EUVE & HST Observations of PKS2155-304 & Constraints on the Ionization through the Galactic Halo

The BL Lacertae object, PKS2155-304, toward l(II=17,b({II)) =-52}, has been observed both with the Extreme Ultraviolet Explorer (EUVE) and the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope (HST). The EUVE observations show no detectable Extreme-UV flux longward of 100 Angstroms, but shortward of 100 Angstroms the decreased interstellar attenuation yields clearly detectable flux from the continuum of PKS2155-304. The EUVE detection together with the deduced H I column density from the G140L data for H I Lyalpha obtained with the HST/GHRS are used to obtain constraints upon the ionization of hydrogen and helium through the galactic halo toward PKS2155-304. Constraints for the ionization of H and He are presented based upon assumptions for the intrinsic EUV fluxes of the observed BL Lacertae object. More definitive constraints will be provided with further EUVE observations.