Metals in the z~3 intergalactic medium: results from an ultra-high
  signal-to-noise ratio UVES quasar spectrum
  In this work, we investigate the abundance and distribution of metals in the
intergalactic medium (IGM) at $\langle z \rangle \simeq 2.8$ through the
analysis of an ultra-high signal-to-noise ratio UVES spectrum of the quasar
HE0940-1050. In the CIV forest, our deep spectrum is sensitive at $3\,\sigma$
to lines with column density down to $\log N_{\rm CIV} \simeq 11.4$ and in 60
per cent of the considered redshift range down to $\simeq11.1$. In our sample,
all HI lines with $\log N_{\rm HI} \ge 14.8$ show an associated CIV absorption.
In the range $14.0 \le \log N_{\rm HI} <14.8$, 43 per cent of HI lines has an
associated CIV absorption. At $\log N_{\rm HI} < 14.0$, the detection rates
drop to $<10$ per cent, possibly due to our sensitivity limits and not to an
actual variation of the gas abundance properties. In the range $\log N_{\rm HI}
\ge 14$, we observe a fraction of HI lines with detected CIV a factor of 2
larger than the fraction of HI lines lying in the circum-galactic medium (CGM)
of relatively bright Lyman-break galaxies hosted by dark matter haloes with
$\langle M\rangle \sim10^{12}$ M$_{\odot}$. The comparison of our results with
the output of a grid of photoionization models and of two cosmological
simulations implies that the volume filling factor of the IGM gas enriched to a
metallicity $\log Z/Z_{\odot} \ge -3$ should be of the order of $\sim 10-13$
percent. In conclusion, our results favour a scenario in which metals are found
also outside the CGM of bright star-forming galaxies, possibly due to pollution
by lower mass objects and/or to an early enrichment by the first sources.
