J. Michael Shull , Steven V. Penton , John T. Stocke, PASA, 16 (1), in press.
Next Section: Theoretical Implications Title/Abstract Page: The Low-Redshift Intergalactic Medium Previous Section: Introduction | Contents Page: Volume 16, Number 1 |
HST Survey of low-z LyAbsorbers
The frequency of low-z Ly lines with
mÅ reported by the HST/FOS Key Project,
(Bahcall et al. 1996), was considerably higher than a simple extrapolation from the high-redshift forest. These higher-N absorbers exhibit associations with galaxies (
kpc) about half the time (Lanzetta et al. 1995).
), to probe their association with galaxies, and to measure their clustering and large-scale structure. Toward 15 targets, we detected Ly systems (plus a number of high-velocity clouds and associated Ly absorbers) over a cumulated pathlength
km s-1. In cycle 7, we will observe 14 more sightlines with the Space Telescope Imaging Spectrograph (STIS) to double our Ly sample. The locations of Ly absorbers toward two of our first sightlines are shown in Figure 1.
In our first 4 sightlines, the frequency of absorbers with N
cm-2 was
, corresponding to a local space density,
for absorber radius
. This space density is times that of bright (L*) galaxies, but similar to that of dwarf galaxies with
. >From a statistical, nearest-neighbor analysis, we found that the Ly 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).
and offset distance
is seen at heliocentric velocity cz = 1955 km s-1, remarkably near to that of the Ly absorber. Thus, some of the lower-N 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 absorption lines, down to 20 mÅ (N
cm-2) in some cases. Many of the new sightlines exhibit considerably more Ly absorbers; for these 15 sightlines,
for N
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
cm-2. For a curve of growth with b = 25 km s-1, the 70 Ly absorbers with
follow a distribution
, remarkably close to the slope in the high-z Ly 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).
We turn now to the extraordinary sightline toward PKS 2155-304 (Bruhweiler et al. 1993; Shull et al. 1998). This target exhibits numerous Ly 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
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.
cm-2 and applying corrections for ionization (H/H
for
J0 = 10-23 and 600 kpc cloud depth) and for helium mass (Y = 0.24), the gas mass could total
.
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
absorption is detected (rest equivalent width
mÅ or N
cm-2 at ) at wavelengths corresponding to the strong Ly absorbers near 1281 Å and 1285 Å. Over a range of photoionization models for (H/H) and (Si+2/Si), this limit corresponds to an abundance (Si/H)
for an assumed N
cm-2 and 300-600 kpc cloud depth (Shull et al. 1998). The lack of observed C IV absorption leads to similar limits, [C/H] < 0.005 solar. A rudimentary analysis of the lack of observed D I (Ly) absorption in the blueward wings of the strong H I lines suggests that (D/H)
. 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
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 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 Previous Section: Introduction | Contents Page: Volume 16, Number 1 |
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