
CosmoCoffee

[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 threedimensional distribution of matter at z~2 using high
resolution spectra of QSO pairs and simulated spectra drawn from cosmological
hydrodynamical 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 ObservatoryVery Large Telescope (ESOVLT). 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 Crosscorrelation 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 underdense
regions in the intergalactic medium. 

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Anze Slosar
Joined: 24 Sep 2004 Posts: 205 Affiliation: Brookhaven National Laboratory

Posted: April 06 2010 


This is an interesting paper. They have a bunch of close pairs of quasars and then they do various 2point statistics (which they phrase in somewhat complicated manners, but it is really just correlation function at various angles). They also do some higherorder 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 crosscorrelation functions. Autocorrelation function is essentially correlation function at mu=1 and crosscorrelation 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 autocorrelation function to be always larger than crosscorrelation 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 autocorrelation should be larger than crosscorrelation. The only way to wash it out is to have peculiar velocities. So, from the fact that the autocorr is lower than crosscorr, 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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