2 Previous DLA surveys

 

 

Table 1: Summary of past surveys and their sample definitions. Column 3 tabulates the number of QSOs new to each survey. Therefore, if a particular survey has been supplemented with a previous sample, the total number of QSOs is the sum of the individual entries in Col. 3.

Survey reference	Abbreviation	No. QSOs	Supplemented	Limiting Mag.	Telescope	Resolution
			with	(Band)
Wolfe et al. (1986)	WTSC86	68	...	18.5(V)	Lick	10 Å
Sargent et al. (1989)	SSB89	53	...	N/Aa	Hale 5-m	4 Å
Lanzetta et al. (1991)	LAN91	57	SSB89	Literature	Hale 5-m and Las	4-6 Å
				search	Campanas 2.5-m
Lanzetta et al. (1995)	LWT95	260	LAN91	20(V)b	IUE	7-10 Å
Wolfe et al. (1995)	WLFC95	228	LWT95	18.75(B)	MMT and Las	6-10 Å
					Campanas 2.5-m
Storrie-Lombardi	SLW00	40	Many incl. WTSC86,	19.5(R)b	Keck, AAT	2 Å
& Wolfe (2000)			LWT95, WLFC95		Lick
Péroux et al. (2001a)	PER01	66	SLW00	20.5(R)b	4-m CTIO	5 Å
					WHT

Notes: ^a No broad-band limit is quoted for SSB89, only a derived apparent magnitude from the continuum flux at $\lambda_0 = 1450$Å.
         ^b This is the faintest magnitude in this sample, but it does not indicate a completeness limit.

It is useful at this point to review some of the previous surveys for damped Lyman $\alpha$ systems and summarise their sample definitions and major findings. Relevant data are collected in Table 1.

The first major survey for DLAs was conducted by Wolfe et al. (1986) (WTSC86) from the relatively bright Lick QSO sample. The strategy adopted by those authors was to obtain low resolution spectra for a large number of QSOs in order to identify absorption systems above a certain equivalent width limit (in this case $W_0 \ge 5$ Å). These candidate DLAs were then followed-up with 2 Å resolution spectroscopy revealing that approximately 50% were indeed damped, the remainder being blends of lower column density lines. This strategy of pre-selecting DLA candidates from low resolution spectra, based on rest-frame equivalent width, has come to characterise surveys in subsequent years. The approach allows a large sample of QSOs to be considered, maximising efficient use of observing time. The WTSC86 survey represents a landmark in absorption line studies, being the first major compilation of spectra and line lists for DLAs. One of the main results was that the incidence of DLAs was found to be greater than expected if the absorption were due to galaxies with the same cross section for H I absorption as present-day spirals (Wolfe 1988). In addition, the procedures for identifying DLAs were established and the N(H I) definition of a damped system set by this work, $N\rm (H~I)\ge 2\times 10^{20}$ cm^-2, has been upheld in all subsequent studies.

A survey of similar size to the WTSC86 sample was conducted a few years later by Sargent et al. (1989) (SSB89), although the initial focus of this work was on Lyman limit systems (LLSs). All of the quasars in the SSB89 sample were observed at a resolution of 4 Å, considerably higher than the first pass made by Wolfe et al. (1986). However, SSB89 estimate that their DLA sample is probably only complete for systems with $N\rm (H~I)\ge 10^{21}$ cm^-2, although several lower column density lines were identified. Combining the SSB89 dataset with a similar number of QSOs found in the literature, Lanzetta et al. (1991) adopted the strategy of WTSC86 in making a 5 Å equivalent width cut to select 89 DLA candidates from their sample of 101 QSOs.

