Physical conditions in the ISM of intensely star-forming galaxies at
  redshift~2
  We analyze the physical conditions in the interstellar gas of 11 actively
star-forming galaxies at z~2, based on integral-field spectroscopy from the
ESO-VLT and HST/NICMOS imaging. We concentrate on the high H-alpha surface
brightnesses, large line widths, line ratios and the clumpy nature of these
galaxies. We show that photoionization calculations and emission line
diagnostics imply gas pressures and densities that are similar to the most
intense nearby star-forming regions at z=0 but over much larger scales (10-20
kpc). A relationship between surface brightness and velocity dispersion can be
explained through simple energy injection arguments and a scaling set by nearby
galaxies with no free parameters. The high velocity dispersions are a natural
consequence of intense star formation thus regions of high velocity dispersion
are not evidence for mass concentrations such as bulges or rings. External
mechanisms like cosmological gas accretion generally do not have enough energy
to sustain the high velocity dispersions. In some cases, the high pressures and
low gas metallicites may make it difficult to robustly distinguish between AGN
ionization cones and star formation, as we show for BzK-15504 at z=2.38. We
construct a picture where the early stages of galaxy evolution are driven by
self-gravity which powers strong turbulence until the velocity dispersion is
high. Then massive, dense, gas-rich clumps collapse, triggering star formation
with high efficiencies and intensities as observed. At this stage, the intense
star formation is likely self-regulated by the mechanical energy output of
massive stars.
