FAINT OBJECT CAMERA FAR-ULTRAVIOLET OBJECTIVE PRISM OBSERVATIONS OF 12 Z-GREATER-THAN-3 QUASARS

We present the first results of an exploratory objective prism survey of the far-ultraviolet (lambdalambda1200-1800) spectra of high-redshif t quasars obtained with the Faint Object Camera on-board the Hubble Sp ace Telescope. The prime objective of this survey is to identify one o r more candidates among the known quasars at z(em) > 3 with sufficient flux at far-ultraviolet wavelengths to enable detailed follow-up obse rvations of redshifted intergalactic He II lambda304 absorption using the HST grating spectrographs. FOC prism observations of 12 prime cand idates selected for redshift (z(em) congruent-to 3.03-3.66), brightnes s (V less than or similar 18.5) and the appearance of their optical ab sorption spectra are presented and discussed. As anticipated beforehan d, cumulative neutral hydrogen Lyman continuum absorption from the var ious classes of intervening absorption systems presents a serious obst acle for observing high redshift quasars at extreme ultraviolet rest e nergies. At our limiting sensitivity of F(lambda) congruent-to 2-4 x 1 0(-16) ergs s-1 cm-2 angstrom-1, 10 of the 12 objects observed show no far-UV flux shortward of lambda1800 observed wavelength. Two objects, Q0114-089 (UM 670; z(em) = 3.16) and Q1442+102 (OQ 172; z(em) = 3.53) are detected, but only down to lambda congruent-to 330 angstrom and l ambda congruent-to 345 angstrom rest wavelength. No object is reliably detected at emitted He II lambda304. The severity of the cumulative L yman continuum absorption is, however, such that these preliminary fin dings are still statistically consistent with the hypothesis that lumi nous quasars are intrinsically bright at extreme ultraviolet energies at flux levels comparable to those suggested by extrapolation of the p ower law continua seen at longer wavelengths. A larger number of high- redshift quasars need to be observed in the far-UV before the prospect s for observing intergalactic He II lambda304 absorption with HST can be fully appraised.