Detecting the IGM metal enrichment with the 2-point correlation function of the flux. Application to the UVES deep spectrum

Abstract

The distribution and the abundance of metals in the intergalactic medium (IGM) have strong implications on galaxy formation and evolution models. The ionic transitions of heavy elements in quasar spectra can probe both the mechanisms and the sources of chemical pollution, but high resolution, high signal-to-noise ratio (S/N) spectra are required, as the IGM absorbers can be too weak for direct detection. In this work, we investigate the IGM metallicity, focusing on the detection of the weak absorption lines. We exploited the cosmological tool of the two-point correlation function (TPCF) and applied it to the transmitted flux in the CIV forest 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 to reveal the underlying IGM signal. For each test, we generated 1000 mock spectra with shuffled line positions to derive an estimate for the TPCF noise level. The TPCF shows a clear peak at the characteristic velocity separation of the CIV doublet, but when deabsorbing the CGM contribution (i.e. all metal lines and CIV lines associated with logN(HI)>14.0), the peak is no longer significant at 1$\sigma$, although 7 weak CIV systems remain visible. Even adding up to 135 additional weak mock CIV systems (logN(HI)<14.0) to the spectrum does not produce a significant CIV peak. Eventually, when we create a synthetic spectrum with gaussian distributed noise and 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. 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 would possibly change when combining more than one line of sight.

15 pages, 10 figures, accepted for publication in A&A, abstract abridged

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