Issue |
A&A
Volume 444, Number 2, December III 2005
|
|
---|---|---|
Page(s) | 461 - 479 | |
Section | Extragalactic astronomy | |
DOI | https://doi.org/10.1051/0004-6361:20041570 | |
Published online | 25 November 2005 |
The dust obscuration bias in damped Lyman α systems
1
Osservatorio Astronomico di Trieste – Istituto Nazionale di Astrofisica, Via Tiepolo 11, 34131 Trieste, Italy e-mail: vladilo@ts.astro.it
2
European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching-bei-München, Germany e-mail: cperoux@eso.org
Received:
1
July
2004
Accepted:
4
October
2005
We present a new study of the effects
of quasar obscuration on the
statistics of Damped Ly α (DLA) systems.
We show that the extinction
of any Galactic or extragalactic region,
, increases linearly with the column density of zinc,
with a turning point
,
above which background sources are suddenly obscured.
We derive a relation
between the extinction of a DLA system and its
column density,
, metallicity, Z,
fraction of iron in dust,
, and redshift, z.
From this relation we estimate the fraction of DLA systems missed as a consequence of their own extinction
in magnitude-limited surveys. We derive a method for recovering the true
frequency distributions of
and Z in DLAs,
and fZ,
using the biased distributions measured in the redshift range
where the observations have sufficient statistics (
).
By applying our method we find that the
well-known empirical thresholds of DLA column densities,
atoms cm-2 and
atoms cm-2, can be
successfully explained in terms of the obscuration effect
without tuning of the local dust parameters. The obscuration has a modest effect on the distribution of quasar apparent magnitudes,
but plays an important role in shaping the statistical distributions of DLAs.
The exact estimate of the bias is still limited by the paucity of the data
(
40 zinc measurements at
).
We find that the fraction of DLAs missed as a consequence of obscuration is ~30% to 50%, consistent with the results of surveys of radio-selected quasars. By modelling the metallicity distribution with a Schechter function
we find that the mean metallicity can be ∼5 to 6 times higher than the value commonly reported for DLAs at
.
Key words: ISM: dust, extinction / galaxies: ISM / galaxies: high-redshift / quasars: absorption lines
© ESO, 2005
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