Large turbulent reservoirs of cold molecular gas around high-redshift
  starburst galaxies
  Starburst galaxies at the peak of cosmic star formation are among the most
extreme starforming engines in the universe, producing stars over ~100 Myr. The
star formation rates of these galaxies, which exceed 100 $M_\odot$ per year,
require large reservoirs of cold molecular gas to be delivered to their cores,
despite strong feedback from stars or active galactic nuclei. Starburst
galaxies are therefore ideal targets to unravel the critical interplay between
this feedback and the growth of a galaxy. The methylidyne cation, CH$^+$, is a
most useful molecule for such studies because it cannot form in cold gas
without supra-thermal energy input, so its presence highlights dissipation of
mechanical energy or strong UV irradiation. Here, we report the detection of
CH$^+$(J=1-0) emission and absorption lines in the spectra of six lensed
starburst galaxies at redshifts z~2.5. This line has such a high critical
density for excitation that it is emitted only in very dense ($>10^5$
cm$^{-3}$) gas, and is absorbed in low-density gas. We find that the CH$^+$
emission lines, which are broader than 1000 km s$^{-1}$, originate in dense
shock waves powered by hot galactic winds. The CH$^+$ absorption lines reveal
highly turbulent reservoirs of cool ($T\sim 100$K), low-density gas, extending
far outside (>10 kpc) the starburst cores (radii <1 kpc). We show that the
galactic winds sustain turbulence in the 10 kpc-scale environments of the
starburst cores, processing these environments into multi-phase,
gravitationally bound reservoirs. However, the mass outflow rates are found to
be insufficient to balance the star formation rates. Another mass input is
therefore required for these reservoirs, which could be provided by on-going
mergers or cold stream accretion. Our results suggest that galactic feedback,
coupled jointly to turbulence and gravity, extends the starburst phase instead
of quenching it.
