Lyman-Alpha Forest Correlations Using Neighbor Counts

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Abstract
We present a novel technique for calculating the two-point autocorrelation function of the Lyα forest based on the relation between the two-point correlation function and the neighbor probability distribution functions. The technique appears to reduce the scatter in estimates of the correlation function by a factor of ~2 from the traditional pair-counting method. We apply the technique to the Lyα forest line lists determined from the spectra of seven z > 2 QSOs observed at high resolution (Δv < 25 km s-1). Of these, only two spectra, those of Q0055-259 and Q0014+813, appear to be sufficiently free of systematics to obtain meaningful estimates of the correlation function. We find positive correlations, with a maximum amplitude of 0.5-1, on the scale of 0.5-3 h-1 Mpc (comoving), or 100-600 km s-1, in the forests of both QSOs. The technique also finds strong evidence for anticorrelation on the scale of 3-6 h-1 Mpc. The strength of the positive correlations is comparable to that predicted from the primordial power spectrum inferred from optically selected galaxy redshift surveys. If the anticorrelations are physical, it becomes unclear whether the detected clustering is consistent with current models of structure formation with a homogeneous photoionizing UV metagalactic background. Instead, the anticlustering may require inhomogeneity in the UV background and possibly in the process of reionization itself.
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