Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

The diversity of quasars unified by accretion and orientation


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.

This is a preview of subscription content

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Distribution of quasars in the EV1 plane.
Figure 2: Average [O iii] profiles in the EV1 plane.
Figure 3: Cross-correlation functions between different quasar subsamples and a galaxy sample.
Figure 4: The effect of orientation on FWHM.


  1. Boroson, T. A. & Green, R. F. The emission-line properties of low-redshift quasi-stellar objects. Astrophys. J. 80 (suppl.). 109–135 (1992)

    CAS  Article  Google Scholar 

  2. Sulentic, J. W., Zwitter, T., Marziani, P. & Dultzin-Hacyan, D. Eigenvector 1: an optimal correlation space for active galactic nuclei. Astrophys. J. 536, L5–L9 (2000)

    ADS  CAS  Article  Google Scholar 

  3. Wang, T., Brinkmann, W. & Bergeron, J. X-ray properties of active galactic nuclei with optical FeII emission. Astron. Astrophys. 309, 81–96 (1996)

    ADS  CAS  Google Scholar 

  4. Boroson, T. A. Black hole mass and Eddington ratio as drivers for the observable properties of radio-loud and radio-quiet QSOs. Astrophys. J. 565, 78–85 (2002)

    ADS  CAS  Article  Google Scholar 

  5. Laor, A. The soft X-ray properties of a complete sample of optically selected quasars. II. Final results. Astrophys. J. 477, 93–113 (1997)

    ADS  CAS  Article  Google Scholar 

  6. Schneider, D. P. et al. The Sloan Digital Sky Survey quasar catalog. V. Seventh data release. Astron. J. 139, 2360–2373 (2010)

    ADS  CAS  Article  Google Scholar 

  7. Shen, Y. et al. A catalog of quasar properties from Sloan Digital Sky Survey data release 7. Astrophys. J. 194 (suppl.). 45 (2011)

    ADS  Article  Google Scholar 

  8. Stern, J. & Laor, A. Type 1 AGN at low z—III. The optical narrow-line ratios. Mon. Not. R. Astron. Soc. 431, 836–857 (2013)

    ADS  CAS  Article  Google Scholar 

  9. Zhang, K., Wang, T.-G., Gaskell, C. M. & Dong, X.-B. The Baldwin effect in the narrow emission lines of active galactic nuclei. Astrophys. J. 762, 51 (2013)

    ADS  Article  Google Scholar 

  10. Baldwin, J. A. Luminosity indicators in the spectra of quasi-stellar objects. Astrophys. J. 214, 679–684 (1977)

    ADS  CAS  Article  Google Scholar 

  11. Dopita, M. A. & Sutherland, R. S. Spectral signatures of fast shocks. II. Optical diagnostic diagrams. Astrophys. J. 455, 468–479 (1995)

    ADS  CAS  Article  Google Scholar 

  12. Wright, E. L. et al. The Wide-field Infrared Survey Explorer (WISE): mission description and initial on-orbit performance. Astron. J. 140, 1868–1881 (2010)

    ADS  Article  Google Scholar 

  13. Antonucci, R. Unified models for active galactic nuclei and quasars. Annu. Rev. Astron. Astrophys. 31, 473–521 (1993)

    ADS  CAS  Article  Google Scholar 

  14. Urry, C. M. & Padovani, P. Unified schemes for radio-loud active galactic nuclei. Publ. Astron. Soc. Pacif. 107, 803–845 (1995)

    ADS  Article  Google Scholar 

  15. Peterson, B. M. et al. Central masses and broad-line region sizes of active galactic nuclei. II. A homogeneous analysis of a large reverberation-mapping database. Astrophys. J. 613, 682–699 (2004)

    ADS  CAS  Article  Google Scholar 

  16. Laor, A. On black hole masses and radio loudness in active galactic nuclei. Astrophys. J. 543, L111–L114 (2000)

    ADS  Article  Google Scholar 

  17. Vestergaard, M. & Peterson, B. M. Determining central black hole masses in distant active galaxies and quasars. II. Improved optical and UV scaling relationships. Astrophys. J. 641, 689–709 (2006)

    ADS  CAS  Article  Google Scholar 

  18. Bardeen, J. M., Bond, J. R., Kaiser, N. & Szalay, A. S. The statistics of peaks of Gaussian random fields. Astrophys. J. 304, 15–61 (1986)

