An Unbiased Search for Molecular Clouds in the Southern Galactic Warp

Abstract

Abstract We made an unbiased search for molecular clouds in the Galactic Warp. This survey, covering an area of 56 square degrees at $l = 252^{\circ}$ to $266^{\circ}$ and $b =-5^{\circ}$ to $-1^{\circ}$, revealed 70 molecular clouds, while only 6 clouds were previously known in this sector at $R \gtrsim 14.5 \,\mathrm{kpc}$. The mass of the clouds is in a range from $7.8 \times 10^{2} \,{{{M}_{\odot}}}$ to $8.4 \times 10^{4} \,{{{M}_{\odot}}}$, significantly less than the most massive giant molecular clouds in the inner disk, $\sim 10^{6} \,{{{M}_{\odot}}}$, while the cloud mass spectrum characterized by a power law is basically similar to other parts of the Galaxy. The $X$ factor, $N(\mathrm{H}_{2}) / \int T({{12\atop} \mathrm{CO}}) \,dV$, derived from the molecular clouds in the Warp, is estimated to be $3.5(\pm1.8)$-times larger than that in the inner disk. The total molecular mass in the Warp is estimated to be $7.3 \times 10^5 \,{{{M}_{\odot}}}$, and the total mass in the far-outer Galaxy ($R \gtrsim 14.5 \,\mathrm{kpc}$) can be estimated to be $2 \times 10^7 \,{{{M}_{\odot}}}$. The spatial correlation between the CO and H i distribution appears to be 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 the star-formation efficiency for high-mass stars in the Warp is found to be smaller than those in other molecular clouds in the Galaxy.