The expected number of DLAS (N(H I
cm-2) and LLS (
H I
cm-2) in the
redshift interval covered by our 7 spectra as
computed from their number density as a function of
redshift -
,
(Storrie-Lombardi & Wolfe 2000)-
is of 1 and 9, respectively.
We detect 3 DLASs and 8 LLSs indicating that our lines of
sight are not strongly biased toward an overabundance of
high column density systems.
The investigation of the nearby lines of sight at
the redshift of each of the previous systems, gives the
following results.
Of the three DLASs: 2 coincide with metal systems with
C IV rest equivalent width
Å, and 1 is at less than 1000 km s-1 from the
emission redshift of the paired QSO, which in turn is
marking the presence of a high matter density peak
(see Ellison et al. 2001).
The transverse spatial separation over which these
coincidences happen varies between
5 and 9 h-1 Mpc.
As for the 8 LLSs: 4 of them form two
coinciding pairs at
and 2.12 in the
spectra of UM680 and UM681, their transverse spatial
separations are
920 and 940 h-1 kpc,
respectively. The LLS at
in the
spectrum of Q2138-4427 shows a coinciding metal system in
the spectrum of Q2139-4434 at
a transverse spatial separation
Mpc. However, only low-ionisation transitions are
observed and no C IV. Furthermore, the H I Lyman-
of the
latter system is outside our spectral range .
The remaining 3 Lyman limit systems have corresponding
Lyman-
absorptions without associated metals within 3000 km s-1.
In summary, we measure a
coincidence within 1000 km s-1 between high density
systems, in 5 cases out of 10.
We exclude the coincidence at
in the
triplet, since it was not possible to determine the
H I column density of the metal system.
Figure 15 shows a pictorial description of the
observed coincidences as a function of redshift; while in
Table 4 we report the main properties of the
matching absorption systems.
In order to approximately compute the significance of our
result, we consider the number density of C IV systems
with rest equivalent width
Å as a
function of redshift (Steidel 1990). The chance
probability (in the hypothesis of null clustering) to
detect a C IV absorption line within 1000 km s-1,
between z =2 and 3, is
.
If we assume that a binomial random process rules the
detection or the non-detection of a coincidence, the
a posteriori probability in the studied case is
<
.
The clustering signal is indeed highly
significant.
Going back to our sample, the two coincidences
in the spectra of UM680, UM681 at Mpc
are closely related to the emitting quasars.
As recently claimed for associated absorption lines
(e.g. Srianand & Petitjean 2000; de Kool et al. 2001; Hamann et al. 2001), the observed
absorption systems could arise in gas expelled
by a galactic "superwind'' in a luminous starburst
associated with the formation of the quasar itself.
Superwinds contain cool dense clouds which justify the
presence of low ionisation lines, embedded in a hot
(
107 K) X-ray-emitting plasma (see Heckman et al. 1996, and
references therein). In low redshift galaxies,
outflow velocities of
102-103 km s-1 and column
densities
cm-2 have
been measured which are consistent with the observed
values.
The remaining three coincident systems involve DLASs and
are characterized by larger QSO pair separations.
Damped systems at high redshifts are thought to
arise in large disks (e.g. Wolfe 1995) or in
multiple protogalactic clumps (Haehnelt et al. 1998;
Ledoux et al. 1998; McDonald & Miralda-Escudé 1999).
In either case they trace high matter density peaks and
they are possibly associated with Lyman-break galaxies
(Møller et al. 2002).
The representation of these kind of objects in
hydrodynamical simulations (e.g. Jenkins et al. 1998; Cen 1998)
shows that they lie in knots of Mpc
scale from which filaments several Mpc in length depart
in a spider-like structure. Star formation takes place in
the central condensation but also in some denser blobs
of matter along the filaments.
The correlation on large scales observed around the DLAS
in our sample
finds a likely explanation in this scenario (see the
discussion in Francis et al. 2001b).
For comparison, Lyman break galaxies at
,
which
are thought to have masses
,
show correlation lengths
Mpc
(Giavalisco et al. 1998; Porciani & Giavalisco 2001; Arnouts et al. 2002).
Copyright ESO 2002