Implications of an updated ultraviolet background for the ionization mechanisms of intervening Ne viii absorbers

Abstract 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 ≥5 × 10−5 cm−3 above which the observed $N({Ne \small{VIII}})/\rm N({O \small {VI}})$ can be reproduced by pure collisional processes. If the gas is of near solar metallicity (as measured for the low ions) then the cooling time-scales will be small (