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Title:
The Lyalpha forest at 1.5 < z < 4
Authors:
Kim, T.-S.; Cristiani, S.; D'Odorico, S.
Affiliation:
AA(Dipartimento di Astronomia dell'Universitàdi Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy), AB(ST European Coordinating Facility, ESO, Karl-Schwarzschild-Strasse 2, 85748, Garching b. München, Germany ; Dipartimento di Astronomia dell'Universitàdi Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy), AC(European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748, Garching b. München, Germany )
Publication:
Astronomy and Astrophysics, v.373, p.757-781 (2001) (A&A Homepage)
Publication Date:
07/2001
Origin:
A&A
A&A Keywords:
COSMOLOGY: OBSERVATIONS, QUASARS: ABSORPTION LINES
DOI:
10.1051/0004-6361:20010650
Bibliographic Code:
2001A&A...373..757K

Abstract

Using high resolution (R ~ 45 000), high S/N ( ~ 20-50) VLT/UVES data, we have analyzed the Lyalpha forest of 3 QSOs in the neutral hydrogen (H I) column density range Nion HI = 1012.5-16 cm-2 at 1.5 < z < 2.4. We combined our results with similar high-resolution, high S/N data in the literature at z > 2.4 to study the redshift evolution of the Lyalpha forest at 1.5 < z < 4. We have applied two types of analysis: the traditional Voigt profile fitting and statistics on the transmitted flux. The results from both analyses are in good agreement: 1. The differential column density distribution function, f(Nion HI), of the Lyalpha forest shows little evolution in the column density range Nion HI = 1012.5-14 cm-2, f(Nion HI) ~ Nion HI-beta , with beta ~ 1.4-1.5 at 1.5 < z < 4 and with a possible increase of beta to beta ~ 1.7 at z < 1.8. A flattening of the power law slope at lower column densities at higher z can be attributed to more severe line blending. A deficiency of lines with Nion HI > 1014 cm-2 is more noticeable at lower z than at higher z. The one-point function and the two-point function of the flux confirm that strong lines do evolve faster than weak lines; 2. The line number density per unit redshift, dn/dz, at Nion HI = 1013.64-16 cm-2 is well fitted by a single power law, dn/dz ~ (1+z){2.19 +/- 0.27}, at 1.5 <z < 4. In combination with the HST results from the HST QSO absorption line key project, the present data indicate that a flattening in the number density evolution occurs at z ~ 1.2. The line counts as a function of the filling factor at the transmitted flux F in the range 0 < F < 0.9 are constant in the interval 1.5 < z < 4. This suggests that the Hubble expansion is the main drive governing the forest evolution at z > 1.5 and that the metagalactic UV background changes more slowly than a QSO-dominated background at z < 2; 3. The observed cutoff Doppler parameter at the fixed column density Nion HI = 1013.5 cm-2, bc, 13.5, shows a weak increase with decreasing z, with a possible local bc, 13.5 maximum at z ~ 2.9; 4. The two-point velocity correlation function and the step optical depth correlation function show that the clustering strength increases as z decreases; 5. The evolution of the mean H I opacity, /lineτion HI, is well approximated by an empirical power law, /lineτion HI ~ (1+z)3.34 +/- 0.17, at 1.5 < z < 4; 6. The baryon density, Omegab, derived both from the mean H I opacity and from the one-point function of the flux is consistent with the hypothesis that most baryons (over 90%) reside in the forest at 1.5 < z < 4, with little change in the contribution to the density, Omega , as a function of z. Based on public data released from the UVES commissioning at the VLT/Kueyen telescope, ESO, Paranal, Chile.Tables~A.1, A.2 and A.3 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/373/757

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