Sl. Ellison et al., The CORALS survey I: New estimates of the number density and gas content of damped Lyman alpha systems free from dust bias, ASTRON ASTR, 379(2), 2001, pp. 393-406
We present the first results from the Complete Optical and Radio Absorption
Line System (CORALS) survey. We have compiled a homogeneous sample of radi
o-selected QSOs from the Parkes Catalogue and searched for damped Lyman alp
ha systems (DLAs) towards every target, irrespective of its optical magnitu
de. This approach circumvents selection effects - particularly from interve
ning dust - which have long been suspected to affect DLA surveys in optical
ly-selected, magnitude-limited QSO samples. The CORALS data set consists of
66 z(em) greater than or equal to 2.2 QSOs in which 22 DLAs with absorptio
n redshifts 1.8 less than or equal to z(abs) less than or equal to z(em) ha
ve been identified over a total redshift interval Deltaz = 55.46. Three of
the DLAs are classified as "associated" systems with z(abs) similar to z(em
); of the 19 intervening DLAs, 17 are new discoveries. In this first paper
of the CORALS series we describe the sample, present intermediate resolutio
n spectroscopy and determine the population statistics of DLAs. We deduce a
value of the neutral gas mass density traced by DLAs (expressed as a fract
ion of the closure density) log Omega (DLA)h = -2.59(-0.24)(+0.17), and a n
umber density of DLAs per unit redshift n(z) = 0.31(-0.08)(+0.09), both at
a mean redshift [z] = 2.37. Both values are only marginally higher than tho
se 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
n(z) is greater in fainter (B >20) QSOs, the effect is only at the similar
to1 sigma level and we have not uncovered a previously unrecognised populat
ion of high column density (N(H I) > 10(21) cm(-2)) DLAs in front of faint
QSOs. These conclusions are tentative because of the limited size of our da
ta set; in particular the distribution of column densities is poorly sample
d at the high end where a much larger survey of radio-selected QSOs is requ
ired the improve the statistics.