Australia Telescope National Facility

The Low-Redshift Intergalactic Medium
J. Michael Shull , Steven V. Penton , John T. Stocke, PASA, 16 (1), in press.

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Theoretical Implications

A primary theoretical issue is whether low-z clouds have any relation to the evolution of the baryons in the high-z forest. A quick estimate suggests that the low-z absorbers could contain a substantial (25%) fraction of the total baryons estimated from Big Bang nucleosynthesis,

$\Omega_{\rm BBN} = (0.0343 \pm 0.0025) h_{75}^{-2}$ (Burles & Tytler 1998). Consider those Ly $\alpha$ systems with N

$_{\rm HI} \geq 10^{13}$ cm^-2, for which one can derive the space density $\phi_0$ ,

$\begin{displaymath} \frac {d{\cal N}} {dz} = \phi_0 (\pi R_0^2) \frac {c}{H_0} \approx 100 \; . \end{displaymath}$

(1)

The major uncertainty in deriving absorber masses is the ionization correction, which depends on the profile of gas density around the cloud centers. Assume, for simplicity, that

n_H(r) = n₀ (r/r₀)^-2 and adopt photoionization equilibrium with photoionization rate

$\Gamma_{\rm HI}$ and a case-A hydrogen recombination rate coefficient,

$\alpha_{H}^{(A)}$ , at 20,000 K. The ionizing radiation field is

$J_{\nu} = J_0 (\nu / \nu_0)^{-\alpha_s}$ with

$\alpha_s \approx 1.8$ and

$J_0 = (10^{-23}~{\rm ergs~cm}^{-2}~{\rm s}^{-1}~{\rm Hz}^{-1}~ {\rm sr}^{-1}) J_{-23}$ . The H I column density integrated through the cloud at impact parameter b is,

$\begin{displaymath} {\rm N}_{\rm HI}(b) = \frac { \pi n_0^2 r_0^4 \alpha_H^{(A)} (1 + 2n_{\rm He}/n_{\rm H}) } {2 \Gamma_{\rm HI} b^3 } \; . \end{displaymath}$

(2)

We can solve for n₀r₀² and find the total gas mass within

$b = (100~{\rm kpc})b_{100}$ for a fiducial column density N

$_{\rm HI} = (10^{14}~{\rm cm}^{-2}) N_{14}$ ,

$\begin{displaymath} M_{\rm cl}(b) = [4 \pi n_0 r_0^2 b (1.22 m_H)] = (1.6 \ti... ...{9}~M_{\odot}) N_{14}^{1/2} J_{-23}^{1/2} b_{100}^{5/2} \; , \end{displaymath}$

(3)

which yields a cloud closure parameter in baryons,

$\begin{displaymath} \Omega_b \approx \phi_0(b) M_{\rm cl}(b) = (0.008 \pm 0.004) J_{-23}^{1/2} N_{14}^{1/2} b_{100}^{1/2} h_{75}^{-1} \;. \end{displaymath}$

(4)

For the spherical-cloud model, the radiation field, cloud size, and column-density distribution probably each contribute 30-40% to the uncertainty in $\Omega_b$ , while temperature T_e and ionizing spectral index $\alpha_s$ contribute 10%, for an overall uncertainty of 50%. However, as with the high-z forest, the greatest uncertainty in $\Omega_b$ lies in the cloud geometry and radial profile. These parameters can only be understood by building up statistics through many sightlines, particularly multiple targets that probe the same cloud structures.

We have also increased our understanding of the metagalactic ionizing background radiation and the ``Gunn-Peterson'' opacities,

$\tau_{\rm HI}(z)$ and

$\tau_{\rm HeII}(z)$ . Using a new cosmological radiative transfer code and IGM opacity model, Fardal, Giroux, & Shull (1998) model the ionization fractions of H I and He II in a fluctuating radiation field due to quasars and starburst galaxies. In this work, we have calculated the metagalactic ionizing radiation field, $J_{\nu}(z)$ , using QSO and stellar emissivities and including cloud diffuse emission and new (somewhat lower) IGM opacities derived from Keck Ly $\alpha$ forest spectra.

**Figure 5:** Spectrum, $J_{\nu}(z)$ , of ionizing background from redshift
$z = 5 \rightarrow 0$ from new opacity and radiative transfer models (Fardal, Giroux, & Shull 1998; Shull et al. 1999).
$\begin{figure} \centerline{\vbox{ \psfig{figure=shullF5.ps,height=7cm} }} \end{figure}$

Figure 5 illustrates the evolution of $J_{\nu}$ from

$z = 5 \rightarrow 0$ , peaking at $z \approx 3$ . At z < 2, the absorption breaks at 1 Ryd (H I) and 4 Ryd (He II) become much less prominent and $J_{\nu}$ drops rapidly. At low redshift (z < 0.5), $J_{\nu}$ depends both on the local (Seyfert) luminosity function and on the opacity model. We have recomputed the ionizing radiation field at $z \approx 0$ (Shull et al. 1999) using a new opacity model from HST absorption data and extrapolated EUV emissivities of QSOs and low-redshift Seyferts from our IUE-AGN database (Penton & Shull, unpublished). We find

J₀ = (1.3^+0.8_-0.5) x 10^-23 ergs cm^-2 s^-1 Hz^-1 sr^-1 at z = 0, very close to our adopted scaling parameter, J_-23 = 1. We clearly still have an enormous amount to learn about the nature and distribution of the low-redshift Ly $\alpha$ clouds. It seems likely that future studies may uncover valuable information about their connection to large-scale structure and to the processes of galaxy formation and evolution.

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