Implications of an updated ultraviolet background for the ionization
  mechanisms of intervening Ne VIII absorbers
  Ne VIII absorbers seen in QSO spectra are useful tracers of warm ionized gas,
when collisional ionization is the dominant ionization process. While
photoionization by the ultraviolet background (UVB) is a viable option, it
tends to predict large line-of-sight thickness for the absorbing gas. Here, we
study the implications of the recently updated UVB at low-z to understand the
ionization mechanisms of intervening Ne VIII absorbers. With the updated UVB,
one typically needs higher density and metallicity to reproduce the observed
ionic column densities under photoionization. Both reduce the inferred
line-of-sight thicknesses of the absorbers. We find a critical density of
$\geq5\times10^{-5}$ cm$^{-3}$ above which the observed N(Ne VIII)/N(O VI) can
be reproduced by pure collisional processes. If the gas is of near solar
metallicity (as measured for the low ions) then the cooling timescales will be
small (<$10^{8}$ yrs). Therefore, a continuous injection of heat is required in
order to enhance the detectability of the collisionally ionized gas. Using
photoionization models we find that in almost all Ne VIII systems the inferred
low ion metallicity is near solar or supersolar. If we assume the Ne VIII phase
to have similar metallicities then photoionization can reproduce the observed
N(Ne VIII)/N(O VI) without the line-of-sight thickness being unreasonably large
and avoids cooling issues related to the collisional ionization at these
metallicities. However the indication of broad Ly$\alpha$ absorption in a
couple of systems, if true, suggests that the Ne VIII phase is distinct from
the low ion phase having much lower metallicity.
