The bispectrum of the Lyman - α forest at z ∼ 2 − 2 . 4 from a Large sample of UVES QSO Absorption Spectra ( LUQAS ) ⋆

We present a determination of the bispectrum of the flux in the Lyman-α forest of QSO absorption spectra obtained from LUQAS which consists of spectra observed with the high resolution Echelle spectrograph UVES. Typical errors on the observed bispectrum as obtained from a jack-knife estimator are ∼ 50 %. For wavenumbers in the range 0.03 s/km < k < 0.1 s/km the observed bis-pectrum agrees within the errors with that of the synthetic absorption spectra obtained from numerical hydro-simulations of a ΛCDM model with and without feedback from star formation. Including galactic feedback changes the bispectrum by less than 10%. At smaller wavenumbers the associated metal absorption lines contribute about 50% to the bispectrum and the observed bispectrum exceeds that of the simulations. At wavenumbers k < 0.03 s/km second-order perturbation theory applied to the flux spectrum gives a reasonable (errors smaller than 30%) approximation to the bispectra of observed and simulated absorption spectra. The bispectrum of the observed absorption spectra also agrees, within the errors, with that of a randomized set of absorption spectra where a random shift in wavelength has been added to absorption lines identified with VPFIT. This suggests that for a sample of the size presented here, the errors on the bispectrum are too large to discriminate between models with very different 3D distribution of Lyman-α absorption. If it were possible to substantially reduce these errors for larger samples of absorption spectra, the bispectrum might become an important statistical tool for probing the growth of gravitational structure in the Universe at redshift z ∼ > 2.