Quasars are rapidly accreting supermassive black holes at the centres of massive galaxies. They display a broad range of properties across all wavelengths, reflecting the diversity in the physical conditions of the regions close to the central engine. These properties, however, are not random, but form well-defined trends. The dominant trend is known as ‘Eigenvector 1’, in which many properties correlate with the strength of optical iron and [O iii] emission1,2,3. The main physical driver of Eigenvector 1 has long been suspected4 to be the quasar luminosity normalized by the mass of the hole (the ‘Eddington ratio’), which is an important parameter of the black hole accretion process. But a definitive proof has been missing. Here we report an analysis of archival data that reveals that the Eddington ratio indeed drives Eigenvector 1. We also find that orientation plays a significant role in determining the observed kinematics of the gas in the broad-line region, implying a flattened, disk-like geometry for the fast-moving clouds close to the black hole. Our results show that most of the diversity of quasar phenomenology can be unified using two simple quantities: Eddington ratio and orientation.
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Support for the work of Y.S. was provided by NASA through Hubble Fellowship grant number HST-HF-51314.01, awarded by the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy for NASA, under contract number NAS 5-26555. L.C.H. acknowledges support from the Kavli Foundation, Peking University, and the Chinese Academy of Science through grant number XDB09030102 (Emergence of Cosmological Structures) from the Strategic Priority Research Program. This work makes extensive use of SDSS-I/II and SDSS-III data (http://www.sdss.org/ and http://www.sdss3.org/).
The authors declare no competing financial interests.
Extended data figures and tables
The core component (a) and the wing component (b) are shown for each composite spectrum shown in Fig. 2. Error bars are 1σ measurement errors estimated using Monte Carlo trials of mock spectra generated using the estimated flux error arrays of the co-added spectra. Both luminosities are normalized to the quasar continuum luminosity L5,100 Å, hence reflecting the strength of [O iii]. The core [O iii] shows a prominent anti-correlation with both L5,100 Å and RFe ii, while the wing [O iii] shows weaker anti-correlations with L5,100 Å and RFe ii. For both [O iii] components there is no correlation with FWHMHβ, as shown in Figs 1 and 2. The Baldwin effect and EV1 correlation for [O iii] shown in Fig. 1 and Fig. 2 are then primarily associated with the core [O iii] component. The difference between the core and wing [O iii] components may suggest different excitation mechanisms for both components.
a, FWHM against luminosity for core [O iii]. b, FWHM against luminosity for wing [O iii]. c, Velocity offset against luminosity for core [O iii]. d, Velocity offset against luminosity for wing [O iii]. Error bars are 1σ measurement errors estimated using Monte Carlo trials of mock spectra generated using the estimated flux error arrays of the co-added spectra. The most significant correlations are the correlation between luminosity and the core [O iii] FWHM, and the correlations between the wing [O iii] blueshift and L/RFe ii. The former correlation is consistent with the scenario that more luminous quasars are on average hosted by more massive galaxies with deeper potential wells, hence having larger core [O iii] widths. The latter correlations are consistent with the scenario that the wing [O iii] component is associated with outflows.
Extended Data Figure 3 Composite SDSS quasar spectra for several other lines in the same RFe ii–FWHMHβ bins as defined in Fig. 1.
a, Hβ and [O iii]. b, Mg ii. c, [O ii] 3,727 Å. d, [Ne v] 3,426 Å. As in Fig. 2, each composite spectrum has been normalized by the continuum such that the integrated line intensity reflects the strength of the line. The composite spectra for the Hβ region are generated using the pseudo-continuum-subtracted spectra, while for each of the other three lines (Mg ii, [O ii] and [Ne v]) the composite spectrum is the median spectrum created using the full SDSS spectra and normalized at a nearby continuum window.
