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>New HST Observations of the Halo Gas of NGC 3067: Limits on the Extragalactic Ionizing Background at Low Redshift and the Lyman Continuum Escape Fraction
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New HST Observations of the Halo Gas of NGC 3067: Limits on the Extragalactic Ionizing Background at Low Redshift and the Lyman Continuum Escape Fraction
We present ultraviolet spectroscopy from HST/GHRS and reanalyze existing Hα images of the quasar/galaxy pair 3C 232/NGC 3067 and of the halo gas associated with NGC 3067. The spectra permit measurement of, or limits on, the column densities of Fe I, Fe II, Mg I, and Mg II in the absorbing cloud. Two distinct models of the extragalactic radiation field are considered: (1) the ionizing spectrum is dominated by a power-law extragalactic continuum, and (2) the power-law spectrum contains a Lyman break, implying enhanced flux longward of 912 ? relative to the hydrogen-ionizing flux. Models of the second type are required to fully explore the ionization balance of the Fe and Mg in the model cloud. The Hα images constrain the escape fraction of Lyman continuum photons from the galaxy to fesc ≤ 0.02. With the assumption that the cloud is shielded from all galactic contributions, we can constrain the intensity and shape of the extragalactic continuum. For an AGN-dominated power-law extragalactic spectrum, we derive a limit on the extragalactic ionizing flux Φion ≥ 2600 photons cm-2 s-1, or I0 ≥ 10-23 ergs cm-2 s-1 Hz-1 sr-1 for a ν-1.8 ionizing spectrum and a cloud of constant density. When combined with previous upper limits from the absence of Hα recombination emission from intergalactic clouds, our observations require 2600 ≤ Φion ≤ 10,000 photons cm-2 s-1. We show that if galactic contributions to the incident radiation are important, it is difficult to constrain Φion. These results demonstrate that galactic halo opacities and their wavelength dependence are crucial to understanding the abundance of low-ionization metals in the IGM.
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