Detecting metal enrichment of the intergalactic medium with the two-point correlation function of the flux: Application to the UVES deep spectrum

Context. The distribution and the abundance of metals in the intergalactic medium (IGM) have strong implications for galaxy formation and evolution models. The ionic transitions of heavy elements in quasar spectra can be used to probe both the mechanisms and the sources of chemical pollution. However, the need for high-resolution and high signal-to-noise ratio (S/N) spectra makes it challenging to characterize the process of IGM metal enrichment since the IGM absorbers are too weak for direct detection. Aims. The aim of this work is to investigate the IGM metallicity, focusing on the detection of the weak absorption lines. Methods. We exploited the cosmological tool of the two-point correlation function (TPCF) and applied it to the transmitted flux in the C IV forest region of the ultra-high S/N UVES spectrum of the quasar HE0940-1050 (z ∼ 3). We also ‘deabsorbed’ the strongest circum-galactic medium (CGM) systems in order to reveal the underlying IGM signal. For each of our tests, we created a catalogue of 1000 mock spectra in which we shuffled the position of the absorption lines to derive an estimate for the TPCF noise level. Results. The TPCF shows a clear peak at the characteristic velocity separation of the C IV doublet. However, when the CGM contribution is removed (i.e. when all metal lines and C IV lines associated with log N HI > 14.0 are deabsorbed), the peak is no longer significant at 1 σ, even though seven weak C IV systems are still detectable by eye. Even after including up to 135 additional weak mock C IV systems (log N HI < 14.0) in the spectrum, we are not able to detect a significant C IV peak. Eventually, when we create a synthetic spectrum with gaussian distributed noise and the same S/N as the complete spectrum, we remove the signal caused by the spectral intrinsic features and thus find a peak compatible with a metallicity of −3.80 < [C/H] < − 3.50. Conclusions. We conclude that the TPCF method is not sensitive to the presence of the weakest systems in the real spectrum, despite the extremely high S/N and high resolution of the data. However, the results of this statistical technique could change when combining more than one line of sight.