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

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HST Survey of low-z Ly $\alpha$ Absorbers

The frequency of low-z Ly $\alpha$ lines with

$W_{\lambda} \geq 320$ m�� reported by the HST/FOS Key Project,

$d{\cal N}/dz = (24.3 \pm 6.6)(1+z)^{0.58 \pm 0.50}$ (Bahcall et al. 1996), was considerably higher than a simple extrapolation from the high-redshift forest. These higher-N $_{\rm HI}$ absorbers exhibit associations with galaxies (

$D \leq 200h_{75}^{-1}$ kpc) about half the time (Lanzetta et al. 1995).

**Figure 1:** Pie-diagram distributions of recession velocity and RA of bright (CfA survey) galaxies and four Ly $\alpha$ absorbers toward Mrk�501 and Mrk�421 (Shull, Stocke, & Penton 1996). Two of these systems lie in voids; the nearest bright galaxies lie
>4 h₇₅^-1 Mpc from the absorber.
$\begin{figure} \centerline{\vbox{ \psfig{figure=shullF1.ps,height=10cm} }} \end{figure}$

In HST cycles 4-6, our group began GHRS studies of lower-N $_{\rm HI}$ absorbers toward 15 bright targets (Stocke et al. 1995; Shull, Stocke, & Penton 1996). These low-z targets were chosen because of their well-mapped distributions of foreground galaxies (superclusters and voids). Our studies were designed to measure the distribution of Ly $\alpha$ absorbers in redshift ( $z \leq 0.08$ ) and column density (

$12.5 \leq \log {\rm N}_{\rm HI} \leq 16$ ), to probe their association with galaxies, and to measure their clustering and large-scale structure. Toward 15 targets, we detected $\sim70$ Ly $\alpha$ systems (plus a number of high-velocity clouds and associated Ly $\alpha$ absorbers) over a cumulated pathlength

$(c \Delta z) \approx 114,000$ km�s^-1. In cycle 7, we will observe 14 more sightlines with the Space Telescope Imaging Spectrograph (STIS) to double our Ly $\alpha$ sample. The locations of Ly $\alpha$ absorbers toward two of our first sightlines are shown in Figure 1.

In our first 4 sightlines, the frequency of absorbers with N

$_{\rm HI} \geq 10^{13}$ cm^-2 was

$\langle d{\cal N}/dz \rangle \approx (90 \pm 20)$ , corresponding to a local space density,

$\phi_0 = (0.7~{\rm Mpc}^{-3}) R_{100}^{-2} h_{75}$ for absorber radius

$(100~{\rm kpc})R_{100}$ . This space density is $\sim40$ times that of bright (L_*) galaxies, but similar to that of dwarf galaxies with

$L \approx 0.01 L_*$ . >From a statistical, nearest-neighbor analysis, we found that the Ly $\alpha$ clouds have some tendency to associate with large structures of galaxies and to ``avoid the voids''. However, for the lower column systems, the nearest bright galaxies are often too far to be physically associated in hydrostatic halos or disks (Maloney 1993; Dove & Shull 1994). Of 10 absorption systems first analyzed (Shull et al. 1996), 3 lie in voids, with the nearest bright galaxies several Mpc distant. In several cases, we identified dwarf H�I galaxies within 100-300 kpc using the VLA (Van Gorkom et al. 1996).

**Figure 2:** Overlay of galaxy field around Mrk�335, showing a dwarf galaxy at 1955 km�s^-1 at nearly the same redshift as the 1970 km�s^-1 Ly $\alpha$ absorber (
$N_{\rm HI} = 10^{13.5}$ cm^-2). The offset distance is

$\sim95 h_{75}^{-1}$ kpc.
$\begin{figure} \centerline{\vbox{ \psfig{figure=shullF2.ps,height=7.cm} }} \end{figure}$

Figure 2 shows one system toward Mrk�335, where a dwarf galaxy with

$M_{\rm HI} \approx (7 \times 10^7~M_{\odot}) h_{75}^{-2}$ and offset distance

$\sim(100~{\rm kpc})h_{75}^{-1}$ is seen at heliocentric velocity cz = 1955�km�s^-1, remarkably near to that of the Ly $\alpha$ absorber. Thus, some of the lower-N $_{\rm HI}$ absorbers appear to be associated with dwarf galaxies, although much better statistics are needed.

In HST cycle 6, we observed 7 more sightlines with the GHRS/G160M. With better data, we were able to detect weaker Ly $\alpha$ absorption lines, down to 20 m�� (N

$_{\rm HI} = 10^{12.6}$ cm^-2) in some cases. Many of the new sightlines exhibit considerably more Ly $\alpha$ absorbers; for these 15 sightlines,

$\langle d{\cal N}/dz \rangle = 200 \pm 40$ for N

$_{\rm HI} \geq 10^{12.6}$ cm^-2 or one line every 1500 km�s^-1. Although there is wide variation, this frequency is almost 3 times the value (one every 3400 km�s^-1) reported earlier (Shull et al. 1996) for N

$_{\rm HI} \geq 10^{13}$ cm^-2. For a curve of growth with b = 25 km�s^-1, the 70 Ly $\alpha$ absorbers with

$12.6 \leq \log {\rm N}_{\rm HI} \leq 14.0$ follow a distribution

$f(\geq N_{\rm HI}) \propto N_{\rm HI}^{-0.63 \pm 0.04}$ , remarkably close to the slope in the high-z Ly $\alpha$ forest. These results have been corrected for incompleteness at low equivalent widths, for line blending, and for the GHRS sensitivity function versus wavelength (Penton, Stocke, & Shull 1999).

