Complete Optical And Radio Absorption Line System Research Articles

Assessing the dust selection bias in quasar absorbers at : Zn/Fe abundances in a radio-selected sample

The Complete Optical and Radio Absorption Line System (CORALS) survey has previously been used to demonstrate that the number density, gas and metals content of z>1.6 damped Lyman alpha systems is not significantly under-estimated in magnitude limited surveys. In this paper, a sample of strong MgII absorbers selected from the optically complete 0.7 < z < 1.6 CORALS sample of Ellison et al. is used to assess the potential of dust bias at intermediate redshifts. From echelle spectra of all CORALS absorbers with MgII lambda 2796 and FeII lambda 2600 rest equivalent widths > 0.5 A in the redshift range 0.7 < z < 1.6, we determine column densities of Zn, Cr, Fe, Mn and Si. The range of dust-to-metals ratios and inferred number density of DLAs from the D-index are consistent with optical samples. We also report the discovery of 4 new absorbers in the echelle data in the redshift range 1.7 < z< 2.0, two of which are confirmed DLAs and one is a sub-DLA, whilst the Lya line is not covered for the fourth.

The Optical-Infrared Colors of CORALS QSOs: Searching for Dust Reddening Associated with High-Redshift Damped Lyα Systems

The presence of dust in quasar absorbers, such as damped Lyα (DLA) systems, may cause the background QSO to appear reddened. We investigate the extent of this potential reddening by comparing the optical-to-infrared colors of QSOs with and without intervening absorbers. Our QSO sample is based on the Complete Optical and Radio Absorption Line System (CORALS) survey of Ellison and coworkers. The CORALS data set consists of 66 radio-selected QSOs at zem ≥ 2.2 with complete optical identifications. We have obtained near-simultaneous B- and K-band magnitudes for a subset of the CORALS sample and supplemented our observations with further measurements published in the literature. In total, we have B - K colors for 42 of the 66 QSOs, of which 14 have intervening DLA systems. To account for redshift-related color changes, the B - K colors are normalized using the Sloan Digital Sky Survey QSO composite. The mean normalized B - K color of the DLA subsample is +0.12, whereas the mean for the no-DLA sample is -0.10; both distributions have rms scatters of ~0.5. Neither a Student's t-test nor a Kolmogorov-Smirnov test indicate that there is any significant difference between the two color distributions. Based on simulations that redden the colors of QSOs with intervening DLA systems, we determine a reddening limit that corresponds to E(B - V) < 0.04 (SMC-like extinction) at 99% confidence (3 σ), assuming that E(B - V) is the same for all DLA systems. Finally, we do not find any general correlation between absorber properties (such as [Fe/Zn] or neutral hydrogen column density) and B - K color. The two reddest QSOs with DLA systems in our sample have H I column densities that differ from each other by an order of magnitude and moderate gas-to-dust ratios as inferred from chemical abundances. One of these two QSOs shows evidence for strong associated absorption from X-ray observations, an alternative explanation for its very red color. We conclude that the presence of intervening galaxies causes a minimal reddening of the background QSO.

Zn and Cr abundances in damped Lyman alpha systems from the CORALS survey

We present metal abundances in 15 damped Lyα systems (DLAs) from the Complete Optical and Radio Absorption Line System (CORALS) survey, designed to be free from any biasing effects due to extinction of QSOs by dust in intervening absorbers. It has long been suggested that such biasing may explain differences in metallicity between damped Lyα systems and coeval luminous galaxies, and between model predictions and observations. We use our measured zinc and chromium abundances (combined with those for five more CORALS DLAs from the literature, giving us a very nearly complete sample) to test whether the metallicity and degree of dust depletion in CORALS DLAs are significantly different from those of existing, larger, samples of DLAs drawn from magnitude limited, optical surveys. We find that the column density weighted metallicity of CORALS DLAs, (� (Zn/H)DLA� ) = −0.88 ± 0.21 in the redshift interval 1.86 < zabs < 3.45, is only marginally higher than that of a control sample from the by Kulkarni et al., (� (Zn/H)DLA� ) = −1.09 ± 0.10. With the present limited statistics this difference is not highly significant. Furthermore, we find no evidence for increased dust depletions in CORALS DLAs - their (Cr/Zn) ratios conform to the known trend of increasing depletion (decreasing (Cr/Zn)) with increasing metallicity, and we have encountered no cases where Cr is as depleted as in local cold interstellar clouds. These results, when combined with the earlier findings of the CORALS survey reported by Ellison et al. in 2001, make it difficult to invoke a dust-induced bias to explain the generally low level of chemical evolution exhibited by most DLAs. Rather, they indicate that large scale optical QSO surveys give a fair census of the population of high redshift absorbers.

