Computer Science
Scientific paper
Feb 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999noao.prop..259c&link_type=abstract
NOAO Proposal ID #1999A-0259
Computer Science
Scientific paper
Until recently, determinations of the size of the objects that produce Ly(alpha) forest absorption in QSO spectra were uncertain by more than a factor of 30. Our recent observations, noting the number of absorption lines in common for a pair of QSO sightlines separated by 40 h^-1kpc, now peg the size of these objects (for column densities N_HI ~ 10^14 cm^-2), at ~300 h^-1kpc across, to within a factor of two (95% confidence). This is a surprisingly large size. Given the size and N_HI, we can determine the H I mass of the absorbers. Given the ionizing radiation density (determined with the ``proximity effect''), we can determine the total hydrogen (H I + H II) mass of the absorbers. Furthermore, knowing the absorber size and their number per unit z along the line of sight, we can determine their number density in space and their (large) contribution to the universal closure density Ω, which is predicted to vary with z. These size measures can be compared directly to simulated forest spectra from numerical models, and can determine which models are in best agreement, and which choice of ionization flux and fluctuation power spectrum corresponds to reality. A key factor is how observed absorbers correspond to model clouds, and this is pegged by the size of the aborbers. The size evolution with redshift depends on the scale of collapse, which varies between models and at different z. We propose observing two z~3 QSO pairs with separations of 33-62 arcsec (120-220 h^-1kpc at z=3), exactly the scale of the probable absorber size, in order to determine the extent of absorbers and all of the implied properties to high accuracy, and to determine which absorber model survives. This is best done at high z, where numerical models are simplest due to less gravitational collapse. Absorber size at z~ 3 can also be compared to existing estimates at z~ 2 and z~ 1 to determine how the Ly (alpha) forest has contributed to Ω over time.
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