Statistics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7345s&link_type=abstract
HST Proposal ID #7345
Statistics
Hst Proposal Id #7345 Quasar Absorption Lines
Scientific paper
We propose to use the STIS+E140M grating to complete our study of the local Ly-alpha forest at high spectral resolution to better understand the origin and physical conditions of these clouds and their relationship {if any} to galaxies. Our targets are 14 very bright AGN at cz 20, 000kms. Our Ly-alpha cloud program differs in 3 important respects from the Absorption-Line Key Project and subsequent FOS projects: {1} It probes the very low column-density regime of Ly-alpha clouds, which may represent the low-z analogs of the low- metallicity Ly-alpha clouds at high-z; {2} It is restricted to cz<20, 000kms, allowing detailed ground-based studies of nearby galaxies with spectroscopy, H Alpha imaging, and 21-cm maps to very low luminosities; {3} It yields line profiles {Doppler widths, ionH1 columns, components} at a resolution of 25kms { 6 times the FOS resolution} for cloud model discrimination. Our previous GHRS/G160M work showed that, unlike the high-N_H clouds, the low-N_H Ly-alpha clouds cluster only weakly with galaxies. However, there is no convincing case, among 24 GHRS clouds at cz<20, 000kms examined thus far, for an individual cloud/galaxy association. We detect 3-4 clouds in voids but our current statistics suggest that Ly-alpha clouds ``avoid the voids''. Our proposed observations totalling 250, 000kms of coverage will more than double the number { 60} of low column density Ly-alpha lines, double the number of nearby galaxy halos probed at D<=250 kpc for weak Ly-alpha lines, and double the pathlength through voids.
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