4 Observations and spectral analysis

Before commencing our observing campaign, we first searched the literature to determine which QSOs had already been observed at sufficiently high spectral resolution, S/N and over the correct wavelength range to match the rest of our DLA survey. The eight QSOs for which adequate spectra were found in the literature were not re-observed. The remaining 58 QSOs were subject to an extensive observing campaign that spanned five semesters on three different telescopes; relevant details are presented in Table 2. References for the eight QSO spectra observed by others are given in Table 3.

 

 

Table 2: Summary of observations CORALS survey. Many of the bright targets were observed first with low resolution at the ESO 3.6-m and QSOs with strong absorbers followed-up with AAT observations, whilst others were observed directly with the AAT. All faint targets (B > 20) were observed only with the VLT.

Telescope	Date	Resolution	No. of
	(no. nights)	(at 4000 Å)	QSOs observed
ESO 3.6-m	Sep. 1998 (2)	6-8 Å	16
ESO 3.6-m	Feb. 1999 (2)	7 Å	12
AAT	Dec. 1998 (3a)	3 Å	11
AAT	Apr. 1999 (2.5b)	3 Å	10
AAT	Oct. 1999 (2)	3 Å	12
VLT	Oct. 2000 (0.5c)	4.5 Å	9
VLT	Mar. 2001 (2)	4.5 Å	10
VLT	June 2001 (0.5d)	4.5 Å	1

Notes: ^a One night lost to bad weather.
         ^b The April AAT run consisted of 5 half nights.
         ^c 6 hours of service time.
         ^d 6 hours of Director's discretionary time.

Our observing strategy has been to obtain "snapshot'' spectra of as many bright (typically B < 20) QSOs as possible at low resolution with the ESO 3.6-m in order to pre-select targets with candidate DLAs for higher resolution follow-up at the AAT. Targets fainter than this limit were observed directly with the VLT. We briefly review the main characteristics of the observations.

 

 

Table 3: Complete list of all CORALS QSOs and the tally of DLAs identified. References refer to QSOs for which optical spectroscopy had already been obtained prior to our survey, as follows: 1 - Lanzetta et al. (1991); 2 - Pettini et al. (1997); 3 - Ledoux et al. (1998); 4 - Lu et al. (1996); 5 - M. Giavalisco, private communication; 6 - Tytler et al. (1996); 7 - Songaila (1998); 8 - Turnshek et al. (1989).

QSO	$z_{\rm em}$	B mag	DLA?	N(H I) (1020cm-2)	$z_{\rm abs}$	Ref
B0017-307	2.666	19.0	N	...	...
B0039-407	2.478	18.5	N	...	...
B0104-275	2.492	18.5	N	...	...
B0113-283	2.555	19.0	N	...	...
B0122-005	2.280	18.5	N	...	...
B0244-128	2.201	18.5	N	...	...
B0256-393	3.449	19.6	N	...	...
B0325-222	2.220	19.0	N	...	...
B0329-255	2.685	17.1	N	...	...	1
B0335-122	3.442	21.5	Y	6.0	3.178
B0347-211	2.944	21.1	Y	2.0	1.947
B0405-331	2.570	19.0	Y	4.0	2.570a
B0420+022	2.277	19.5	N	...	...
B0422-389	2.346	18.0	N	...	...
B0432-440	2.649	19.6	Y	6.0	2.297
B0434-188	2.702	20.0	N	...	...
B0438-436	2.863	19.5	Y	6.0	2.347
B0451-282	2.560	19.0	N	...	...
B0458-020	2.286	20.0	Y	45.0	2.039	2
B0528-250	2.765	19.0	Y	5.6	2.141	3
			Y	15.8	2.811a	4
B0537-286	3.110	20.0	Y	2.0	2.974
B0601-172	2.711	20.0	N	...	...
B0610-436	3.461	19.0	N	...	...	5
B0819-032	2.352	18.2	N	...	...
B0834-201	2.752	19.0	N	...	...
B0913+003	3.074	21.7	Y	5.5	2.744
B0919-260	2.300	19.0	N	...	...
B0933-333	2.906	20.0	Y	3.0	2.682
B1010-427	2.954	17.5	N	...	...
B1055-301	2.523	19.5	Y	35.0	1.904
B1136-156	2.625	20.0	N	...	...
B1147-192	2.489	19.4	N	...	...
B1149-084	2.370	18.5	N	...	...
B1228-113	3.528	22.0	Y	4.0	2.193
B1228-310	2.276	19.0	N	...	...
B1230-101	2.394	19.8	Y	3.0	1.931
B1251-407	4.464	23.7	Y	4.0	3.533
			Y	2.0	3.752
B1256-243	2.263	19.5	N	...	...
B1318-263b	2.027	20.4	N	...	...
B1351-018	3.710	20.9	N	...	...
B1354-107	3.006	19.2	Y	2.5	2.501
			Y	6.0	2.966a
B1402-012	2.518	18.2	N	...	...
B1406-267	2.430	21.8	N	...	...
B1418-064	3.689	18.5	Y	2.5	3.449
B1430-178	2.331	19.0	N	...	...
B1535+004	3.497	24.1	N	...	...
B1556-245	2.813	18.5	N	...	...
B1635-035	2.871	21.8	N	...	...

