FUSE-Observations of the Hell 304 A Forest in two QSOs from the Hamburg Quasary Surveys and the fluctuating metagalactic UV background radiation

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We apply for support of an empirical investigation of the ionizing metagalactic UV background. For z < 3 the ionizing background radiation is due to both QSOs and stars. Recently there has been growing evidence, in particular from high resolution Hell 304 A forest observations, that the shape of the ionizing background might fluctuate on length scales as short as 1 Mpc (comoving). If true, this would have severe consequences for the determination of the heavy element content of QSO metal line systems. We plan to investigate the Hell (and simultaneous HI) spectra of the only two bright QSOs for which FUSE could take high resolution Hell spectra (both HE 2347-4342 and HS 1700+6416 are from the Hamburg QSO surveys) by more elaborate methods than hitherto applied. In particular we shall test the evaluation methods on artificial data from simulations of the Lya forest (from large scale structure simulations). We shall also study the population of sources along the two lines of sight both by direct imaging and by studies of the ionization in strong metal line systems. The question of the influence of background radiation fluctuations on metal abundances in the general IGM will be considered. Finally, realistic transfer calculations of QSO radiation in the cosmic web will be performed.