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Register a new userRevisited abundance diagnostics in quasars: Fe II/Mg II ratios
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Both the Fe II UV emission in the 2000- 3000 A region [Fe II (UV)] and resonance emission line complex of Mg II at 2800 A are prominent features in quasar spectra. The observed Fe II UV/ Mg II emission ratios have been proposed as means to measure the buildup of the Fe abundance relative to that of the alpha-elements C, N, O, Ne and Mg as a function of redshift. The current observed ratios show large scatter and no obvious dependence on redshift. Thus, it remains unresolved whether a dependence on redshift exists and whether the observed Fe II UV/ Mg II ratios represent a real nucleosynthesis diagnostic. We have used our new 830-level model atom for Fe+ in photoionization calculations, reproducing the physical conditions in the broad line regions of quasars. This modeling reveals that interpretations of high values of Fe II UV/ Mg II are sensitive not only to Fe and Mg abundance, but also to other factors such as microturbulence, density, and properties of the radiation field. We find that the Fe II UV/ Mg II ratio combined with Fe II (UV)/ Fe II (Optical) emission ratio, where Fe II (Optical) denotes Fe II emission in 4000 - 6000 A can be used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios for the physical conditions relevant to the broad-line regions (BLRs) of quasars. This has extreme importance for quasar observations with the Hubble Space Telescope and also with the future James Webb Space Telescope.
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To investigate the chemical abundance of broad-line region clouds in quasars at high redshifts, we performed near-infrared spectroscopy of six luminous quasars at z ~ 2.7 with the WINERED spectrograph mounted on the New Technology Telescope (NTT) at the La Silla Observatory, Chile. The measured Fe II/Mg II flux ratios nearly matched with the published data for 0.7 < z < 1.6, suggesting that there is no evolution over a long period of cosmic time, which is consistent with previous studies. To derive the chemical abundances from the measured equivalent widths (EWs), their dependence on nonabundance factors must be corrected. In our previous paper, we proposed a method to derive the [Mg/Fe] abundance ratio and the [Fe/H] abundance by correcting the dependence of EW(Mg II) and EW(Fe II) on the Eddington ratio. To the best of our knowledge, that was the first report to discuss the star-formation history through a direct comparison with chemical evolution models. In the present study, we further investigated the dependence of EWs on luminosity, which is known as the Baldwin effect (BEff). Additional correction for the BEff significantly affects the derived chemical abundances for the six luminous quasars at z ~ 2.7, and the resultant abundances agree well with the prediction of chemical evolution models. Given that most distant quasars found thus far are biased toward luminous ones, the correction of the measured EWs for the BEff is crucial for extending the chemical evolution study to higher redshifts.
We investigate the relationships between Mg ii {lambda}2798 emission line properties, as well as between these properties and inclination angle and Fe ii strength, in a lobe-dominated quasar sample. We find no correlation between Mg ii line width and inclination, unlike previous studies of the general quasar population. This suggests that the Mg ii emission region in these objects is not of a thin disk geometry, however the average equivalent width of the line negates a spherical alternative. A positive correlation between Mg ii equivalent width and inclination cannot be ruled out, meaning there is no strong evidence that Mg ii emission is anisotropic. Since thin disk emission would be highly directional, the geometric implications of these two findings are compatible. The lack of line width correlation with inclination may also indicate that Mg ii is useful for estimating black hole masses in lobe-dominated quasar samples, since it is unbiased by orientation. Some quasars in our sample have almost edge-on viewing angles and therefore cannot have a smooth toroidal obscurer co-planar with the accretion disk. Alternatives may be a distorted dusty disk or a clumpy obscurer. This could result from the sample selection bias towards high inclination objects, rather than intrinsic differences between lobe-dominated and typical quasars. Five objects have visible [O iii] allowing equivalent width calculation, revealing it to be higher than in typical quasars. Since these objects are of high inclination, this finding supports the positive correlation between [O iii] equivalent width and inclination found in a previous study.