Absorption systems in radio-selected QSO surveys

Radio-selected samples of quasars with complete optical identifications offer an ideal dataset with which to investigate dust bias associated with intervening absorption systems. Here, we review our work on the Complete Optical and Radio Absorption Line System (CORALS) survey whose aim is to quantify this bias and assess the impact of dust on absorber statistics. First, we review previously published results on the number density and gas content of high column density absorbers over the redshift range . We then present the latest results from CORALS which focus on measuring the metal content of our unbiased absorber sample and an investigation of their optical–IR colours. Overall we find that although dust is unarguably present in absorption galaxies, the level appears to be low enough that the statistics of previous magnitude limited samples have not been severely affected and that the subsequent reddening of background QSOs is small.

The Number Density of 0.6 &lt;z&lt; 1.7 MgiiSystems from CORALS: Observational Biases at Intermediate Redshift

The goal of the Complete Optical and Radio Absorption Line System (CORALS) survey is to quantify the potential impact on QSO absorber statistics from dust in intervening galaxies. Dust may introduce a selection bias in surveys that are based on magnitude-limited QSO samples, leading to an underestimate of absorber number density, n(z). Here we present the results of the second phase of the CORALS survey, which extends our previous work on z > 1.8 damped Lyα systems (DLAs) to search for strong metal line systems (candidate DLAs) in the range 0.6 < z < 1.7. We have identified 47 Mg II systems with rest-frame equivalent widths EW(Mg λ2796) > 0.3 Å in our sample of 75 radio-selected quasars. The total redshift path covered by the survey is Δz = 35.2, 58.2, and 63.8 for EW(Mg λ2796) > 0.3, 0.6, and 1.0 Å thresholds, respectively (5 σ). Our principal and most robust result is that the n(z) value of low-redshift Mg II systems determined for the CORALS survey is in excellent agreement with that of optically selected, magnitude-limited QSO samples. We use empirically determined Mg II equivalent width statistics to estimate the likely number of DLAs in this sample. The statistically inferred number density of DLAs, n(z) = 0.16, is consistent with other low-redshift samples, although the large 1 σ error bars permit up to a factor of 2.5 more DLAs in CORALS. However, confirmation of the DLA candidates, precise evaluation of their n(z) values, and measurement of their H I column densities awaits UV observations with the Hubble Space Telescope. Finally, we report an excess of intermediate-redshift Mg II systems observed toward bright QSOs, which could be due to a lensing amplification bias. However, there is also evidence that this excess could simply be due to more sensitive EW detection limits toward brighter QSOs. We also emphasize that absorber statistics determined from magnitude-limited surveys reach a steady value if the completeness limit is significantly fainter than the fiducial value of the quasar luminosity function.

The CORALS survey I: New estimates of the number density and gas content of damped Lyman alpha systems free from dust bias

We present the first results from the Complete Optical and Radio Absorption Line System (CORALS) survey. We have compiled a homogeneous sample of radio-selected QSOs from the Parkes Catalogue and searched for damped Lyman alpha systems (DLAs) towards every target, irrespective of its optical magnitude. This approach circumvents selection effects -particularly from intervening dust -which have long been suspected to affect DLA surveys in optically-selected, magnitude-limited QSO samples. The CORALS data set consists of 66 QSOs in which 22 DLAs with absorption redshifts have been identified over a total redshift interval . Three of the DLAs are classified as "associated" systems with ; of the 19 intervening DLAs, 17 are new discoveries. In this first paper of the CORALS series we describe the sample, present intermediate resolution spectroscopy and determine the population statistics of DLAs. We deduce a value of the neutral gas mass density traced by DLAs (expressed as a fraction of the closure density) , and a number density of DLAs per unit redshift , both at a mean redshift . Both values are only marginally higher than those measured in optically selected samples of QSOs. Taking into account the errors, we conclude that dust-induced bias in previous surveys may have led to an underestimate of these quantities by at most a factor of two. While is greater in fainter () QSOs, the effect is only at the ~1σ level and we have not uncovered a previously unrecognised population of high column density ( cm-2) DLAs in front of faint QSOs. These conclusions are tentative because of the limited size of our data set; in particular the distribution of column densities is poorly sampled at the high end where a much larger survey of radio-selected QSOs is required the improve the statistics.