4 The cosmological mass-density of the O VI phase

With the present STIS spectra of HE 0515-4414 the redshift range $1.21 \leq z \leq 1.73$ has been covered for the first time at sufficiently high resolution to undertake a sensitive search for O VI absorbers. We have detected 6 O VI systems. Two of them (z = 1.697, 1.736) are either associated with the QSO or in the proximity zone of the extremely luminous QSO. The system z = 1.416 is marginal, since only the 1031Å line is detected. Including the latter, we have 4 detections in the range z = 1.21to 1.67 which yield a number density of O VI absorbers with $W_{\rm rest} \geq$ 25mÅ of ${\rm d}N/{\rm d}z \leq 10$. Compared with the findings by Tripp et al. (2000) of ${\rm d}N/{\rm d}z = 48$ at $\bar{z} \simeq 0.21$, the number density at $\bar{z} \simeq 1.44$ is roughly a factor of 5 lower. Tripp et al. (2000) compared their finding of a high number density of weak O VI absorbers ( $W_{\rm rest} \geq$ 30 mÅ) in H 1821+643 and PG 0953+415 with other classes of absorbers and found that the weak O VI number density is more comparable to that of the low z weak Ly $\alpha$ absorbers - which have ${\rm d}N/{\rm d}z \approx 100$ for $W_{\rm rest} \geq$ 50mÅ - than to other types of metal absorbers like Mg II. In HE 0515-4414 we have at least 42 Ly $\alpha$ systems (the exact number being unknown due to the line blending problem) with $W_{\rm rest} \geq$ 50mÅ in the range $1.21 \leq z \leq 1.67$ which yields roughly ${\rm d}N/{\rm d}z = 90$. Among these, roughly half of them are strong, saturated Ly $\alpha$ lines with a detected Ly $\beta$ line. Again, while our STIS spectrum of HE 0515-4414 confirms the number density of Ly $\alpha$ absorbers found previously (see Weymann et al. 1998), the number of O VI absorbers with $W_{\rm rest} \geq 25$mÅ is lower than the number of Ly $\alpha$ absorbers with $W_{\rm rest} \geq 50$mÅ by a factor of 10. It is noteworthy that, except the z = 1.674 system, O VI is detected in lower column density Ly $\alpha$ absorbers (log $N_{\rm H} \leq 14$). Following the calculations by Tripp et al. (2000) and earlier work by Storrie-Lombardi et al. (1996) and Burles & Tytler (1996), the mean cosmological mass-density of O VI absorbers can be written in units of the critical density $\rho_{\rm c}$ as

$$\Omega_{\rm b} (O{\sc vi}) = \frac{\mu\,m_{\rm H}\,H_{0}}{\rh... ...}_{O{\sc vi}} \frac{\sum_i N_i ({\rm O{\sc vi}})} {\Delta X},$$ (3)

where [O/H] is the assumed oxygen abundance in the O VI absorbing gas, f(O VI) is the fraction of oxygen in O VI, $\sum_{i} N_i(O{\sc vi})$ is the total O VI column density from all absorbers, and $\Delta X$ is the absorption distance (Bahcall & Peebles 1969).

Over the redshift interval z = 1.21 to z = 1.67 we have $\Delta X = 0.72$ for q₀ = 1/2. $\sum_i N_i ({\rm O{\sc vi}})$ is $2.1 \times 10^{14}\,{\rm cm^{-2}}$ (Table 1). Assuming $f(O{\sc vi}) = 0.2$, following Tripp et al. (2000) and Tripp & Savage (2000), which is close to the maximum for both collisional ionization and photoionization, we obtain a lower limit $\Omega_{\rm b}$(O VI) $\geq 3 \times 10^{-5}$ $[{\rm (O/H)/(O/H)}_{\odot}]^{-1}\,h^{-1}_{75}$. The only reliably measured heavy element abundances at $\bar{z} = 1.4$ are from DLAs. Typically the metal abundance (e.g. from Zn) is 1/10 solar (Pettini et al. 1999; Vladilo et al. 2000). There is, however, no guarantee that these abundances apply also to the O VI absorbers among the low column density systems. Assuming 1/10 solar for the oxygen abundance, we have $\Omega_{\rm b} ({\rm O{\sc vi}}) \geq 3\times 10^{-4}\, h^{-1}_{75}$. With the same assumptions Tripp et al. (2000) derived a value $\geq 4\times 10^{-3}\, h^{-1}_{75}$. Using a somewhat different formalism for the derivation of $\Omega_{b}(O{\sc vi})$, namely Eq. (6) from Tripp & Savage (2000), we get with the same assumptions $\Omega_{\rm b}$(O VI) $\geq 1.5 \times 10^{-4}\, h^{-1}_{75}$. Both from the number counts of the O VI systems and the estimate of the mean O VI density the unavoidable conclusion seems to be that at $\bar{z} = 1.5$, the baryon content of the O VI phase contains a factor of $\geq 10$ less material than at $\bar{z} = 0.21$.