|Annu. Rev. Astron. Astrophys. 2005. 43:
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Surveys for damped Ly systems have the greatest impact if they represent a fair sample of the neutral gas in the Universe, allowing a clear probe of the evolution with redshift of the neutral hydrogen content and the metallicity of neutral gas. However, it has long been a major concern that the sample of damped Ly systems suffers from "dust bias," i.e., the absence from a magnitude-limited QSO sample of those QSOs that suffer obscuration from dusty foreground damped Ly systems, leading to underrepresentation of dusty damped Ly systems in the overall sample. The easiest way to probe the existence and abundance of dust in damped Ly systems would be to find the 2175 Å bump feature superimposed in absorption on background QSO spectra. While Junkkarinen et al. (2004) found at least one strong example, this does not appear to be the rule (Pei, Fall & Bechtold 1991). Without such sharp features to look for and given the wide range of intrinsic QSO spectral slopes, reddening from dust in damped Ly systems must be searched for statistically by checking if the sample of QSOs with foreground damped Ly systems is redder on average than a "control sample" of QSOs without foreground damped Ly systems.
10.1. Observational Estimates of Reddening
Ostriker & Heisler (1984) pointed out that optically selected QSO samples are biased toward those QSOs with little foreground dust extinction. Fall & Pei (1989) showed that dust in damped Ly systems did not appear to cause the famous drop in the QSO number abundance at z 3. Pei, Fall & Bechtold (1991) detected reddening from damped Ly systems at the 4 confidence level and inferred dust-to-gas ratios between 1/20 and 1/5 that of the Galaxy, enough to explain the lack of observed Lyman emission from damped Ly systems. This led to the prediction that 10-70% of QSOs are missing from optically selected samples, leading to an order of magnitude uncertainty in g, < Z>, and other quantities estimated from damped Ly systems (Fall & Pei 1993). However, the dust-to-gas ratios estimated from high-resolution echelle spectroscopy of QSOs with foreground damped Ly systems are lower than the dust-to-gas ratios predicted by Fall & Pei (1993), reducing the uncertainty in quantities such as g to factors of 2-3. Pettini et al. (1997a) combined a metallicity of 1/15 solar with a dust-to-metals ratio of 1/2 that in the Milky Way to find a typical damped Ly system dust-to-gas ratio of 1/30 Galactic. Using an SMC reddening curve, they predicted a dust extinction of only 0.1 magnitudes at 1500 Å in the spectrum of background QSOs due to damped Ly system dust. If a nucleosynthetic floor exists in damped Ly systems at [Si/Fe] 0.3, then the dust-to-gas ratios are even lower than this, closer to 1/200 Galactic in most systems. Indeed, the detection of reddening due to damped Ly systems by Pei, Fall & Bechtold (1991) conflicts with the recent finding by Murphy & Liske (2004) that E(B - V) < 0.01 magnitudes using 81 damped Ly systems found in a homogeneous set of SDSS Data Release 2 QSOs. The resolution of the conflict is not clear at present.
10.2. Surveys of Radio-Selected QSOs
An insidious (but not physically motivated) possibility would be the existence of gray dust associated with the damped Ly systems that could cause obscuration without the telltale effect of reddening. Even this effect could be overcome by using a radio-selected sample of QSOs. The reason this has not typically been done is twofold: (1) The ability to select QSOs within a preferred redshift range makes optical color selection more efficient. (2) Conducting optical spectroscopic follow-up on a radio-selected sample of QSOs is far more time-consuming precisely because they do not have a strict optical magnitude limit; half of the total exposure time can be required by the dimmest one or two objects. Obviously, dropping those from the survey would defeat the entire purpose of radio selection. One radio-selected sample has been published: the CORALS survey of Ellison et al. (2001) found 19 intervening damped Ly systems toward 66 zem 2.2 radio-selected QSOs from the Parkes quarter-Jansky sample (Shaver et al. 1996), yielding a marginal increase in d / dX and g at < z> = 2.37 versus optically selected QSO samples. These results imply that at most half of damped Ly systems are missing from optically selected samples. For g, the radio sample yields 1.4 × 10-3 as opposed to the value of 6.7 × 10-4 found for optically selected samples at this redshift (Prochaska & Herbert-Fort 2004), but this is only a 1.5 difference given the small sample size.
10.3. Empirical Estimates of Damped Ly System Obscuration
A third way to estimate the effects of dust obscuration by damped Ly systems is to infer this from the observed chemical abundances. Taking the observed H I column densities and the dust-to-gas ratios implied by the depletion patterns of the damped Ly systems (see Equation 7), it is possible to estimate the extinction in the rest-frame UV of the QSO for an assumed extinction curve. Given the lack of the observed 2175 Å bump feature, it appears more reasonable to assume an SMC (Prevot et al. 1984) rather than Galactic (Cardelli, Clayton & Mathis 1989) dust extinction law. Prochaska & Wolfe (2002) used this technique (see their figure 24; see Prochaska 2004 for an update) to correct the observed QSO magnitudes by this inferred extinction and then to compare the implied true magnitude with the magnitude limit of the survey used to search for damped Ly systems (which is typically shallower than the limit of the survey used to discover the QSOs). These quantities were then compared to a bootstrap prediction of how many QSOs are expected to be missing from observed samples due to extinction by foreground damped Ly systems. The typical range of extinction corrections runs from 0 to 0.3 magnitudes, even though half of the QSOs are so much brighter than the survey limit that they could have been seen with up to 1 magnitude of extinction. This shows that damped Ly systems that cause between 0.3 magnitudes and 1 magnitude of extinction are rare and predicts that at most 10% of QSOs are missing from the samples probed for damped Ly systems due to a damped Ly dust bias.