    ADS  CAS  Article  Google Scholar 

  19. Ahn, C. et al. The tenth data release of the Sloan Digital Sky Survey: first spectroscopic data from the SDSS-III Apache Point Observatory Galactic Evolution Experiment. Astrophys. J. 211 (Supp.). 17 (2014)

    Article  Google Scholar 

  20. Shen, Y. et al. Cross-correlation of SDSS DR7 quasars and DR10 BOSS galaxies: the weak luminosity dependence of quasar clustering at z 0.5. Astrophys. J. 778, 98 (2013)

    ADS  Article  Google Scholar 

  21. Wills, B. J. & Browne, I. W. A. Relativistic beaming and quasar emission lines. Astrophys. J. 302, 56–63 (1986)

    ADS  CAS  Article  Google Scholar 

  22. Runnoe, J. C., Brotherton, M. S., Shang, Z., Wills, B. J. & DiPompeo, M. A. The orientation dependence of quasar single-epoch black hole mass scaling relationships. Mon. Not. R. Astron. Soc. 429, 135–149 (2013)

    ADS  CAS  Article  Google Scholar 

  23. Park, D., Kelly, B. C., Woo, J.-H. & Treu, T. Recalibration of the virial factor and MBH-σ* relation for local active galaxies. Astrophys. J. 203 (suppl.). 6 (2012)

    Article  Google Scholar 

  24. Marziani, P. et al. An optical spectroscopic atlas of low-redshift active galactic nuclei. Astrophys. J. 145 (suppl.). 199–211 (2003)

    CAS  Article  Google Scholar 

  25. Kormendy, J. & Ho, L. C. Coevolution (or not) of supermassive black holes and host galaxies. Annu. Rev. Astron. Astrophys. 51, 511–653 (2013)

    ADS  CAS  Article  Google Scholar 

  26. Shen, J., Vanden Berk, D. E., Schneider, D. P. & Hall, P. B. The black hole-bulge relationship in luminous broad-line active galactic nuclei and host galaxies. Astron. J. 135, 928–946 (2008)

    ADS  CAS  Article  Google Scholar 

  27. Bentz, M. C. et al. The radius-luminosity relationship for active galactic nuclei: the effect of host-galaxy starlight on luminosity measurements. II. The full sample of reverberation-mapped AGNs. Astrophys. J. 697, 160–181 (2009)

    ADS  CAS  Article  Google Scholar 

Download references


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 ( and

Author information

Authors and Affiliations



Y.S. and L.C.H. co-developed the idea. Y.S. performed the measurements and analysis. Both authors contributed to the interpretation and manuscript writing.

Corresponding author

Correspondence to Yue Shen.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 Decomposed [O iii] λ = 5,007 Å luminosity.

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 FWHM, 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.

Extended Data Figure 2 Kinematic properties of the decomposed core and wing [O iii] 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–FWHM 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.

Extended Data Figure 4 Distribution in the EV1 plane in terms of C iv properties.

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 FWHM 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.

Extended Data Figure 7 Distribution in the EV1 plane in terms of X-ray properties.

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 FWHM.

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 FWHM 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 FWHM, possibly caused by a broader Eddington ratio distribution at lower luminosities, which introduces additional dispersion in FWHM. LEdd = 1.3 × 1038(MBH/1 ) erg s−1 is the Eddington luminosity of the black hole.

Extended Data Figure 9 Distributions of radio-loud and radio-quiet quasars in EV1 plane.

The radio-loud population shifts to lower RFe ii and larger FWHM, 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 FWHM compared with the lobe-dominant radio quasars, consistent with the hypothesis that orientation leads to variations in FWHM. The points with error bars are the median and 1σ uncertainty for the median in each RFe ii bin.

Extended Data Figure 10 Distribution in the EV1 plane colour-coded by the FWHM/σ ratio.

The distribution has been smoothed over a box of ΔRFe ii = 0.2 and ΔlogFWHM = 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 FWHM 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.

Supplementary information

Supplementary information

This file contains Supplementary text and additional references. (PDF 333 kb)

PowerPoint slides

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Shen, Y., Ho, L. The diversity of quasars unified by accretion and orientation. Nature 513, 210–213 (2014).

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI:

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing