AAS Meeting #193 - Austin, Texas, January 1999
Session 4. Intergalactic Medium
Display, Wednesday, January 6, 1999, 9:20am-6:30pm, Exhibit Hall 1

## [4.01] A Population of Low Metallicity Lyman Alpha Forest Clouds at z~

K.A. Knierman, J.R. Rigby, C.W. Churchill (Penn State)

Using HIRES/Keck and archival FOS/HST spectra, we are investigating the metallicities of low redshift (0.4< z< 1.4) Lyman \alpha Forest clouds along the lines of sight to 18 quasars. We report first results obtained from a pilot study of five of these lines of sight from the QSO Absorption Line Key Project (Jannuzi et al., 1998, ApJS, 118, 1).

We selected Lyman \alpha clouds from the FOS spectra which have no associated metals, are not blended, and include coverage of the Lyman limit break. The last condition enforces N({\rm HI}) < 1016.5 cm-2. From the HIRES spectra, we then obtained equivalent width upper limits of MgII 2796 for the sample of Lyman \alpha clouds and directly converted these to column densities (since the limits are on the linear part of the curve of growth). Using G. Ferland's CLOUDY, we constructed a grid of photoionized clouds by varying the ionization parameters, U, metallicities, Z, and N({\rm HI}) for a Haardt & Madau UV background at z=0.8. MgII is ideal for constraining metallicities because the ionization corrections are not large and they vary only slightly with ionization condition for U < 10-2.

The results somewhat depend upon the assumed Doppler widths of the Lyman \alpha lines, which range from 20 to 60 km/s (Kirkman & Tytler, 1997, ApJ, 484, 672). If b = 20 km/s, then there are 32 clouds in the range 1015.0 < N({\rm HI}) < 1016.5 cm-2, of which 22 have [Mg/H] < -2. If b = 60 km/s, then there are 11 clouds in this N({\rm HI}) range, of which six have [Mg/H] < -2 . The mean upper limit for both samples is [Mg/H] < -1.8.

Our mean upper limit of [Mg/H] < -1.8 is consistent only with the most conservative lower limit, [C/H] > -1.9, reported by Barlow and Tytler (1998, ApJ, 115, 1725), who stacked 15 FOS/HST spectra in a search for CIV detections. That we have measured [Mg/H] < -2 in a substantial fraction of the clouds in our sample suggests that there is a sub--population of low metallicity (less than 0.01 solar) Lyman \alpha clouds at z<1.