An Unbiased Survey for Molecular Clouds in the Southern Galactic Warp

We have made an unbiased survey for molecular clouds in the Galactic Warp. This survey, covering an area of 56 square degrees at l = 252 to 266 and b = −5 to −1, has revealed 70 molecular clouds, while only 6 clouds were previously known in the region. The number of molecular clouds is, then, an order of magnitude greater than previously known in this sector at R > ∼ 14.5 kpc. The mass of the clouds is in a range from 7.8×102M⊙ to 8.4×10 4M⊙, significantly less than the most massive giant molecular clouds in the inner disk, ∼106M⊙, while the cloud mass spectrum characterized by a power law is basically similar to other parts of the Galaxy. The X factor, N(H2)/ ∫ T (CO)dV , derived from the molecular clouds in the Warp is estimated to be 3.5(±1.8) times larger than that in the inner disk. The total molecular mass in the Warp is estimated as 7.3×10 M⊙, and total mass in the far-outer Galaxy (R > ∼ 14.5 kpc) can be estimated as 2×10 7 M⊙. The spatial correlation between the CO and H I distribution appears fairly good, and the mass of the molecular gas is about 1% of that of the atomic gas in the far-outer Galaxy. This ratio is similar to that in the interarm but is ten times smaller than those of the spiral arms. Only 6 of the 70 Warp clouds show signs of star formation at the IRAS sensitivity and star formation efficiency for high-mass stars in the Warp is found to be smaller than those in other molecular clouds in the Galaxy.