AAS 205th Meeting, 9-13 January 2005
Session 129 Absorptions and The Intergalactic Medium
Oral, Wednesday, January 12, 2005, 2:00-3:30pm, Pacific 2/3

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[129.06] Mean amount of Absorption from the Intergalactic Medium

D. Tytler, D. Kirkman, J. M. O'Meara, N. Suzuki, A. Orin, D. Lubin, P. Paschos, T. Jena, W-C Lin, M. Norman (UCSD), A. Meiksin (U of Edinburgh)

We have made the first accurate measurement of the mean amount of absorption from HI in the low density IGM at z=1.9 and the first calibrated measurement at any redshift. We use the spectra of 77 QSOs that we observed with the Kast spectrograph on the Lick Observatory 3m Shane telescope. We made artificial spectra that match the real spectra in most obvious ways: redshift, resolution, S/N, emission lines and absorption lines. We fit the unabsorbed continua to both the real and artificial spectra to calibrate our errors. We also measure the mean amount of absorption from metal lines (2.3 0.5%) and the Lyman-alpha lines of Lyman limit systems (1.0 0.4%). The low density IGM accounts for 78% of the total absorption in the Lyman-alpha forest at z=1.9. We have measured the variation on the amount of absorption on large 150 Mpc scales, and we have found fully hydrodynamic simulations of the IGM that match these and other measurements of Lyman-alpha forest to within the measurement errors. We obtain new joint constraints on a set of cosmological and astrophysical parameters, including the most accurate measurement of the rate of ionization of HI atoms. This work was funded in part by NSF grant AST 0098731.


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