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Exploring the Lyman Forest with VLT/UVES

By Stefano Cristiani, Simone Bianchi and Tae-sun Kim


Abstract. A sample of 8 QSOs with 1.7 < zem < 3.7 has been observed with VLT/UVES at a typical resolution 45000 and S/N ∼ 40−50. Thanks to the two-arm design of the spectrograph, a remarkable efficiency has been achieved below 400nm and above 800nm, which translates immediately in the possibility of obtaining new results, especially at z ∼ < 2.5. We report here new insight gained about the evolution of the number density of Ly-α lines, their column density distribution, the ionizing UV background and the cosmic baryon density. 1 The number density evolution of Ly-α lines The swift increase of the number of absorptions (and the average opacity) with increasing redshift is the most impressive property of the Ly-α forest. Fig. 1 shows the number density evolution of the Ly-α lines [8, 9] in the column density interval 1 NHI = 10 13.64−16 cm −2. The long-dashed line is the maximum-likelihood fit to the data at z> 1.5 with the customary parameterization: N(z) = N0(1 + z) γ = (6.5 ± 3.8)(1 + z) 2.4±0.2. The UVES [2] observations imply that the turn-off in the evolution does occur at z ∼ 1, no

Year: 2001
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