Lack of Iron Abundance Evolution in High-Redshift QSOS

Details Lack of Iron Abundance Evolution in High-Redshift QSOS Lack of Iron Abundance Evolution in High-Redshift QSOS

Apr 1999

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999apj...515..487t&link_type=abstract

The Astrophysical Journal, Volume 515, Issue 2, pp. 487-496.

Astronomy and Astrophysics

Astronomy

29

Cosmology: Observations, Cosmology: Early Universe, Galaxies: Evolution, Galaxies: Quasars: Emission Lines, Galaxies: Quasars: General

Scientific paper

We analyze the spectra of 12 high-redshift QSOs in the region of Mg II lambda2798. Two six-spectrum composites, with =4.47 and 3.35, are compared with the Francis et al. Large Bright Quasar Survey (LBQS) composite (~0.8) and show remarkable similarity. The equivalent width of Mg II is somewhat larger in the LBQS composite than in the high-redshift composites; this may be identified as a weak Baldwin effect. The equivalent width of the Fe II blends to either side is the same in the =3.35 composite as in the LBQS composite and marginally larger in the =4.47 composite. We derive strong confidence limits on the absence of a significant increase in either the Fe II equivalent width or the Fe II/Mg II line ratio from the earliest epoch to the present. Hamann & Ferland predicted that the Fe II/Mg II ratio at high redshift should be a factor of ~3 lower than for low-redshift QSOs if the age of the universe at the earlier epoch is much less than 1 Gyr. This is due to the delayed contribution of supernovae type Ia to the iron abundance. A 1 Gyr minimum age of the universe at z=4.47 gives interesting upper limits to q_0 for a given H_0 (e.g., q_0<0.20 for H_0=75 km s^-1 Mpc^-1). The lack of change in Fe II/Mg II between the two high-redshift epochs indicates that 1 Gyr may be an underestimate of the universe age at z=4.47. We also discuss alternate explanations. The abundance evolution in the central regions of galaxies that host QSOs may follow a significantly different path to that in the solar neighborhood, perhaps from the dominance of short-lived type Ia supernovae progenitors or through the lack of any significant type Ia contribution, in which case we are yet underestimating the efficiency of Fe^+ ions in QSO broad-line clouds for production of the observed Fe II. Cosmologies with terms beyond zero-pressure matter are also briefly discussed. These considerations notwithstanding, the lack of observed Fe II evolution at high redshifts puts unique constraints on luminous QSO broad-line physics, nucleosynthesis in the QSO host galaxies, and/or cosmological parameters.

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