**Figure 3:** HST/GHRS (G160M) spectrum of PKS�2155-304 (Shull et al. 1998) shows multiple Ly $\alpha$ absorption systems between 1281-1290 �� (
cz = 15,700 - 17,500 km�s^-1). Upper limits on Si�III

$\lambda1206.50$ absorption at 1274.7 �� and 1275.2 �� correspond to [Si/H] $\leq0.003$ solar abundance.
$\begin{figure} \centerline{\vbox{ \psfig{figure=shullF3.ps,height=10cm} }} \end{figure}$

We turn now to the extraordinary sightline toward PKS�2155-304 (Bruhweiler et al. 1993; Shull et al. 1998). This target exhibits numerous Ly $\alpha$ absorbers (Fig. 3), including a group of strong systems between cz = 15,700 and 17,500 km�s^-1. The strong absorbers have an estimated combined column density N

$_{\rm HI} = (2-5) \times 10^{16}$ cm^-2, based on Lyman-limit absorption seen by ORFEUS (Appenzeller et al. 1995). Using the VLA (Van Gorkom et al. 1996; Shull et al. 1998), we have identified these absorbers with the very extended halos or intragroup gas associated with four large galaxies at the same redshift (Fig. 4). The offsets from the sightline to these galaxies are enormous.

**Figure 4:** VLA field of 21-cm emission toward PKS�2155-304 at velocities (16,000 - 17,300 km�s^-1) near the Ly $\alpha$ absorbers. Four large H�I galaxies are detected at projected offsets of
(400-800)h₇₅^-1 kpc (Shull et al. 1998). At least two galaxies, to the south and southwest, appear to be kinematically associated with Ly $\alpha$ absorbers at 16,460 and 17,170 km�s^-1.
$\begin{figure} \centerline{\vbox{ \psfig{figure=shullF4.ps,height=9cm} }} \end{figure}$

Despite the kinematic associations, it would be challenging to make a dynamical association with such galaxies. One must extrapolate from the 10²⁰ cm^-2 columns seen in galactic 21-cm emission to the range 10^13-16 cm^-2 probed by Ly $\alpha$ absorption. Much of the strong Ly $\alpha$ absorption may arise in gas of wide extent, $\sim1$ Mpc in diameter, spread throughout the group of galaxies at z = 0.057. Assuming that

$\langle {\rm N}_{\rm HI} \rangle \approx 2 \times 10^{16}$ cm^-2 and applying corrections for ionization (H $^{\circ}$ /H

$\approx 3 \times 10^{-4}$ for

J₀ = 10^-23 and 600 kpc cloud depth) and for helium mass (Y = 0.24), the gas mass could total

$\sim10^{12}~M_{\odot}$ .

These absorbers offer an excellent opportunity to set stringent limits on heavy-element abundances and D/H in low-metallicity gas in the far regions of such galaxies. For example, no Si�III

$\lambda1206.50$ absorption is detected (rest equivalent width

$W_{\lambda} \leq 22$ m�� or N

$_{\rm SiIII} \leq 1.0 \times 10^{12}$ cm^-2 at $4\sigma$ ) at wavelengths corresponding to the strong Ly $\alpha$ absorbers near 1281 �� and 1285 ��. Over a range of photoionization models for (H $^{\circ}$ /H) and (Si⁺²/Si), this limit corresponds to an abundance (Si/H)

$\leq 0.003 ({\rm Si/H})_{\odot}$ for an assumed N

$_{\rm HI} = 2 \times 10^{16}$ cm^-2 and 300-600 kpc cloud depth (Shull et al. 1998). The lack of observed C�IV $\lambda1549$ absorption leads to similar limits, [C/H] < 0.005 solar. A rudimentary analysis of the lack of observed D�I (Ly $\alpha$ ) absorption in the blueward wings of the strong H�I lines suggests that (D/H)

$\leq 2 \times 10^{-4}$ . These limits can be improved with more sophisticated profile fitting and future data from HST/STIS (cycle 8) and FUSE.

The H�I toward PKS�2155-304 appears to represent gas with the lowest detected metallicity. Was this gas was once inside the galaxies at

$cz = 17,000 \pm 1000$ km�s^-1, or is it pristine? We can perhaps answer this question by deeper spectral searches for traces of metals. The origin of the lower-column Ly $\alpha$ systems would seem to be more diverse, possibly arising in extended halos or debris disks of dwarf galaxies, large galaxies, and small groups (Morris & van den Bergh 1994).

Next Section: Theoretical Implications
Title/Abstract Page: The Low-Redshift Intergalactic Medium
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Contents Page: Volume 16, Number 1

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Australia Telescope National Facility

HST Survey of low-z LyAbsorbers

HST Survey of low-z Ly $\alpha$ Absorbers