  One of the outstanding questions in astronomy today is how gas flows from the
circumgalactic medium (CGM) onto the disks of galaxies and then transitions
from the diffuse atomic medium into molecular star-forming cores. For studies
of the CGM, the Next Generation Very Large Array (ngVLA) will have the
sensitivity and resolution to measure the sizes of the neutral atomic hydrogen
(HI) disks of galaxies and complete a census of the HI content around galaxies.
Within galaxies, the ngVLA will be able to resolve HI clouds in large numbers
of galaxies beyond the Local Group providing measurements of the physical
conditions of gas across a wide range of galaxy types. Finally, within our own
Milky Way, the ngVLA will provide a dense grid of HI absorption spectra in the
cold and warm neutral medium constraining the temperature and density of atomic
gas as it transitions into molecular gas. Combined with radio continuum and
molecular line data from the ngVLA plus multi-wavelength data from other
planned facilities, ngVLA will have a key role in understanding star-formation
in the local universe while complementing future studies with the Square
Kilometer Array.
