The MACH HI absorption survey I: Physical conditions of cold atomic gas
  outside of the Galactic plane
  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 $21\rm\,cm$ 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 $21\rm\,cm$
absorption outside the Galactic plane. With matching $21\rm\,cm$ 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 ($c\sim100\%$), whereas high
optical depths ($\tau>1$) are extremely rare ($c\sim0\%$).
