A quasar hiding behind two dusty absorbers. Quantifying the selection
  bias of metal-rich, damped Lyman-alpha absorption systems
  The cosmic chemical enrichment as measured from damped Ly$\alpha$ absorbers
(DLAs) will be underestimated if dusty and metal-rich absorbers have evaded
identification. Here we report the discovery and present the spectroscopic
observations of a quasar, KV-RQ\,1500-0031, at $z=2.520$ reddened by a likely
dusty DLA at $z=2.428$ and a strong MgII absorber at $z=1.603$. This quasar was
identified as part of the KiDS-VIKING Red Quasar (KV-RQ) survey, specifically
aimed at targeting dusty absorbers which may cause the background quasars to
escape the optical selection of e.g. the SDSS quasar survey. For the DLA we
find an HI column density of $\log N$(HI) = $21.2\pm 0.1$ and a metallicity of
[X/H] = $-0.90\pm 0.20$ derived from an empirical relation based on the
equivalent width of SiII$\lambda$1526. We observe a total visual extinction of
$A_V=0.16$ mag induced by both absorbers. We compile a sample of 17 additional
dusty ($A_V > 0.1$ mag) DLAs toward quasars (QSO-DLAs) from the literature for
which we characterize the properties of HI column density, metallicity and
dust. From this sample we also estimate a correction factor to the overall DLA
metallicity budget. We demonstrate that the dusty QSO-DLAs have high metal
column densities ($\log N$(HI) + [X/H]) and are more similar to gamma-ray burst
(GRB)-selected DLAs (GRB-DLAs) than regular QSO-DLAs. We evaluate the effect of
dust reddening in DLAs as well as illustrate how the induced color excess of
the underlying quasars can be significant (up to $\sim 1$ mag in various
optical bands), even for low to moderate extinction values ($A_V \lesssim 0.6$
mag). Finally we discuss the direct and indirect implications of a significant
dust bias in both QSO- and GRB-DLA samples. [Abridged]
