Mathematics – Logic
Scientific paper
Jan 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21546010k&link_type=abstract
American Astronomical Society, AAS Meeting #215, #460.10; Bulletin of the American Astronomical Society, Vol. 42, p.491
Mathematics
Logic
Scientific paper
One of the powers of qso absorption line systems is estimating the metallicities and abundances over cosmological time and in a variety of environments (i.e., DLAs, galaxy halos, and the IGM). Most all metallicity estimates in the literature have not investigated UV photon shielding by optically thick material in the proximity of the absorber being studied. Shielding removes ionizing photons, thereby softening the ionizing spectrum incident on the absorber. This process results in ionization fractions that differ from an unshielded photoionized cloud and can lead to a substantial underestimate in the inferred metallicity. This may have consequences for metallicity estimates, especially in Lyman Limit Systems, which are thought to arise in close proximity to galaxies.
In this work we use the photoionization code Cloudy to investigate the effects of shielding on the observed column densities of MgII, CIV, OVI, and FeII for both the optically thin and optically thick cloud scenarios. We test this for shielding clouds with different HI column density, N(HI) , as a function of total hydrogen number density, nH.
Shielding results in an underestimate of the metallicity relative to unshielded absorbers. Generally, the greater the shield cloud N(HI), the greater the metallicity underestimate for all three ions. For MgII, the greater the shield cloud nH, the greater the metallicity underestimate, while the opposite is true for the higher ionization ions CIV and OVI. The effect is more sensitive to the shield cloud N(HI) than to its nH. The amount of underestimation is more severe for higher N(HI) absorbers.
Churchill Christopher W.
Klimek Elizabeth S.
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