The enrichment of Fe, relative to alpha-elements such as O and Mg, represents a potential means to determine the age of quasars and probe the galaxy formation epoch. To explore how ion{Fe}{2} emission in quasars is linked to physical conditions and abundance, we have constructed a 830-level ion{Fe}{2} model atom and investigated through photoionization calculations how ion{Fe}{2} emission strengths depend on non-abundance factors. We have split ion{Fe}{2} emission into three major wavelength bands, ion{Fe}{2} (UV), ion{Fe}{2}(Opt1), and ion{Fe}{2}(Opt2), and explore how the ion{Fe}{2}(UV)/ion{Mg}{2}, ion{Fe}{2}(UV)/ion{Fe}{2}(Opt1) and ion{Fe}{2}(UV)/ion{Fe}{2}(Opt2) emission ratios depend upon hydrogen density and ionizing flux in broad-line regions (BLRs) of quasars. Our calculations show that: 1) similar ion{Fe}{2}(UV)/ion{Mg}{2} ratios can exist over a wide range of physical conditions; 2) the ion{Fe}{2}(UV)/ion{Fe}{2}(Opt1) and ion{Fe}{2}(UV)/ion{Fe}{2}(Opt2) ratios serve to constrain ionizing luminosity and hydrogen density; and 3) flux measurements of ion{Fe}{2} bands and knowledge of ionizing flux provide tools to derive distances to BLRs in quasars. To derive all BLR physical parameters with uncertainties, comparisons of our model with observations of a large quasar sample at low redshift ($z<1$) is desirable. The STIS and NICMOS spectrographs aboard the Hubble Space Telescope (HST) offer the best means to provide such observations.
We present a study of strong intervening absorption systems in the near-IR spectra of 31 luminous quasars at $z>5.7$. The quasar spectra were obtained with {it Gemini} GNIRS that provide continuous wavelength coverage from $sim$0.9 to $sim$2.5 $mu$m. We detect 32 strong Mg II doublet absorbers with rest-frame equivalent width $W_r$ ($lambda2796$) $>1.0$ AA at $2.2 < z < 6.0$. Each Mg II absorber is confirmed by at least two associated Fe II absorption lines in the rest-frame wavelength range of $sim 1600-2600$ AA. We find that the comoving line density ($dN/dX$) of the strong Fe II-bearing Mg II absorbers decreases towards higher redshift at $z>3$, consistent with previous studies. Compared with strong Mg II absorbers detected in damped Ly$alpha$ systems at 2 $<z<$ 4, our absorbers are potentially less saturated and show much larger rest-frame velocity widths. This suggests that the gas traced by our absorbers are potentially affected by galactic superwinds. We analyze the {it Hubble Space Telescope} near-IR images of the quasars and identify possible associated galaxies for our strong absorbers. There are a maximum of two galaxy candidates found within 5 radius of each absorber. The median F105W-band magnitude of these galaxy candidates is 24.8 mag, which is fainter than the $L^*$ galaxy luminosity at $zsim$ 4. By using our observed $dN/dX$ of strong Mg II absorbers and galaxy candidates median luminosity, we suggest that at high redshift, strong Mg II absorbers tend to have a more disturbed environment but smaller halo size than that at $z <$ 1.
We present spectra of six luminous quasars at z ~ 2, covering rest wavelengths 1600-3200 A. The fluxes of the UV Fe II emission lines and Mg II 2798 doublet, the line widths of Mg II, and the 3000 A luminosity were obtained from the spectra. These quantities were compared with those of low-redshift quasars at z = 0.06 - 0.55 studied by Tsuzuki et al. In a plot of the Fe II(UV)/Mg II flux ratio as a function of the cental black hole mass, Fe II(UV)/Mg II in our z ~ 2 quasars is systematically greater than in the low-redshift quasars. We confermed that luminosity is not responsible for this excess. It is unclear whether this excess is caused by rich Fe abundance at z ~ 2 over low-redshift or by non-abundance effects such as high gas density, strong radiation field, and high microturbulent velocity.
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