High‐Latitude Molecular Clouds in an H i Filament toward the MBM 53, 54, and 55 Complex: Existence of an H 2 Cloud with Low CO Intensity

Abstract

We carried out a CO survey of high Galactic latitude molecular clouds toward an H I filament that contains a molecular cloud complex, MBM 53, 54, and 55, with the NANTEN telescope. Our observation covered the whole area of the H I filament in 12CO (J = 1-0) with a 4′ grid spacing. The filament is found to consist of a number of clumpy molecular clouds, and we identified 110 12CO clouds in the region, whose total mass is estimated to be ∼1200 M⊙. 13CO (J = 1-0) observations were carried out toward the region of high 12CO intensities in order to measure the optical depth of molecular gas. There is no detection in C18O (J = 1-0) line in the observed region, indicating that there are no clouds dense enough to form stars in the near future. These observations spatially resolved the entire gas distribution of MBM 53, 54, and 55 for the first time, and we have found a massive cloud, HLCG 92-35, around (l, b) ∼ (92°, -35°) whose mass is ∼330 M⊙, corresponding to ∼ 1/4 of the total mass. This CO cloud occupies the Galactic western half of a circular H I cloud toward (l, b) = (93°.5, -35°.5), and the H I to CO mass ratio is estimated to be the largest in the observed region. The far-infrared excess over H I emission, which is a good indicator of an existence of molecular hydrogen, toward HLCG 92-35 is the largest in the observed region. The ratio of the luminosity of the infrared excess to CO mass is also significantly larger than those of the other clouds, by a factor of ∼5. These facts indicate that HLCG 92-35 is a CO-forming molecular cloud, which is younger than the MBM clouds in terms of molecular cloud formation. Some past explosive event has been suggested toward the H I filament. Toward HLCG 92-35, the molecular gas distributed along the western edge of the H I cloud, which implies that the molecular gas may be formed by a compression of expanding H I shell.