3 Absorption systems with OVI lines

We searched for O VI lines associated with known Ly $\alpha$ and Ly $\beta$absorbers. Therefore, as a starting point, we tried to identify all Ly $\alpha$ lines. Line identification and the analysis of the Ly $\alpha$ forest will be presented in some detail in a later paper. At the resolution of $\sim$30000 (STIS) and $\sim$50000 (UVES), narrow metal lines can usually be distinguished easily from hydrogen lines. In all Ly $\alpha$ absorption systems with column densities log $N_{\rm H} \geq$ 13.5 we searched for metal lines, in particular for O VI, C IV, N V, Si IV, C III, N III. In a first step, all lines within $\pm 200\, {\rm km\,s^{-1}}$ were considered to be plausibly associated with the Ly $\alpha$/Ly $\beta$ systems. Within this selection criterium we have found 6 systems with probable O VI absorption, listed in Table 1. Due to the moderate S/N ratio of the STIS spectra (between 10 and 20 per resolution element) the detection limit of O VI is estimated to lie between $\log N = 13.3$ at the lower limit of the z range and $\log N = 13.1$ near the quasar.

• z = 1.385: The single O VI line and the C IV doublet correspond to the unsaturated Ly $\alpha$ line at 2899.42 Å. Both O VI and C IV are slightly blueshifted (by $-14\,{\rm km\,s^{-1}}$ and $-17\,{\rm km\,s^{-1}}$, respectively) relative to Ly $\alpha$ and Ly $\beta$ (Fig. 1). The Doppler parameter of the C IV doublet of $8.2\,{\rm km\,s^{-1}}$ is determined reliably with the high-resolution, high S/N UVES spectra, while O VI, less certain, yields $13.5\,{\rm km\,s^{-1}}$. The absorbing cloud at z = 1.385 is a close neighbour to a strong Ly $\alpha$/Ly $\beta$ system at z = 1.386( $+123\,{\rm km\,s^{-1}}$, $\log N_{\rm H} = 15.1$) with no heavy element absorption at all. The z = 1.385 system appears to represent the extremely rare case of a highly ionized cloud with low neutral hydrogen density ( $\log N_{\rm H} = 13.9$). According to the velocity centroids, C IV and O VI are apparently not formed in the same volume. In addition, the velocity shift between H I and C IV/O VI is in favour of different phases (volumes). This behaviour is similar to what Tripp et al. (2000) have found in a system at z = 0.22637in H 1821+643. Because apparently H I, C IV, and O VI are not formed in the same volume, there are no empirical constraints on the ionization mechanism (photoionization versus collisional ionization).

• z = 1.416: The absorber is seen in Ly $\alpha$, Ly $\beta$ and O VI 1031, while the O VI 1037 line is blended with Ly $\varepsilon$ of a strong system at z = 1.674. Since again a velocity shift of $+22\,{\rm km\,s^{-1}}$ is seen between O VI 1031 and Ly $\alpha$/Ly $\beta$, this system can only be considered as marginal.

• z = 1.602: Besides Ly $\alpha$ and Ly $\beta$, only the O VI doublet is detected at a velocity of $+18\,{\rm km\,s^{-1}}$ relative to the hydrogen main component. Notice that in velocity space the O VI doublet is located between two Ly $\alpha$ clouds (cf. Table 1, Fig. 1).

• z = 1.674: This absorbtion system is seen in Ly $\alpha$ down to Ly $\varepsilon$ and exhibits a strong O VI doublet. The C IV doublet in our UVES spectra show that it consists of two components, a narrow ( $b \simeq 9\,{\rm km\,s^{-1}}$), stronger component redshifted by $4.3 \,{\rm km\,s^{-1}}$ relative to hydrogen, and a broad ( $b \simeq 18\,{\rm km\,s^{-1}}$) component at $-12.8\,{\rm km\,s^{-1}}$. Noteworthy are the broad wings of Ly $\alpha$ which can be explained only with an additional extremely broad ( $b > 100\,{\rm km\,s^{-1}}$) unsaturated ( $\log\, N_{\rm H} = 14.1$) component which in velocity nearly coincides with the O VI line and the saturated Ly $\alpha$ Doppler core ( $\log\, N_{\rm H} = 15.1$). The broad wing is not seen clearly in the other Lyman lines, but is still consistent with the lower S/N STIS spectra. This extremely broad component is probably caused by collapsing or expanding structures in the intergalactic medium.

• z = 1.697: This is a system with O VI, C IV, and N V doublets as well as C III 977Å in combination with an unsaturated Ly $\alpha$ line. The high resolution UVES profiles of both C IV and N V show two components (Fig. 1): a strong one, nearly unshifted relative to hydrogen, and a weak one at $\sim$ $-33\,{\rm km\,s^{-1}}$. The O VI 1037Å line is at $+ 0.4\,{\rm km\,s^{-1}}$ with a possible second component at $-32.9\,{\rm km\,s^{-1}}$ (O VI 1031Å is blended with a strong Ly $\alpha$ line at z = 1.2897). It seems a likely supposition that O VI is being formed in the same volume as C IV and N V. If so, the column density ratios O VI/C IV and O VI/N V as well as the high O VI/H I ratio support the idea of photoionization in the proximity zone of the QSO. Notice that HE 0515-4414 is one of the most luminous QSOs in the universe.

• z = 1.736: This is an associated system close to the systemic QSO redshift ( $z_{\rm em} = 1.73$) which shows only the O VI doublet. Both C IV and N V are not detected, in spite of the high S/N of the UVES spectra. Again, the inferred lower limits to the column density ratios O VI/C IV and O VI/N V are consistent only with photoionization.

Figure 1: Selected absorption line profiles of systems with O VI detection. The normalized flux is plotted vs. rest-frame velocity of the hydrogen main component. Long tick marks indicate the position of the primary lines, while short tick marks indicate additional absorption components. It should be noted that some profile ensembles contain lines which do not belong to the same absorption system. The dotted curves represent our fit models.

  

Table 1: Absorption line systems with O VI detections.

$$\begin{tabular}{lrcccrcr@{.}l} \hline {\rule[-2mm]{-1mm}{6mm}... ...m 0.04$ & $20.2 \pm 2.0$ & 42 & $-1$&2 \\ \hline \end{tabular}$$

  

Table 1: continued.

$$\begin{tabular}{lrcccrcr@{.}l} \hline {\rule[-2mm]{-1mm}{6mm}... ... 0.05$ & $22.0 \pm 2.8$ & 32 & $-12$&2 \\ \hline \end{tabular}$$

^*

The Doppler parameter has been fixed to improve the goodness-of-fit.