Dust-to-neutral gas ratio of the intermediate velocity HI clouds derived based on the sub-mm dust emission for the whole sky

Abstract

We derived, as a proxy for the metallicity, the dust-to-HI ratio of the multiple components, the intermediate-velocity clouds (IVCs), the high-velocity clouds (HVCs), and the local HI, by carrying out a multiple-regression analysis of the 21cm HI emission combined with the sub-mm dust optical depth. The method covers over 80 per cent of the sky contiguously at a resolution of 47arcmin and is distinguished from the absorption line measurements toward bright stars covering a tiny fraction of the sky. Major results include that the ratio of the IVCs is in a range of 0.1-1.5 with a mode at 0.6 (relative to the majority of the local ISM, likewise below) and that a significant fraction, ~20 per cent, of the IVCs include dust-poor gas with a ratio of <0.5. It is confirmed that 80 per cent of the HVC Complex C has a ratio of <0.3, and that the Magellanic Stream has a uniform low ratio <0.1. The results prove that some IVCs have low metallicity gas, contrary to the previous absorption line measurements. Considering that the recent works show that the IVCs are interacting and exchanging momentum with the high metallicity Galactic halo gas, we argue that the high metallicity gas contaminates a significant fraction of the IVCs, and the observed fraction of the low metallicity IVCs gives a secure lower limit. Accordingly, we argue the IVCs include a significant fraction of the low metallicity gas supplied from outside the Galaxy as an alternative to the Galactic-fountain model.Comment: 29 pages, 23 figures, submitted to MNRA