The Mass of the HII Envelope

We derive a total flux from the extended filaments of $1.4 \times 10^{-13}$ erg cm$^{-2}$ s$^{-1}$. This implies a luminosity of $2.5 \times 10^{42}$ erg s$^{-1}$ at the redshift distance of the quasar (384 Mpc). Assuming complete ionization and employing a geometry in which the filaments are approximately as deep as they are wide (i.e., 7 pixels $\sim$ 5 kpc), we calculate an rms filament density of $\left<n_e\right> \sim 0.04 \cdot f^{-1/2}$ cm$^{-3}$. For a filling factor $f = 1$, this implies an upper limit on the ionized gas mass of $M\sim6\times10^{10}$ M$_{\sun}$, distributed in a volume of $6.1\times10^4$ kpc$^3$. These estimates are consistent with previously published values (Nørgaard-Nielsen et al. 1986; Macchetto et al. 1990).