Astro2020 Science White Paper: ISM and CGM in external galaxies
  The wavelength range 912-2000A (hereafter far-UV) provides access to
absorption lines of the interstellar medium (ISM), circumgalactic medium (CGM),
and intergalactic medium (IGM) in phases spanning a wide range of ionization,
density, temperature, and molecular gas fraction. Far-UV space telescopes have
enabled detailed studies of the ISM in the Milky Way, paving the way to
understand in particular depletion of elements by condensation onto dust
grains, molecular gas formation pathways, ionization processes, and the
dynamics of shocks. An interesting prospect is to transpose the same level of
details to the ISM in external galaxies, in particular in metal-poor galaxies,
where the ISM chemical composition, physical conditions, and topology
(arrangement of various phases) change dramatically, with significant
consequences on the star-formation properties and on the overall galaxy
evolution. To circumvent current systematic biases in column density
determinations and to examine the ISM enrichment as a function of the
environment, we advocate for a versatile far-UV space mission able to observe
individual O/B stars or stellar clusters in galaxies up to few 100s Mpc away
with a spectral resolution power R~10^{4-5}. Such requirements would also make
it possible to probe multiple quasar lines of sight intersecting the CGM of
galaxies at various redshifts, making it possible to reconcile the various
physical scales and phases and to comprehend the role of gas exchanges and
flows for galaxy evolution.
