Skip to main content

Thank you for visiting nature.com. 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.

  • Article
  • Published:

A universal average spectral energy distribution for quasars from the optical to the extreme ultraviolet

Nature Astronomy volume 7, pages 1506–1516 (2023)Cite this article

Subjects

Abstract

Quasars feature gas swirling towards a supermassive black hole inhabiting a galactic centre. The disk accretion produces enormous amounts of radiation from optical to ultraviolet (UV) wavelengths. Extreme UV (EUV) emission, stemming from the energetic innermost disk regions, has critical implications for the production of broad emission lines in quasars, the origin of the correlation between linewidth and luminosity (or the Baldwin effect) and cosmic reionization. Spectroscopic and photometric analyses have claimed that brighter quasars have on average redder EUV spectral energy distributions (SEDs), which may, however, have been affected by a severe EUV detection incompleteness bias. Here, after controlling for this bias, we reveal a luminosity-independent universal average SED down to a rest frame of ~500 Å for redshift z ≈ 2 quasars over nearly two orders of magnitude in luminosity, contrary to the standard thin disk prediction and the Baldwin effect, which persists even after controlling for the bias. Furthermore, we show that the intrinsic bias-free mean SED is redder in the EUV than previous mean quasar composite spectra, while the intrinsic bias-free median SED is even redder and is unexpectedly consistent with the simply truncated wind model prediction, suggesting prevalent winds in quasars and altered black hole growth. A microscopic atomic origin is probably responsible for both the universality and redness of the average SED.

This is a preview of subscription content, access via your institution

Access options

Buy this article

  • Purchase on Springer Link
  • Instant access to full article PDF
Buy now

Prices may be subject to local taxes which are calculated during checkout

Fig. 1: Rest-frame median UV SEDs.
Fig. 2: The Baldwin effect of the Mg ii line versus the luminosity dependence of the median EUV SED.
Fig. 3: Our intrinsic bias-free average UV SEDs versus previous mean composite quasar spectra.
Fig. 4: Our intrinsic bias-free median UV SED versus the predictions of two static disk models.

Similar content being viewed by others

Data availability

All original data necessary to reproduce the results are publicly available and have been specified with website addresses in Methods. The SDSS DR14Q catalogue is available at https://data.sdss.org/sas/dr14/eboss/qso/DR14Q/DR14Q_v4_4.fits, the GALEX legacy data release GR6plus7 is at https://galex.stsci.edu/GR6/, searching for GALEX counterparts for SDSS quasars can be achieved at https://galex.stsci.edu/casjobs/default.aspx, and the catalogue of GALEX GR6plus7 UV data for SDSS DR14Q quasars is at http://cdsarc.cds.unistra.fr/vizbin/cat/J/MNRAS/493/2745. All data used in this paper are available from the corresponding author upon request.

Code availability

All codes used in this paper are available from the corresponding author upon request.

References

  1. Lynden-Bell, D. Galactic nuclei as collapsed old quasars. Nature 223, 690–694 (1969).

    Article  ADS  Google Scholar 

  2. Shakura, N. I. & Sunyaev, R. A. Black holes in binary systems. Observational appearance. Astron. Astrophys. 24, 337–355 (1973).

    ADS  Google Scholar 

  3. Shields, G. A. Thermal continuum from accretion disks in quasars. Nature 272, 706–708 (1978).

    Article  ADS  Google Scholar 

  4. Malkan, M. A. & Sargent, W. L. W. The ultraviolet excess of Seyfert 1 galaxies and quasars. Astrophys. J. 254, 22–37 (1982).

    Article  ADS  Google Scholar 

  5. Kishimoto, M. et al. The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared. Nature 454, 492–494 (2008).

    Article  ADS  Google Scholar 

  6. Koratkar, A. & Blaes, O. The ultraviolet and optical continuum emission in active galactic nuclei: the status of accretion disks. Publ. Astron. Soc. Pac. 111, 1–30 (1999).

    Article  ADS  Google Scholar 

  7. Neugebauer, G. et al. Continuum energy distributions of quasars in the Palomar-Green Survey. Astrophys. J. Suppl. Ser. 63, 615 (1987).

    Article  ADS  Google Scholar 

  8. Cheng, F. H., Gaskell, C. M. & Koratkar, A. P. The shape of the ultraviolet continuum of quasars and intergalactic dust. Astrophys. J. 370, 487 (1991).

    Article  ADS  Google Scholar 

  9. Zheng, W., Kriss, G. A., Telfer, R. C., Grimes, J. P. & Davidsen, A. F. A composite HST spectrum of quasars. Astrophys. J. 475, 469–478 (1997).

