  We investigate the physical state of HI absorbing gas at low redshift (z =
0.25) using a subset of cosmological, hydrodynamic simulations from the OWLS
project, focusing in particular on broad (b_HI > 40 km/s) HI Lyman-Alpha
absorbers (BLAs), which are believed to originate in shock-heated gas in the
warm-hot intergalactic medium (WHIM). Our fiducial model, which includes
radiative cooling by heavy elements and feedback by supernovae and active
galactic nuclei, predicts that by z = 0.25 nearly 60 per cent of the gas mass
ends up at densities and temperatures characteristic of the WHIM and we find
that half of this fraction is due to outflows. The standard HI observables
(distribution of HI column densities N_HI, distribution of Doppler parameters
b_HI, b_HI - N_HI correlation) and the BLA line number density predicted by our
simulations are in remarkably good agreement with observations.
  BLAs arise in gas that is hotter, more highly ionised and more enriched than
the gas giving rise to typical Lyman-Alpha forest absorbers. The majority of
the BLAs arise in warm-hot (log (T/K) ~ 5) gas at low (log Delta < 1.5)
overdensities. On average, thermal broadening accounts for at least 60 per cent
of the BLA line width, which in turn can be used as a rough indicator of the
thermal state of the gas. Detectable BLAs account for only a small fraction of
the true baryon content of the WHIM at low redshift. In order to detect the
bulk of the mass in this gas phase, a sensitivity at least one order of
magnitude better than achieved by current ultraviolet spectrographs is
required. We argue that BLAs mostly trace gas that has been shock-heated and
enriched by outflows and that they therefore provide an important window on a
poorly understood feedback process.
