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Register a new userA Systematic Analysis of Fe II Emission in Quasars: Evidence for Inflow to the Central Black Hole
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Chen Hu
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Broad Fe II emission is a prominent feature of the optical and ultraviolet spectra of quasars. We report on a systematical investigation of optical Fe II emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan Digital Sky Survey. We have developed and tested a detailed line-fitting technique, taking into account the complex continuum and narrow and broad emission-line spectrum. Our primary goal is to quantify the velocity broadening and velocity shift of the Fe II spectrum in order to constrain the location of the Fe II-emitting region and its relation to the broad-line region. We find that the majority of quasars show Fe II emission that is redshifted, typically by ~ 400 km/s but up to 2000 km/s, with respect to the systemic velocity of the narrow-line region or of the conventional broad-line region as traced by the Hbeta line. Moreover, the line width of Fe II is significantly narrower than that of the broad component of Hbeta. We show that the magnitude of the Fe II redshift correlates inversely with the Eddington ratio, and that there is a tendency for sources with redshifted Fe II emission to show red asymmetry in the Hbeta line. These characteristics strongly suggest that Fe II originates from a location different from, and most likely exterior to, the region that produces most of Hbeta. The Fe II-emitting zone traces a portion of the broad-line region of intermediate velocities whose dynamics may be dominated by infall.
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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.
In X-ray binaries, the frequencies revealed in X-ray quasi-periodic oscillations (QPOs) are often interpreted as characteristic frequencies in the inner accretion disk, though the exact oscillation mechanism is unknown at present. Broadened Fe K-alpha lines are also excellent diagnostics of the inner accretion flow, if their broadening is indeed due to inner disk reflection. Herein, we present two cases where the flux and equivalent width of the Fe K-alpha emission lines in spectra of the Galactic black hole GRS 1915+105 vary with the phase of strong 1 Hz and 2 Hz QPOs in the X-ray flux. These results provide strong evidence that both QPOs and the Fe-alpha lines originate in the inner disk. If the 1 Hz QPO is only a Keplerian orbital frequency, the QPO comes from a distance of 84 +/- 26 R_Schw from the black hole; the 2 Hz QPO implies a radius of 50 +/- 15 R_Schw. At these radii, relativistic shaping of a disk line is inevitable. Moreover, the link holds in radio-bright and radio-faint phases, signaling that in systems like GRS 1915+105, the Fe K-alpha line is a disk line and not a jet line as per SS 433. A particularly interesting possibility is that a stable warp in the inner disk, e.g. due to Lense-Thirring precession, may produce the observed QPOs and line modulations. More broadly, the FeK-QPO link provides an unprecedented mechanism for revealing the inner accretion flow and relativistic regime in accreting systems, in that it gives two measures of radius: for a given disk QPO model, the frequency translates into a specific radius, and relativistic line models yield radii directly.
We present the spectra of 14 quasars with a wide coverage of rest wavelengths from 1000 to 7300 A. The redshift ranges from z = 0.061 to 0.555 and the luminosity from M_{B} = -22.69 to -26.32. We describe the procedure of generating the template spectrum of Fe II line emission from the spectrum of a narrow-line Seyfert 1 galaxy I Zw 1 that covers two wavelength regions of 2200-3500 A and 4200-5600 A. Our template Fe II spectrum is semi-empirical in the sense that the synthetic spectrum calculated with the CLOUDY photoionization code is used to separate the Fe II emission from the Mg II line. The procedure of measuring the strengths of Fe II emission lines is twofold; (1) subtracting the continuum components by fitting models of the power-law and Balmer continua in the continuum windows which are relatively free from line emissions, and (2) fitting models of the Fe II emission based on the Fe II template to the continuum-subtracted spectra. From 14 quasars, we obtained the Fe II fluxes in five wavelength bands, the total flux of Balmer continuum, and the fluxes of Mg II, Halpha, and other emission lines, together with the full width at half maxima (FWHMs) of these lines. Regression analysis was performed by assuming a linear relation between any two of these quantities. Eight correlations were found with a confidence level higher than 99%. The fact that six of these eight are related to FWHM or M_{BH} may imply that M_{BH} is a fundamental quantity that controls Gamma or the spectral energy distribution (SED) of the incident continuum, which in turn controls the Fe II emission. Furthermore, it is worthy of noting that Fe II(O1)/Fe II(U1) is found to tightly correlate with Fe II(O1)/Mg II, but not with Fe II(U1)/Mg II.
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.
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