From follow-up spectroscopy at 2-3 Å resolution of these candidates, Lanzetta et al. (1991) produced the first thorough statistical analysis of damped system number density, the mass density of neutral gas in DLAs and their clustering properties. A few years later, Lanzetta et al. (1995) extended their work to include DLAs at lower redshift by exploiting ultraviolet data obtained from the International Ultraviolet Explorer (IUE) satellite to gain one of the first glimpses into the $z_{\rm abs} < 1.6$ absorber population. These authors found that $\Omega _{\rm DLA}$ decreases significantly from $z \sim 3.5$ to $z \sim 0.01$, and interpreted this redshift evolution as evidence for the consumption of H I gas by star formation.

Wolfe et al. (1995) (WLFC95) used the Large Bright Quasar Survey (LBQS, Hewett et al. 1995 and references therein) to search for DLAs with . A total of 59 DLA candidates out of 228 spectra were pre-selected as having $W_0 \ge 5$ Å. At the time of publishing their paper, Wolfe et al. (1995) had confirmed the identification of 13 DLAs with $N\rm (H~I)\ge 2\times 10^{20}$ cm^-2 from 15 candidates with W₀ > 9 Å. In addition, there were 8 DLA candidates whose equivalent widths exceeded 10 Å and were therefore considered highly likely to be damped systems. All of these 8 candidates have subsequently been confirmed as DLAs with intermediate resolution spectroscopy (Storrie-Lombardi & Wolfe 2000). In addition to the LBQS sample, WLFC95 constructed a "statistical sample'' from the literature consisting of 80 DLAs. From this they confirmed the coincidence which had previously been noted (e.g. Lanzetta 1993) that the mass of H I in DLAs at $z \sim 3$ is similar to the total luminous mass in stars today. Expressing both as fractions of the closure density, $\Omega_{\rm DLA} (z = 3.25) = (5.1 \pm 1.7) \times 10^{-3} ~h^{-1}_{50}$ and $\Omega_{\star}(z = 0) \simeq 5 \times 10^{-3}~ h^{-1}_{50}$ (e.g. Pagel 2000).

In order to extend studies of DLAs to lower redshifts, Rao & Turnshek (2000) have recently published results from an observing campaign with the Hubble Space Telescope (HST). Space-based telescopes are required in order to detect low z DLAs since the Lyman $\alpha$ signature at falls below the atmospheric cut-off. Since such satellite resources are limited, a pre-selection based on Mg II and Fe II line strengths determined from ground-based observations has permitted an efficient screening of the initial QSO sample. A total of 12 DLAs with $z_{\rm abs} < 1.5$was found, thereby significantly improving the statistics in this redshift regime. The puzzling result obtained from these new data is that, contrary to previous indications, $\Omega _{\rm DLA}$ appears to remain approximately constant over all redshifts sampled, from z = 3.5 to 0.1. Using HST archival spectra, Churchill (2001) has also argued that the number density of DLAs implied from the incidence of Mg II systems remains unaltered down to $z_{\rm abs} \sim 0.05$.

Two recent surveys have pushed the bounds to higher redshifts. Storrie-Lombardi & Wolfe (2000) and Péroux et al. (2001a) have recently identified 19 and 15 DLAs respectively at $z_{\rm abs} > 3.5$, including the highest redshift damped absorber known to date ( $z_{\rm abs} = 4.466$, Péroux et al. 2001a; Dessauges-Zavadsky et al. 2001). Again, there is only marginal evidence for evolution; between z = 3.5 and 4.5 $\Omega _{\rm DLA}$ is lower than at more recent epochs, but the effect is only at the $\sim$2$\sigma$ level.

The intention of this (non-exhaustive) review of previous DLA surveys is to illustrate the difficulty of extracting unbiased statistics from samples based on inhomogeneous datasets. Many of these surveys have built on existing samples, taken additional targets from the literature and sometimes have insufficient resolution to reliably determine whether an absorber is damped. The advantage of this approach has been largely a statistical one. The aggregate samples often contain several hundred QSOs and such a large number is indeed required in order to determine meaningful statistics because of the rarity of damped systems. The purpose of CORALS is to provide the first complete and homogeneous survey for DLAs free from any bias that may be inherent in magnitude-limited QSO samples.