5 Damped Lyman alpha systems in the CORALS survey

We adopt the usual definition of a DLA, i.e. N(H I)  $\ge 2 \times 10^{20}$ cm^-2, although our data are of sufficient resolution and S/N to recognise and measure absorbers of somewhat lower N(H I) (Ellison 2000). From the observations of the 66 CORALS QSOs, a total of 22 DLAs have been identified. Three of these have absorption redshifts similar to the emission redshift of the QSO. We follow the standard procedure of excluding DLAs within 3000 km s^-1 of the QSO redshift from our statistical analysis, in order to facilitate comparison with other surveys. However, we note that the $z_{\rm abs} \sim z_{\rm em}$ DLAs are probably similar to intervening absorbers (Møller et al. 1998) and we defer the analysis of the "associated'' CORALS DLAs to a future paper (Ellison et al. in preparation). Of the 19 intervening DLAs, two (B0458-020 and B0528-250a) were already known and have been extensively studied prior to our survey. Profile fits to all the others are shown in Fig. 2. We now briefly discuss each DLA system.

B0335-122

Extended absorption around the DLA trough complicates the fitting of this system. The O I $\lambda$1302 line is used to constrain redshift, but the error on the final fit is relatively high ( $6\pm1 \times 10^{20}$ cm^-2) due the critical, yet uncertain, continuum placement.

B0347-211

The combination of low absorption redshift and faint QSO magnitude have resulted in a low S/N spectrum of this DLA and a relatively poor fit. The redshift is constrained by the Al II $\lambda$1670 line and the N(H I) determined by fitting a damped profile is in good agreement with the column density inferred from the equivalent width measurement.

B0432-440

Although the spectrum is noisy, the column density of this DLA is reasonably constrained by the base of its trough and the shape of its red wing. This system is well-fitted with a column density $N\rm (H~I) = 6 \times 10^{20}$ cm^-2.

B0438-436

Both AAT and VLT spectra were obtained for this QSO. The best fit to the combined spectrum has N(H I) = $6 \times 10^{20}$ cm^-2, although this is constrained mostly by the red wing since there is significant contaminating absorption in the blue wing. The redshift of the Al II $\lambda 1670$ line (with rest-frame equivalent width W₀ = 0.63Å) agrees well with the redshift determined from the centre of the DLA trough.

B0537-286

A relatively high S/N spectrum and simple structure around the absorber permit a good fit to this DLA.

B0913+003

An excellent fit to this DLA is facilitated by the clearly defined damping wings and lack of blending.

B0933-333

Despite blending with a weaker component blueward of the DLA, this system is reasonably fit with profile of N(H I) = $3 \times 10^{20}$ cm^-2.

B1055-301

This DLA has a very large equivalent width ( $W_0 \sim 60$ Å) but is heavily blended with other absorption lines. The fit is only constrained by the base of the absorption which is clearly saturated over 10 Å in the rest frame. Higher order Lyman lines are not available to improve the decomposition of the H I cloud model, although several metal transitions are covered by the AAT spectrum. Strong Si II $\lambda$1526, Al II $\lambda$1671 and Fe II $\lambda$1608 are all observed with redshifts of 1.9037 and rest frame equivalent widths W₀ = 1.2Å, 1.3 Å and 0.8 Å respectively. This provides strong support for the presence of a DLA at the position shown in Fig. 2. However, the column density can only be constrained to within $\pm 15$%: N(H I) = $35\pm5 \times 10^{20}$ cm^-2.

Figure 2: Damped Lyman $\alpha$ profile fits (continuous lines) to all newly discovered intervening DLAs. See Table 3 for the values of N(H I) and $z_{\rm abs}$ corresponding to the theoretical profiles shown. Note that the bottom right-hand panel is on a different wavelength scale due to the large column density of the DLA towards B1055-301.

Figure 2: continued.

B1228-113

Another low redshift system whose spectrum has a low S/N, although the lack of strong nearby Lyman $\alpha$ forest lines results in an acceptable fit.

