Next Generation Very Large Array Memo No. 7 Science Working Group 2:
  "Galaxy Ecosystems": The Matter Cycle in and Around Galaxies
  This white paper discusses how a "next-generation" Very Large Array (ngVLA)
operating in the frequency range 1-116 GHz could be a groundbreaking tool to
study the detailed astrophysics of the "matter cycle" in the Milky Way and
other galaxies. If optimized for high brightness sensitivity, the ngVLA would
bring detailed microwave spectroscopy and modeling of the full radio spectral
energy distribution into regular use as survey tools at resolutions of 0.1- 1
arcseconds. This wavelength range includes powerful diagnostics of density,
excitation, and chemistry in the cold ISM, as well as multiple tracers of the
rate of recent star formation, the magnetic field, shocks, and properties of
the ionized ISM. We highlight design considerations that would make this
facility revolutionary in this area, the foremost of which is a large amount of
collecting area on ~km-length baselines. We also emphasize the strong case for
harnessing the large proposed collecting area of the ngVLA for very long
baseline applications as part of the core design. This would allow measurements
of the three dimensional space motions of galaxies to beyond the Local Group
and mapping of the Milky Way out to the far side of the disk. Finally, we
discuss the gains from the proposed combination of very high resolution and
sensitivity to thermal emission, which include observing the feeding of black
holes and resolving forming protoclusters.
