CHEMICAL EVOLUTION OF THE UNIVERSE AT 0.7 < z < 1.6 DERIVED FROM ABUNDANCE DIAGNOSTICS OF THE BROAD-LINE REGION OF QUASARS

Abstract

We present an analysis of Mg II λ2798 and Fe II UV emission lines for archival Sloan Digital Sky Survey (SDSS) quasars to explore diagnostics of the magnesium-to-iron abundance ratio in a broadline region cloud. Our sample consists of 17,432 quasars selected from the SDSS Data Release 7 with a redshift range of 0.72 < z < 1.63. A strong anticorrelation between Mg II equivalent width (EW) and the Eddington ratio is found, while only a weak positive correlation is found between Fe II EW and the Eddington ratio. To investigate the origin of these differing behaviors of Mg II and Fe II emission lines, we have performed photoionization calculations using the Cloudy code, where constraints from recent reverberation mapping studies are considered. We find from calculations that (i) Mg II and Fe II emission lines are created at different regions in a photoionized cloud, and (ii) their EW correlations with the Eddington ratio can be explained by just changing the cloud gas density. These results indicate that the Mg II/Fe II flux ratio, which has been used as a first-order proxy for the Mg/Fe abundance ratio in chemical evolution studies with quasar emission lines, depends largely on the cloud gas density. By correcting this density dependence, we propose new diagnostics of the Mg/Fe abundance ratio for a broad line region cloud. Comparing the derived Mg/Fe abundance ratios with chemical evolution models, we suggest that α-enrichment by mass loss from metal-poor intermediate-mass stars occurred at z ∼ 2 or earlier.

The Fe ii/Mg ii Flux Ratio of Low-luminosity Quasars at z ∼ 3

“…Fe II/Mg II rs= -0.409 rs= -0.582 rs= -0.500 p= 0.212 p= 0.060 p= 0.117 Fe II/Mg II w/ SDSS rs= -0.309 rs= 0.189 rs= 0.486 p= 0.000 p= 0.000 p= 0.000 N V/C IV rs= 0.400 rs= 0.670 rs= 0.642 p= 0.197 p= 0.017 p= 0.024 and AGN properties of low-z quasars show similar trend with those in Dong et al (2011) and Sameshima et al (2017). In contrast, the Fe II/Mg II of our sample of 12 high-z quasars show no clear dependency on AGN properties, presumably due to the small sample size, while their flux ratios are consistent with those of low-z AGNs.…”

“…Using the photoionization code, Cloudy (Ferland et al 1998), several theoretical works also showed that the Fe II/Mg II flux ratio depends on various physical parameters such as the turbulence and the BLR gas density (e.g., Verner et al 2003;Baldwin et al 2004;Sameshima et al 2017). To investigate the Fe II/Mg II correlation with Eddington ratio, Sameshima et al (2017) hypothesized that gas density correlates with Eddington ratio and discussed the possibility of the BLR radius-Eddington ratio relation (Rees et al 1989), while the origin of gas density-Eddington ratio relation has not been discussed. By proposing the correction for the gas density dependence, they discussed the intrinsic Fe/Mg abundance ratio.…”

Semi-empirical metallicity calibrations based on ultraviolet emission lines of type-2 AGNs

“…Other studies, as for example Hamann et al (1997), concluded that BLRs, in fact, present super-solar abundances (Z > 5 Z⊙) and an enhancement of N abundances (see also Peimbert 1968;Alloin 1973;Storchi-Bergmann & Pastoriza 1990;Storchi-Bergmann 1991;Dietrich et al 2003;Bradley et al 2004). More recently, Sameshima et al (2017) derived the metallicity, traced by the Fe/H and Mg/Fe abundance ratios, in the BLRs of a sample of about 17 500 Quasars (0.7 z 1.7), whose observational data were taken from the Sloan Digital Sky Survey (SDSS, York et al 2000). To obtain the metallicity, these authors compared observational fluxes and equivalent widths of Feii (calculated by integrating the fitted Feii template in the 2200Å−3090Å wavelength range) and Mgiiλ2798 lines with photoionization model results.…”

“…In order to compare our NLRs Z estimations with those for BLRs, a histogram containing the metallicity distribution for our sample, derived using the Eq. 8, and the results of Sameshima et al (2017) is presented in Fig. 10.…”

The evolution of chemical abundance in quasar broad line region

“…Many studies have suggested that the broad emission-line flux ratios of quasars at the rest-frame ultraviolet and optical wavelength can provide accurate chemical abundance measurements in quasar BLRs (Hamann et al 2002;Warner et al 2003;Dietrich et al 2003;Nagao et al 2006;Matsuoka et al 2011;Marziani et al 2015;Sameshima et al 2017). The theoretical accuracy of any broad emission-line flux ratios as metallicity indicators depends on a variety of factors such as the central radiation field, temperature sensitivity of the emission-line ratio, the similarity of the ionization potentials and critical densities, the extent to which the line-emitting regions overlap spatially (Hamann et al 2002).…”