Statistics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6593s&link_type=abstract
HST Proposal ID #6593
Statistics
Hst Proposal Id #6593 Qso Absorption Lines
Scientific paper
We propose to use the GHRS+G160M grating to study 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 7 very bright AGN with cz 10,000 kms. Our program on Ly-alpha clouds differs in 3 important respects from the Absorption- Line Key Project and subsequent GHRS and FOS low-resolution projects: {1} It probes the very low column-density regime of Ly-alpha clouds, which may represent the low-redshift analogs of the very low metallicity high-redshift Ly-alpha clouds; {2} It is restricted to cz < 20,000 kms, 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, multiple components} at a resolution of 15 kms { 10 times the FOS resolution} for cloud model discrimination. Our previous GHRS/G160M work showed that, unlike the high column density 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 < 20,000 kms examined thus far, for a cloud/galaxy association within 200 kpc. We detect 3-4 clouds in voids but our current statistics suggest that Ly-alpha clouds ``avoid the voids''. Our proposal will nearly double the number of high S/N Ly-alpha lines, double the number of nearby galaxy halos probed at D <= 750 kpc for weak Ly-alpha lines, and nearly double the pathlength through voids.
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