A high-resolution study of the evolution of the Lyman α forest in the redshift interval $0.9 < z < 1.7$

Abstract

Abstract. Spectroscopy with HST/STISEchelle and VLT/UVES of the bright QSO HE 0515−4414 (z em = 1.73, B = 15.0) offers for the first time the opportunity to study the Lyman α forest in the redshift range 0.9 < z < 1.7 at a resolution ≤10 km s −1 . The number density evolution of the Lyman α lines is well described by the power law approach dn/dz ∝ (1 + z) γ . We derive γ = 2.23 ± 1.21 for the strong lines (13.64 ≤ log N H i ≤ 16.00), in agreement with the Lyman α forest evolution for z > 1.7. The expected slow-down in their evolution does not appear earlier than z ∼ 1. For the weak lines (13.10 ≤ log N H i ≤ 14.00) we find that the HE 0515−4414 data for z > 1 follow the trend with γ = 1.40 known from z > 1.7 observations, i.e. we confirm the difference in evolution between weak and strong lines. We use the two-point velocity correlation function (TPCF) to search for clustering of the Lyman α lines, yet we detect no excess in the TPCF on scales up to 10 000 km s −1 .

The Low‐Redshift Lyα Forest toward PKS 0405−123

“…The z < 0:127 half of the 13:1 < log N H i < 14:0 sample is consistent with both the redshift densities of the STIS E140M samples of Sembach et al (2004) and Richter et al (2004) at z P 0:12 and the 1:0 P z P 1:7 data of Janknecht et al (2002), despite containing one to two voids, depending on how they are defined. The z < 0:127 redshift density toward PKS 0405À123 is also within 1.6 of the result of Penton et al (2004), which covers over 5 times the redshift path that our sample does, but may suffer from blending problems.…”

“…Most of the H i column densities (10/13) at 0:127 < z < 0:175 are in the range 13:3 log N H i 14:0, which corresponds to / % 3 10 . We note that the difference in dN/dz within the PKS 0405À123 sight line is significantly greater than that for PG 1259+593 , as well as for the HE 0515À4414 sight line (0:99 < z < 1:68; Janknecht et al 2002). The PKS 0405À123 sight line could be an extreme example of cosmic variance, which will be considered in more detail in the discussion (x 7).…”

“…The H i column density distributions at low and high redshift appear consistent with each other (Penton et al 2004), whereas clustering in the Ly forest at low z is expected (Davé et al 1999) but has been challenging to detect. Among recent studies, Janknecht et al (2002) found no evidence for clustering via the two-point correlation function for velocity scales Áv < 1000 km s À1 for a sample of 235 Ly forest lines over column densities 13:1 < log N H i < 16:4 at redshifts 0:9 < z < 1:7. However, Penton et al (2004) found a 5.3 signal for clustering at Áv < 190 km s À1 (and 7.2 for Áv < 260 km s À1 ) at 0:002 < z < 0:069 for 187 Ly absorbers with rest equivalent width W 0 > 0:065 8 (log N H i k 13:1).…”

“…Our clustering signal of (Áv < 250 km s À1 ) ¼ 1:2 is weaker in comparison. However, Janknecht et al (2002) found no Ly-Ly correlations in STIS+VLT echelle data of HE 0515À4414 (0:9 < z < 1:7), using 235 Ly lines with log N H i ! 13:1.…”

“…Clustering in the Ly forest has been studied in a number of cases (Janknecht et al 2002 and references therein), with weak clustering indicated on velocity scales of Áv < 500 km s…”