B1230-101

Constrained mostly by its fit to the red wing, this DLA at $z_{\rm abs} = 1.931$ has several metal lines associated with it. The AAT spectrum covers both Fe II $\lambda$1608 (W₀ = 520 mÅ) and Si II $\lambda$1526 (W₀ = 720 mÅ); the latter lies just blueward of a strong, resolved, C IV doublet at $z_{\rm abs} = 1.899$. There is a second C IV system at $\lambda_{\rm obs} \sim 4540$ Å associated with the DLA itself.

B1251-407a,b

As can be seen in Fig. 1, this QSO has two prominent absorption lines at approximately 5510 and 5780Å, corresponding to Lyman $\alpha$ at $z_{\rm abs} = 3.533$ and 3.752 respectively. The former is well reproduced by a damped profile with N(H I) = $4 \times 10^{20}$ cm^-2. The latter has a large equivalent width ( W₀ = 13.7Å), but also steep sides and cannot be fitted satisfactorily with a single absorption component. Closer inspection reveals structure within the core of the damped Lyman $\alpha$ line (see Fig. 2). We consider this feature to be a composite consisting of a DLA at $z_{\rm abs} = 3.752$ with N(H I) = $2 \times 10^{20}$ cm^-2 flanked by two lower column density components. This interpretation is supported by the presence of Si II $\lambda$1526 absorption at the same redshift as the DLA. In any case, both DLAs in this QSO are excluded from our discussion of the sample statistics below because we consider only the redshift interval $1.8 < z_{\rm abs} < 3.5$ (see Sect. 6). The two other moderately large EW systems at $\lambda = 4640$and 4830Å respectively (see Fig. 1), are not DLAs, but blends of lower column density lines.

B1354-107a

Two strong absorption features are seen towards this QSO. Absorber "a'' has a redshift of $z_{\rm abs} = 2.501$ and is well fitted by a damped profile with $N\rm (H~I) = 2.5 \times 10^{20}$ cm^-2. Absorber "b'', with $N\rm (H~I) = 6 \times 10^{20}$ cm^-2 and $z_{\rm abs} = 2.966$, is classified as a $z_{\rm abs} \sim z_{\rm em}$ DLA and is not included in the present analysis.

B1418-064

At $z_{\rm abs} = 3.449$, this is the highest redshift DLA to be included in our statistical analysis; the Lyman $\alpha$ line is well reproduced by a damped profile with N(H I) = 2.5 $\times 10^{20}$ cm^-2. Our limited spectral coverage redward of Lyman $\alpha$ emission reveals associated Si II $\lambda$1304 and O I $\lambda$1302 absorption lines with W₀ = 290 and 390mÅ respectively (although the O I transition is probably blended with an unidentified line and the equivalent width measurement is therefore an overestimate).

B2311-373

A column density of N(H I) = 3 $\times 10^{20}$ cm^-2 provides a satisfactory fit to the base and both wings of this DLA. Si II $\lambda$1526 is the only metal transition identified in the AAT spectrum with W₀ = 500 mÅ.

B2314-409a,b

As is the case for B1055-301, there is extended H I absorption in this spectrum. The spike at $\lambda_0 \sim 1220$ Å in Fig. 2 could be either noise or residual continuum flux between two closely spaced components. The latter interpretation is supported by the presence of two Si II $\lambda$1526 absorption lines at redshifts $z_{\rm abs} = 1.857$ and 1.875 respectively. These redshifts match well the blue and red components of the Lyman $\alpha$ absorption feature, as indicated by the fit shown in Fig. 2. The metal lines are stronger in the $z_{\rm abs} = 1.857$ system, where we also detect Fe II  $\lambda 1608$ and Al II  $\lambda 1671$; the $z_{\rm abs} = 1.875$component may well be a very low metallicity system given that Fe II  $\lambda 1608$ has $W_0 \leq 90$mÅ. This is the second case of a multiple DLA, the first being a triple DLA (CTQ247) spread over $\sim$6000 km s^-1, discovered by Lopez et al. (2000).