disk can be studied unhindered; d) most of the mass of the neutral interstellar medium lies in this region; e) due to differences in environmental conditions prevalent far from the galactic center, the physical properties of the molecular clouds might be different from those located inside the solar circle.

Kutner & Mead (1981) were the first to report the detection of extensive low-level CO emission from molecular clouds well outside the solar circle, from a survey of specific strips in latitude, in the longitude range - . From the study and mapping of 32 clouds obtained from a more extensive CO survey of the third galactic quadrant, May et al. (1985; 1988) found that most of the distant CO emission falls in a pattern inclined to the galactic equator, providing the first evidence that molecular clouds without optically identified HII regions follow the galactic warp. A similar conclusion was obtained by Mead (1988) and Mead & Kutner (1988). These early studies of the warp of the molecular disk have been corroborated and extended by Wouterloot et al. (1990) using CO emission associated with selected IRAS point sources obtained by Wouterloot & Brand (1989).

Several studies of individual molecular clouds in the outer Galaxy have been carried out to derive their main physical parameters. May et al. (1985) studied 32 molecular clouds in the third galactic quadrant finding a shortage of very massive clouds () as compared with the inner Galaxy. Cohen et al. (1985) and Grabelsky et al. (1987) detected 26 clouds delineating the Carina arm beyond the solar circle. Mead & Kutner (1988) mapped 31 clouds in the first galactic quadrant outside the solar circle, reporting a lower excitation and kinetic temperatures for these objects as compared with those for the inner Galaxy. Carpenter et al. (1990) mapped 18 molecular clouds associated with bright far-infrared sources finding a tight, near-linear correlation between CO luminosity and cloud mass. Digel et al. (1990) observed 32 clouds, related to the Outer arm in the first galactic quadrant, concluding that these clouds are times less luminous in CO than those in the inner Galaxy. Sodroski (1991) studying 35 clouds located in the second, third and fourth galactic quadrants, also found that these outer Galaxy objects are under-luminous in CO by a factor of 2 - 3 as compared with the inner Galaxy ones. All the above studies have included molecular clouds located up to 14 kpc from the galactic center. Recently, however, Brand & Wouterloot (1994) detected 11 molecular clouds with galactocentric distances larger than 18 kpc.

A deep CO survey of the third galactic quadrant with improved resolution and sensitivity has been completed by our group (May et al. 1993). While the low-resolution CO survey of the third quadrant (May et al. 1988) allowed the detection of 32 molecular clouds beyond 2 kpc from the Sun and up to 14 kpc from the galactic center, the deep CO survey of the same quadrant has allowed us to detect and map 177 molecular clouds beyond 2 kpc from the Sun and with galactocentric distances of up to 19 kpc. In spite of its low resolution the first CO survey of the third galactic quadrant led to: 1) the discovery of the Molecular Ridge in Vela (Murphy & May 1991); 2) the evidence for a warp and flaring in the molecular disk from a preliminary analysis of the data (May et al. 1985); 3) the presence of molecular counterparts to Linblad's (1967) arms and OB associations Canis Major OB2, Puppis OB2, and Puppis OB3 (Murphy 1985; May et al. 1988); and 4) the completion of the composite CO survey of the entire galactic plane (Dame et al. 1987).

The main purpose of this paper is to present the physical properties of these 177 molecular clouds and to compare these results with previous works on the outer Galaxy.

Observations of the () line at 115 GHz were made with the Southern Millimeter-wave Telescope at Cerro Tololo, Chile. Suitable for large scale surveys, this 1.2 m diameter instrument has, at 115 GHz, a full beam-width at half maximum (FWHM) of () and a main beam efficiency of 0.82 (Cohen 1983; Bronfman et al. 1988).

The data used in this work are taken from a deep CO survey of the third galactic quadrant containing about 8000 spectra (May et al. 1993). Because of the wide strip in latitude () needed to follow the emission detected in the low resolution survey, the large extent in longitude to be covered ( - ), and the sensitivity required to detect the weak emission of molecular clouds in the outer Galaxy, a general sampling interval of was used to complete the survey in a reasonable amount of time. However, where weak emission corresponding to molecular clouds farther than 12 kpc from the galactic center was detected, a sampling interval of was used to map each individual cloud.

The observations are fully described by May et al. (1993); we include here only a brief description of them. The receiver front-end mixer was a Schottky diode cooled to 77 K with liquid nitrogen. The receiver noise temperature, excluding the atmospheric contribution, was about 380 K (SSB). Most of the observations were carried out with a 256-channel filter bank of 500 kHz channel-width, providing a 1.3 km s^-1 velocity resolution at 115.3 GHz and a spectral range of 333 km s^-1. A narrow-band filter bank of the same number of channels but of 100 kHz channel-width, giving a spectral range of 66.6 km s^-1, was used to complete the survey in the area closest to the galactic anticenter, and to finish the mapping of some individual clouds. Position switching was used for all the observations.

Integration time (6-12 minutes) for each spectrum was automatically set to achieve a noise level K rms at a velocity resolution of 1.3 km s^-1. All observations were made above in elevation and under good weather conditions only. Table 1 summarizes the observational parameters.

Table 1. Observational parameters

© European Southern Observatory (ESO) 1997

Online publication: April 8, 1998
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