Notes: ^a $z_{\rm abs} \sim z_{\rm em}$; ^b The original redshift of this target was incorrect in our list, the
VLT spectrum confirms the true redshift which is less than the survey cut-off at z_em =2.2.

 

Table 3: continued.

QSO	$z_{\rm em}$	B mag	DLA?	N(H I) (1020cm-2)	$z_{\rm abs}$	Ref
B1701+016	2.842	21.7	N	...	...
B1705+018	2.575	18.9	N	...	...
B1937-101	3.780	19.0	N	...	...	6
B2000-330	3.780	18.5	N	...	...	7
B2126-158	3.275	17.5	N	...	...	7
B2149-307	2.330	17.5	N	...	...
B2212-299	2.703	17.8	N	...	...
B2215+020	3.550	21.5	N	...	...
B2224+006	2.248	21.7	N	...	...
B2245-059	3.295	19.5	N	...	...
B2245-328	2.268	16.5	N	...	...
B2256+017	2.663	19.0	N	...	...	8
B2311-373	2.476	18.5	Y	3.0	2.182
B2314-340	3.100	18.5	N	...	...
B2314-409	2.448	18.0	Y	4.0	1.857
			Y	2.0	1.875
B2315-172	2.462	19.5	N	...	...
B2325-150	2.465	19.5	N	...	...
B2351-154	2.665	19.0	N	...	...

4.1 ESO 3.6-m observations

We obtained low resolution spectra of 28 QSOs using the EFOSC2 spectrograph on the ESO 3.6-m telescope during four nights between Sep. 1998 and Feb. 1999. The spectral resolution is approximately 7 Å with a 1 arcsec slit and B600 grism, covering 3270-5240 Å. For a few QSOs, we also took spectra using R600 grism with the similar spectral resolution to cover 4320-6360 Å. The seeing conditions remained around 1.2 arcsec, although the sky was not photometric during all four nights. However, since we normalise our spectra to fit the DLA profile, the accurate flux calibration is not critical for the purpose of this paper.

4.2 AAT observations

The RGO spectrograph was used with the TEK CCD, 25 cm camera and 600 V grating. The seeing at Siding Spring was highly variable through these observations, ranging from 0.8 to 2.0 arcsecs, often with large fluctuations during a given night. However, the slit width was fixed at 1.5 arcsec which projected onto 1.8 pixels to give a FWHM resolution of 3Å. The grating angle was chosen to cover the entire wavelength range of each QSO from at least 3400 Å (corresponding to the wavelength of Lyman $\alpha$ $\lambda 1216$ at $z_{\rm abs} = 1.8$) to $z_{\rm em}$. The grating angle varied slightly for each run depending on the QSO subset to be observed, but was typically around $19.74^{\circ}$, corresponding to a central wavelength of $\lambda = 4000$Å. At this setting, the wavelength range is $\sim$3200Å-4800Å. For the few bright, high redshift ( ) targets observed with the AAT, a second grating setting, with a central wavelength $\lambda = 5500$Å, was required to cover the spectrum up to 6300 Å.

4.3 VLT observations

The VLT observations were executed with FORS1 in a combination of service and visitor mode. The 600B grating was used for all targets, with additional wavelength coverage provided by the 600R grating for the $z_{\rm em} = 4.458$ QSO B1251-407. A slit width of 0.7 arcsec provided a FWHM resolution of 4.5 Å and a wavelength coverage of 3360-5760Å. The 600R grating gave additional coverage (only required for the highest redshift QSO in our sample) over 5200-7300Å with a resolution of 3.6 Å. Despite the faintness of several of the VLT targets (down to B = 24), all acquisitions could be executed in "fast'' mode and without blind offsets.

4.4 Data reduction and column density fitting

We applied the same reduction procedure to all of the data. The standard IRAF routines were used. First, all of the images were trimmed and the bias level was subtracted using the over-scan regions. High S/N flat-field images were combined to a single image, which was then smoothed using a box median filter of 1 $\times$ 50 pixels. The orientation of the box median filter is chosen such that we preserve all of the variations along the dispersion axis. Then, the normalized 2-D flat-field image was used to remove pixel-to-pixel variations in the quasar images. The task APALL was used to perform the optimal extraction of the 1-D spectra. Wavelength calibration images were typically taken both before and after each QSO image. The comparison lamps are CuAr at AAT, NeHe at ESO 3.6 m and NeHgCd on FORS1 at VLT. The dispersion solutions were obtained using a 4th order Legendre polynomial and the RMS error of the fitting was less than 0.05 Å. Finally, we performed error-weighted summation of all of the wavelength calibrated spectra for each QSO. The final error array was the quadratic sum of the individual error spectra.

The spectra of all 58 QSOs observed by us are presented in Fig. 1. Once extracted, the spectra were inspected for the presence of DLAs - the Lyman $\alpha$ signature clearly visible as a broad, saturated absorption feature. If a DLA was identified, the spectrum was normalised by dividing through by the QSO continuum and then fitted with a Lyman $\alpha$ profile using the Starlink package DIPSO to determine the redshift and column density of the DLA. The complete list of CORALS QSOs and identified DLAs can be found in Table 3.