Physics
Scientific paper
Jul 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999hst..prop.8125s&link_type=abstract
HST Proposal ID #8125
Physics
Hst Proposal Id #8125 Quasar Absorption Lines
Scientific paper
The origin of the low-redshift LyAlpha clouds remains a mystery, made more tantalizing by the possibility that they contain a significant fraction of baryons. Our GHRS/G160M study of PKS 2155-304 {Shull et al. 1998} identified a cluster of strong LyAlpha absorbers between 16, 000 and 18, 500 km s^-1 with N_ HI 2 * 10^16 cm^-2 {Ly-edge estimate from ORFEUS}. From expected positions of Si III and C IV absorption, we set 4Sigma limits Si/C < 0.003 and C/H < 0.005 solar abundance. Using the VLA, we identify these absorbers with the extended halos or intragroup gas surrounding four H I galaxies offset by {400-800}h_75^-1 kpc. PKS 2155-304 is a FUSE Key Science target for studies of extragalactic D/H and will produce accurate N_ HI from higher Lyman series and Lyman limits. Here, we propose to obtain a STIS/E140M spectrum {1150-1740 Angstrom} at 10 km s^-1 resolution to detect or limit metal absorption lines {C IV, Si III, Si IV, N V, C II, Si II, S II} down to below 10^-3 solar abundance {4 mAngstrom EW limit at S/N 30} as well as D I LyAlpha. These data will ascertain whether the IGM in these absorbers is primordial and thus will probe the origin and chemical evolution of gas at large distances from galaxies. The combination of a bright target {V=13.1} and high-N_HI absorbers make this an ideal situation for studying metals in LyAlpha clouds. These clouds have the potential for being the most metal-free gas ever studied.
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