Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006head....9.0727m&link_type=abstract
American Astronomical Society, HEAD meeting #9, #7.27; Bulletin of the American Astronomical Society, Vol. 38, p.353
Astronomy and Astrophysics
Astronomy
Scientific paper
The ratio of Fe II to Mg II in high redshift quasars permits a probe of the chemical evolution in the Universe. However, this ratio can also depend on the physical conditions in the emitting gas as well as the abundances.
In a study of 900 quasars with narrow Mg II lines from the Sloan Digital Sky Survey, we found a significant dispersion of Fe II/Mg II ratios, including a long tail in the distribution of objects with extremely large Fe II/Mg II ratios. There is evidence that the ratio is sensitive to density, as objects with weak C II] λ2325, a semiforbidden line with critical density of 3.2×109 cm-3, is weak or absent in objects with large Fe II/Mg II ratios.
At the same time, the spectral energy distribution may influence the ratio. The nearby quasar PHL 1811 has been shown to be intrinsically X-ray weak, and its UV and optical spectra are dominated by low-ionization line emission with a high Fe II/Mg II ratio. The strong low-ionization line emission is expected because the X-ray weak SED lacks the high-energy photons required to excite high-ionization lines. The photoionized gas can only cool by emitting low-ionization lines; Fe II emission, with its many multiplets, is an important coolant.
To test the influence of the SED on the Fe II/Mg II ratio, we obtained Chandra snapshots of six high and six low Fe II/Mg II-ratio objects. Four of the twelve are significantly X-ray weak (two are undetected), and three of those are strong Fe II/Mg II emitters. Although our sample is small, the fact that half of the high Fe II/Mg II emitters are significantly X-ray weak suggests that the SED can be involved in determining this ratio.
Leighly Karen M.
Moore John R.
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