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

Abstract: We derived the dust-to-HI ratio of the intermediate-velocity clouds (IVCs), the high-velocity clouds (HVCs), and the local HI gas, 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 galaxies and 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 solar-neighbourhood value, 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 50 per cent of the HVC Complex C has a ratio of <0.3, and that the Magellanic Stream has the lowest ratio with a mode at ~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. Accordingly, we argue that the IVCs include a significant fraction of the low metallicity gas supplied from outside the Galaxy as an alternative to the Galactic-fountain model.