The MACH HI absorption survey I: Physical conditions of cold atomic gas outside of the Galactic plane

Abstract

Tracing the transition between the diffuse atomic interstellar medium (ISM) and cold, dense gas is crucial for deciphering the star formation cycle in galaxies. Here we present MACH, a new survey of cold neutral hydrogen (HI) absorption at $21\mathrm{cm}$ by the Karl G. Jansky Very Large Array. We target 42 bright background sources with $60<l<110^{\circ }$, $30<b<62^{\circ }$, significantly expanding the sample of publicly-available, sensitive $21\mathrm{cm}$ absorption outside the Galactic plane. With matching $21\mathrm{cm}$ emission data from the EBHIS survey, we measure the total column density and cold HI fraction, and quantify the properties of individual HI structures along each sightline via autonomous Gaussian decomposition. Combining the MACH sample with results from recent HI absorption surveys, we produce a robust characterization of the cool atomic medium at high and intermediate Galactic latitudes. We find that MACH HI has significantly smaller column density relative to samples at similar latitudes, and the detected cold HI structures have smaller line widths, temperatures and turbulent Mach numbers, suggesting that MACH probes a particularly quiescent region. Using all available observations, we compute the cumulative covering fraction ($c$) of cold HI at local velocities outside the disk: structures with $\tau >0.001$ are ubiquitous ($c\sim 100\%$), whereas high optical depths ($\tau >1$) are extremely rare ($c\sim 0\%$).

arXiv comment33 pages, 16 figures. Accepted for publication in ApJS

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