Papers › Examining the Decline in the C IV Content of the Universe over 4.3 < z < 6.3 using the...
Examining the Decline in the C IV Content of the Universe over 4.3 < z < 6.3 using the E-XQR-30 Sample
Rebecca L. Davies, Emma Ryan-Weber, Valentina D'Odorico, Sarah E. I. Bosman, Romain A. Meyer, George D. Becker, Guido Cupani, Laura C. Keating, Manuela Bischetti, Frederick B. Davies, Anna-Christina Eilers, Emanuele Paolo Farina, Martin G. Haehnelt, Andrea Pallottini, Yongda Zhu
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Intervening CIV absorbers are key tracers of metal-enriched gas in galaxy halos over cosmic time. Previous studies suggest that the CIV cosmic mass density (Ω_(CIV)) decreases slowly over 1.5 ≲z≲ 5 before declining rapidly at z≳ 5, but the cause of this downturn is poorly understood. We characterize the Ω_(CIV) evolution over 4.3 ≲z≲ 6.3 using 260 absorbers found in 42 XSHOOTER spectra of z∼ 6 quasars, of which 30 come from the ESO Large Program XQR-30. The large sample enables us to robustly constrain the rate and timing of the downturn. We find that Ω_(CIV) decreases by a factor of 4.8 ± 2.0 over the ~300 Myr interval between z∼ 4.7 and z∼ 5.8. The slope of the column density (log N) distribution function does not change, suggesting that CIV absorption is suppressed approximately uniformly across 13.2 ≤ log N/cm⁻² < 15.0. Assuming that the carbon content of galaxy halos evolves as the integral of the cosmic star formation rate density (with some delay due to stellar lifetimes and outflow travel times), we show that chemical evolution alone could plausibly explain the fast decline in Ω_(CIV) over 4.3 ≲z≲ 6.3. However, the CIV/CII ratio decreases at the highest redshifts, so the accelerated decline in Ω_(CIV) at z≳ 5 may be more naturally explained by rapid changes in the gas ionization state driven by evolution of the UV background towards the end of hydrogen reionization.
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