Lyman-Alpha Forest Correlations Using Neighbor Counts

We present a novel technique for calculating the two-point autocorrelationfunction of the Lyman-alpha forest based on the relation between the two-pointcorrelation function and the Neighbor Probability Distribution Functions. Thetechnique appears to reduce the scatter in estimates of the correlationfunction by a factor of about two from the traditional pair-counting method. Weapply the technique to the Lyman-alpha forest line lists determined from thespectra of seven z > 2 QSOs observed at high resolution ( < 25 km/s ). Ofthese, only two spectra, those of Q0055-259 and Q0014+813, appear to besufficiently free of systematics to obtain meaningful estimates of thecorrelation function. We find positive correlations, with a maximum amplitudeof 0.5 - 1, on the scale of 0.5 - 3 h^{-1} Mpc (comoving), or 100 - 600 km/s,in the forests of both QSOs. The technique also finds strong evidence foranticorrelation on the scale of 3 - 6 h^{-1} Mpc. The strength of the positivecorrelations is comparable to that predicted from the primordial power spectruminferred from optically-selected galaxy redshift surveys. If theanticorrelations are physical, it becomes unclear whether the detectedclustering is consistent with current models of structure formation with ahomogeneous photoionizing UV metagalactic background. Instead, theanti-clustering may require inhomogeneity in the UV background, and possibly inthe process of reionization itself.Comment: 18 pages, uuencoded, compressed, postscript, figures included. To appear in the Astrophysical Journal Letter