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

Details 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 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

1999-07-13

arxiv.org/abs/astro-ph/9907174v2

Astronomy and Astrophysics

Astrophysics

25 Pages LaTex, 8 PostScript Figures, accepted for publication in AJ, Nov. 99 issue

Scientific paper

10.1086/301106

We present UV spectroscopy from HST/GHRS and reanalyze existing H_alpha 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 A relative to the hydrogen-ionizing flux. The H_alpha images constrain the escape fraction of Lyman continuum photons from the galaxy to f_esc <= 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 Phi_ion >= 2600 photons cm^-2 s^-1, or I_0 >= 10^-23 erg cm^-2 s^-1 Hz^-1 sr^-1 for an ionizing spectrum with power-law index of 1.8 and a cloud of constant density. When combined with previous upper limits from the absence of H_alpha recombination emission from intergalactic clouds, our observations require 2600 <= Phi_ion <= 10000 photons cm^-2 s^-1. We show that if galactic contributions to the incident radiation are important, it is difficult to constrain Phi_ion. 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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