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标题：High-resolution HI and CO observations of high-latitude intermediate-velocity clouds
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作者：T. Röhser ; T. Röhser ; J. Kerp 等
期刊名称：Astronomy & Astrophysics
印刷版ISSN：0004-6361
电子版ISSN：1432-0746
出版年度：2016
卷号：592
页码：1-14
DOI：10.1051/0004-6361/201526801
语种：English
出版社：EDP Sciences
摘要：Context. Intermediate-velocity clouds (IVCs) are HI halo clouds that are likely related to a Galactic fountain process. In-falling IVCs are candidates for the re-accretion of matter onto the Milky Way. Aims. We study the evolution of IVCs at the disk-halo interface, focussing on the transition from atomic to molecular IVCs. We compare an atomic IVC to a molecular IVC and characterise their structural differences in order to investigate how molecular IVCs form high above the Galactic plane. Methods. With high-resolution HI observations of the Westerbork Synthesis Radio Telescope and 12CO(1 → 0) and 13CO(1 → 0) observations with the IRAM 30 m telescope, we analyse the small-scale structures within the two clouds. By correlating HI and far-infrared (FIR) dust continuum emission from the Planck satellite, the distribution of molecular hydrogen (H2) is estimated. We conduct a detailed comparison of the HI, FIR, and CO data and study variations of the XCO conversion factor. Results. The atomic IVC does not disclose detectable CO emission. The atomic small-scale structure, as revealed by the high-resolution HI data, shows low peak HI column densities and low HI fluxes as compared to the molecular IVC. The molecular IVC exhibits a rich molecular structure and most of the CO emission is observed at the eastern edge of the cloud. There is observational evidence that the molecular IVC is in a transient and, thus, non-equilibrium phase. The average XCO factor is close to the canonical value of the Milky Way disk. Conclusions. We propose that the two IVCs represent different states in a gradual transition from atomic to molecular clouds. The molecular IVC appears to be more condensed allowing the formation of H2 and CO in shielded regions all over the cloud. Ram pressure may accumulate gas and thus facilitate the formation of H2. We show evidence that the atomic IVC will evolve also into a molecular IVC in a few Myr.
关键词：instrumentation: high angular resolution;Galaxy: halo;ISM: clouds;ISM: structure;ISM: molecules