R. Baade

The UV spectrum of HS 1700+6416 II. Fuse observations of the He II Lyman alpha forest

Aims. We present the far-UV spectrum of the quasar HS 1700+6416 taken with FUSE. This QSO provides the second line of sight with the He ii absorption resolved into a Lyα forest structure. Since HS 1700+6416 is slightly less redshifted (zem = 2.72) than HE 2347-4342, we only probe the post-reionization phase of He ii, seen in the evolution of the He ii opacity, which is consistent with as imple power law. Methods. The He ii/H i ratio η is estimated using a line profile-fitting procedure and an apparent optical depth approach, respectively. The expected metal line absorption in the far-UV is taken into account as well as molecular absorption of galactic H2. About 27% of the η values are affected by metal line absorption. In order to investigate the applicability of the analysis methods, we create simple artificial spectra based on the statistical properties of the H i Lyα forest. Results. The analysis of the artificial data demonstrates that the apparent optical depth method as well as the line profile-fitting procedure lead to confident results for restricted data samples only (0.01 ≤ τHi ≤ 0.1 and 12.0 ≤ logNHi ≤ 13.0, respectively). The reasons are saturation in the case of the apparent optical depth and thermal line widths in the case of the profile fits. Furthermore, applying the methods to the unrestricted data set may mimic a correlation between the Heii/Hi ratio and the strength of the H i absorption. For the restricted data samples a scatter of 10−15% in η would be expected even if the underlying value is constant. The observed scatter is significantly larger than expected, indicating that the intergalactic radiation background is indeed fluctuating. In the redshift range 2.58 < z < 2.72, where the data quality is best, we find η ∼ 100, suggesting a contribution of soft sources like galaxies to the UV background.

Detection of molecular hydrogen at z¿=¿1.15 toward HE 0515¿4414

A new molecular hydrogen cloud is found in the sub-damped Lya absorber [log N(H I) = 19.88 ′ 0.05] at the redshift z a b s = 1.15 toward the bright quasar HE 0515-4414 (z e m = 1.71). More than 30 absorption features in the Lyman band system of H 2 are identified in the UV spectrum of this quasar obtained with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope. The H 2 -bearing cloud shows a total H 2 column density N(H 2 ) = (8.7 + 8 . 7 -4.0) x 10 1 6 cm - 2 and a fractional molecular abundance f H 2 = (2.3 + 2 . 3 -1.1) x 10 - 3 derived from the H 2 lines arising from the J = 0 - 5 rotational levels of the ground electronic vibrational state. The estimated rate of photodissociation at the cloud edge I 0 ≤ 1.8 x 10 - 8 s - 1 is much higher than the mean Galactic disk value, I M W ∼ 5.5 x 10 - 1 1 s - 1 . This may indicate an enhanced star-formation activity in the z = 1.15 system as compared with molecular clouds at z ∼ 3 where I ∼ I M W . We also find a tentative evidence that the formation rate coefficient of H 2 upon grain surfaces at z = 1.15 is a factor of 10 larger than a canonical Milky Way value, R M W 3 x 10 - 1 7 cm 3 s - 1 . The relative dust-to-gas ratio estimated from the [Cr/Zn] ratio is equal to k = 0.89 ′ 0.19 (in units of the mean Galactic disk value), which is in good agreement with a high molecular fraction in this system. The estimated line-of-sight size of L ∼ 0.25 pc may imply that the H 2 is confined within small and dense filaments embedded in a more rarefied gas giving rise to the z = 1.15 sub-damped Lya absorber.

HE 0515–4414: an unusual sub-damped Ly α system revisited

Aims. We examine the ionization, abundances, and differential dust depletion of metals, the kinematic structure, and the physical conditions in the molecular hydrogen-bearing sub-damped Ly α system toward HE 0515−4414. Methods. We used the STIS and VLT UVES spectrographs to obtain high-resolution recordings of the damped Ly α profile and numerous associated metal lines. Observed element abundances were corrected with respect to dust depletion effects. Results. The sub-damped Ly α absorber at redshift z = 1.15 is unusual in several aspects. The velocity interval of associated metal lines extends for 700 km s −1 . In addition, saturated H i absorption is detected in the blue damping wing of the NHi = 8 × 10 19 cm −2 main component. The column density ratios of associated Al ii ,A liii ,a nd Feii lines indicate that the absorbing material is ionized. Nineteen of a total of 31 detected metal line components are formed within peripheral H ii regions, while only 12 components are associated with the predominantly neutral main absorber. The bimodal velocity distribution of metal line components suggests two interacting absorbers. For the main absorber, the observed abundance ratios of refractory elements to Zn range from a Galactic warm disk [Si/Zn]g = −0.40 ± 0.06, [Fe/Zn]g = −1.10 ± 0.05 to halo-like and essentially undepleted patterns. The dust-corrected metal abundances indicate a nucleosynthetic odd-even effect and might imply an anomalous depletion of Si relative to Fe for two components, but otherwise they do correspond to solar ratios. The intrinsic average metallicity is almost solar [Fe/H]m = −0.08 ± 0.19, whereas the uncorrected average is [Zn/H]g = −0.38 ± 0.04. The ion abundances in the periphery conform to solar element composition. Conclusions. The detection of H ii, as well as the large variation in dust depletion for this sight line, raises the question whether in future studies of damped Ly α systems ionization and depletion effects have to be considered in more detail. Ionization effects, for instance, may mimic an enrichment of α elements. An empirical recipe for detecting H ii regions is provided.

The associated system of HE 2347-4342

We present an analysis of the complex associated system of the high-redshift QSO HE 2347-4342. Absorption features of

High-resolution OVI absorption line observations at 1.2

\ion{H}{i}

\leq z \leq

,

1.7 in the bright QSO HE 0515-4414

\ion{C}{iii}

Fine-structure diagnostics of neutral carbon toward HE 0515-4414

,

VLA Observations of ζ Aurigae: Confirmation of the Slow Acceleration Wind Density Structure

\ion{C}{iv}

A high-resolution study of the evolution of the Lyman

,