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[1004.0221] High resolution spectroscopy of the three dimensional cosmic web with close QSO groups
Authors:M. Cappetta, V. D'Odorico, S. Cristiani, F. Saitta, M. Viel
Abstract:We study the three-dimensional distribution of matter at z~2 using high resolution spectra of QSO pairs and simulated spectra drawn from cosmological hydro-dynamical simulations. We present a sample of 15 QSOs, corresponding to 21 baselines of angular separations evenly distributed between ~1 and 14 arcmin, observed with the Ultraviolet and Visual Echelle Spectrograph (UVES) at the European Southern Observatory-Very Large Telescope (ESO-VLT). The observed correlation functions of the transmitted flux in the HI Lya forest transverse to and along the line of sight are in agreement, implying that the distortions in redshift space due to peculiar velocities are relatively small and - within the relatively large error bars - not significant. The clustering signal is significant up to velocity separations of ~300 km/s, corresponding to about 5 h^{-1} comoving Mpc. Compatibility at the 2 sigma level has been found both for the Auto- and Cross-correlation functions and for the set of the Cross correlation coefficients. The analysis focuses in particular on two QSO groups of the sample. Searching for alignments in the redshift space between Lya absorption lines belonging to different lines of sight, it has been possible to discover the presence of a wide HI structures extending over about ten Mpc in comoving space, and give constraints on the sizes of two cosmic under-dense regions in the intergalactic medium.
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Anze Slosar

Joined: 24 Sep 2004
Posts: 204
Affiliation: Brookhaven National Laboratory

PostPosted: April 06 2010  Reply with quote

This is an interesting paper. They have a bunch of close pairs of quasars and then they do various 2-point statistics (which they phrase in somewhat complicated manners, but it is really just correlation function at various angles). They also do some higher-order statistics.

What I find most intriguing is their claim that peculiar velocities in absorbing gas are smaller than 100 km/s. It is not clear, where exactly is this number coming from. Figure 4 compares auto and cross-correlation functions. Auto-correlation function is essentially correlation function at mu=1 and cross-correlation is integrated over all the other angles (and would give monopole in the limit of many pairs, but they are not quite there). In any case, in the presence of just Kaiser compression, one would expect auto-correlation function to be always larger than cross-correlation function. Since they have high SNR spectra, they must be dominated by the sample variance, but since the two have been measured over the *same* volume, this errors is irrelevant and I think that auto-correlation should be larger than cross-correlation. The only way to wash it out is to have peculiar velocities. So, from the fact that the auto-corr is lower than cross-corr, I'd say you could put a lower limit on the pec velocities, not the upper one. But I guess you can always check this with simulations....

Also, it would be good to get some more formal chi2 to compare simulations with data... As always with corr func - errorbars are heavily correlated (as one can see from the fact that scatter in the points is always much smaller than errorbars).
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