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Session 85 - Quasar Absorption Line Systems.
Display session, Wednesday, January 17
North Banquet Hall, Convention Center

[85.04] A Search for He I Absorption in Ly\alpha Forest Clouds

R. D. Cohen, E. A. Beaver, F. W. Hamann, V. T. Junkkarinen, R. W. Lyons (CASS/UCSD)

We have obtained high signal--to--noise spectra of the QSO PG1630+377 (V=16.0, z=1.478) with the GHRS and FOS on the HST to search for He I \lambda584 absorption from Ly\alpha forest and metal--line clouds. This bright object is particularly well suited for this search. At these relatively low redshifts, confusion of He lines with other lines is minimized. However, the redshift is too low to search for confirming He I \lambda537 lines. As discussed by Reimers amp; Vogel (Aamp;A, 276, L13, 1993), He I absorption is unsuitable for measurement of the He abundance in intergalactic clouds, but is diagnostic of the ionizing flux and conditions in the clouds.

Despite the relatively low density of lines, most of the possible detections of He I are consistent with chance coincidences with Ly\alpha forest lines. We have a positive detection of He I only in the one Lyman limit system present in our spectra. In some cases, sensitive limits can be set on the column density of He I in Ly\alpha forest clouds. Results will be interpreted with the aid of photoionization models and current models of the intergalactic background.

These observations are part of the GHRS GTO program and are supported by NASA grant NAG 5--1858.

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