A sample of low-redshift quasars with both Hβ and C iv measurements is shown, colour-coded by the C iv strength. A clear trend of decreasing C iv strength with RFe ii is seen, consistent with that seen for the other forbidden lines. The typical 1σ measurement uncertainty in C iv equivalent width is about 7% (relative to the measurement), and hence is negligible compared to the strong EV1 trend observed.
Extended Data Figure 5 Distributions of SDSS quasars in the EV1 plane in terms of the optical–infrared (r − W1) colour.
r is the SDSS r band (6,166 Å) and W1 is the WISE W1 band (3.4 μm). a, r − W1 for quasars with 0.4 < z < 0.8, for which the band-shifting effect is small. We see a trend of increasing mid-infrared emission relative to optical emission with increasing RFe ii. b, A similar result, using the excess colour, Δ(r − W1), which is the deviation of r − W1 colour from the mean colour at each redshift. Using Δ(r − W1) removes the redshift dependence of colours, and we can apply this to all quasars in our sample. This test suggests that the torus emission is enhanced in quasars with larger RFe ii. Given that we have argued that RFe ii is a good indicator for the Eddington ratio, this result suggests that quasars with higher Eddington ratios have stronger torus emission, which may have implications for the formation mechanism of the dusty torus.
Extended Data Figure 6 A detailed look at the median excess optical-WISE colour Δ(r − W1) in the EV1 plane.
The same bins as defined in Fig. 1 are used. Error bars are the 1σ uncertainty in the median, estimated by the standard deviation divided by the square root of the number of objects in the bin. At fixed RFe ii, we see increasing relative torus emission when FWHMHβ increases. This is consistent with the orientation scenario: larger FWHMs indicate more edge-on systems, which suffer more from geometric reduction (the cosI factor) and/or dust extinction in the optical than in the infrared parts of the spectrum.
The subset of our SDSS quasars with available measurements of their soft X-ray photon index ΓX are shown. ΓX increases (becomes softer) with increasing RFe ii, consistent with earlier findings3,5. CSC refers to objects from the Chandra Source Catalog and XMM refers to objects from the XMM-Newton Serendipitous Catalog. The contours are the distribution of all SDSS quasars in our sample, as in Fig. 1.
Extended Data Figure 8 The same EV1 plane as in Fig. 1 in logarithmic FWHMHβ.
The dashed lines show the running median value as a function of RFe ii and the dotted lines show the 16% and 84% percentiles, for objects in different luminosity bins. The distribution of FWHMHβ at fixed RFe ii roughly follows a log-normal distribution, with a dispersion of about 0.15−0.25 dex, which we argued comes mostly from orientation-induced variations. Lower-luminosity objects tend to have slightly larger dispersion in FWHMHβ, possibly caused by a broader Eddington ratio distribution at lower luminosities, which introduces additional dispersion in FWHMHβ. LEdd = 1.3 × 1038(MBH/1 ) erg s−1 is the Eddington luminosity of the black hole.
The radio-loud population shifts to lower RFe ii and larger FWHMHβ, compared with the radio-quiet population. We further divide the radio-loud quasars into core-dominant and lobe-dominant subsets, but we caution that our morphological classification is very crude, and there is potentially a large mixture of true morphological types between the two subsamples. The core-dominant (more pole-on) radio quasars have systematically smaller FWHMHβ compared with the lobe-dominant radio quasars, consistent with the hypothesis that orientation leads to variations in FWHMHβ. The points with error bars are the median and 1σ uncertainty for the median in each RFe ii bin.
The distribution has been smoothed over a box of ΔRFe ii = 0.2 and ΔlogFWHMHβ = 0.2. We show only points for which there are more than 50 objects in the smoothing box to average. The black open circles show the median FWHMHβ at fixed RFe ii (using all objects in that bin), with the error bars indicating the 1σ uncertainty for the median. The transition in FWHM/σ reflects the change in orientation of the broad-line region disk relative to the line of sight.
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Shen, Y., Ho, L. The diversity of quasars unified by accretion and orientation. Nature 513, 210–213 (2014). https://doi.org/10.1038/nature13712
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