Astron. Astrophys. 342, 395-407 (1999)

4. The Lyman limit system at

As expected, the velocity profiles of Si II 1260, 1193, 1190, 1526, 1304, Si III 1206, Si IV 1393, 1402, C II 1334, C IV 1548, 1550 and Al II 1670 detected in the high-resolution optical spectra show a complex structure of the LLS at (see Fig. 3). Inspection by eye of the velocity profiles reveals at least 11 different components spread over a velocity range of 200 km s^-1. The velocity offsets between the individual components range from 10 to 32 km s^-1. We have chosen the minimum number of components for which the fit of the complex cannot substantially be improved. The excellent coincidence between the total column densities derived from fitting of individual components with the total column densities obtained from the apparent optical depth method (Savage & Sembach, 1991) implies that there are no important contributions from hidden blended components unresolved at our resolution (but see Sect. 5 below). No edge leading asymmetry of the profiles is visible, as is found for the majority of damped Ly absorbers (Wolfe 1995), favouring gaseous clumps instead of pointed motion like infall, outfall or rotation of gaseous clouds. Towards negative velocities absorption by low ions (C II and Si II ) is dominating, while at the highest positive velocities absorption by highly ionized elements (C IV and Si IV ) is dominating. Absorption lines of singly ionized elements show similar velocity profiles as do those of the high ions. The O I 1302 absorption line profile differs from those of the other elements, but it might be influenced by H I Ly absorption.

Fig. 3. Velocity profiles of Voigt profile fits (dotted lines) to absorption lines of the LLS in optical spectra of HS 1103+6416. The 1 error for the normalized flux is given, too. The ticks mark the positions of individual components. km s-1 corresponds to . The C IV 1548 absorption profile is blended with C IV 1550 absorption from the complex absorber system at . A comparison of the Si II line profiles reveals that Si II 1190, 1193 must be blended, too.

No absorption is seen in the optical data for C I 1560, 1656, N I 1199, 1200, N V 1238, 1242, Si I 1631, 1845, Al I 1762, 1765, Mg I 1827, Zn I 1589 or Ni II 1741. Al III 1854 falls in a gap between the orders. Very weak and noisy absorption is visible for Al III 1862.

In the ultraviolet spectra prominent absorption by oxygen and carbon is visible, i.e. C II 903, C III 977 and O II 834, O III 507, 702, 832 and O IV 787 (see Fig. 4). O V 629 is located at the Lyman edge of the LLS at . A strong absorption line with perfect wavelength agreement is also detected for O VI 1031, but, unfortunately, O VI 1037 is located in a complex blend. Weak absorption at the expected positions of N II 1084, N III 685, 685.5 and N IV 765 resonance lines is visible, too.

Fig. 4. The dotted lines represent the model results for the strongest absorption lines of the LLS in the ultraviolet spectra of HS 1103+6416. The 1 error for the normalized flux is given, too. The x-axis is given in Å with 5 Å distance between the tickmarks. The expected positions of O VI 1031, 1037 and S V 786 absorption lines are indicated by tickmarks.

Further interesting ions which are only seen in absorption in the UV part of QSO spectra are those of neon and sulphur. Both lines of the Ne III 488, 489 doublet are located at the edge of a blend. Unfortunately, Ne IV 543, 542, 541, Ne V 568 and Ne VI 399, 401, 558 resonance lines fall in noisy regions of the spectrum. Ne V 480 might arise in a blend with O III 702 of the LLS at . Absorption by Ne VII 465 is possible at the edge of a blend. Absorption lines detected at the expected positions of Ne VIII 770, 780 are offset by 0.43 and 0.65 Å, respectively, and are more likely identified with hydrogen lines. Regions of the spectrum were sulphur lines are expected are also shown in Fig. 4. S VI 933, 944 lines are located in blends.

© European Southern Observatory (ESO) 1999

Online publication: February 22, 1999
helpdesk.link@springer.de