    Article  ADS  Google Scholar 

  10. Telfer, R. C., Zheng, W., Kriss, G. A. & Davidsen, A. F. The rest-frame extreme-ultraviolet spectral properties of quasi-stellar objects. Astrophys. J. 565, 773–785 (2002).

    Article  ADS  Google Scholar 

  11. Shull, J. M., Stevans, M. & Danforth, C. W. HST-COS Observations of AGNs. I. Ultraviolet composite spectra of the ionizing continuum and emission lines. Astrophys. J. 752, 162 (2012).

    Article  ADS  Google Scholar 

  12. Stevans, M. L., Shull, J. M., Danforth, C. W. & Tilton, E. M. HST-COS observations of AGNs. II. Extended Survey of ultraviolet composite spectra from 159 active galactic nuclei. Astrophys. J. 794, 75 (2014).

    Article  ADS  Google Scholar 

  13. Lusso, E. et al. The first ultraviolet quasar-stacked spectrum at z ≃ 2.4 from WFC3. Mon. Not. R. Astron. Soc. 449, 4204–4220 (2015).

    Article  ADS  Google Scholar 

  14. Kuhn, O., Elvis, M., Bechtold, J. & Elston, R. A Search for signatures of quasar evolution: comparison of the shapes of the rest-frame optical/ultraviolet continua of quasars at z > 3 and z ~ 0.1. Astrophys. J. Suppl. Ser. 136, 225–264 (2001).

    Article  ADS  Google Scholar 

  15. Davis, S. W., Woo, J.-H. & Blaes, O. M. The UV continuum of quasars: models and SDSS spectral slopes. Astrophys. J. 668, 682–698 (2007).

    Article  ADS  Google Scholar 

  16. Courvoisier, T.J.-L. & Clavel, J. Observational constraints on disc models for quasars and Seyfert galaxies. Astron. Astrophys. 248, 389 (1991).

    ADS  Google Scholar 

  17. Krolik, J. H. & Kallman, T. R. The effects of thermal accretion disk spectra on the emission lines from active galactic nuclei. Astrophys. J. 324, 714 (1988).

    Article  ADS  Google Scholar 

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

    Article  ADS  Google Scholar 

  19. Vanden Berk, D. E. et al. Extreme ultraviolet quasar colours from GALEX observations of the SDSS DR14Q catalogue. Mon. Not. R. Astron. Soc. 493, 2745–2764 (2020).

    Article  ADS  Google Scholar 

  20. Miralda-Escude, J. & Ostriker, J. P. What produces the ionizing background at large redshift? Astrophys. J. 350, 1 (1990).

    Article  ADS  Google Scholar 

  21. Netzer, H., Laor, A. & Gondhalekar, P. M. Quasar discs. III: line and continuum correlations. Mon. Not. R. Astron. Soc. 254, 15–20 (1992).

    Article  ADS  Google Scholar 

  22. Zheng, W. & Malkan, M. A. Does a luminosity-dependent continuum shape cause the Baldwin effect? Astrophys. J. 415, 517 (1993).

    Article  ADS  Google Scholar 

  23. He, Z. et al. Variation of ionizing continuum: the main driver of broad absorption line variability. Astrophys. J. Suppl. Ser. 229, 22 (2017).

    Article  ADS  Google Scholar 

  24. Jiang, L. et al. Definitive upper bound on the negligible contribution of quasars to cosmic reionization. Nat. Astron. 6, 850–856 (2022).

    Article  ADS  Google Scholar 

  25. Pâris, I. et al. The Sloan Digital Sky Survey quasar catalog: fourteenth data release. Astron. Astrophys. 613, A51 (2018).

    Article  ADS  Google Scholar 

  26. Martin, D. C. et al. The Galaxy Evolution Explorer: a space ultraviolet survey mission. Astrophys. J. 619, L1–L6 (2005).

    Article  ADS  Google Scholar 

  27. Trammell, G. B. et al. The UV properties of SDSS-selected quasars. Astron. J. 133, 1780–1794 (2007).

    Article  ADS  Google Scholar 

  28. Krawczyk, C. M. et al. Mean spectral energy distributions and bolometric corrections for luminous quasars. Astrophys. J. Suppl. Ser. 206, 4 (2013).

    Article  ADS  Google Scholar 

  29. Rakshit, S., Stalin, C. S. & Kotilainen, J. Spectral properties of quasars from Sloan Digital Sky Survey Data Release 14: the catalog. Astrophys. J. Suppl. Ser. 249, 17 (2020).

    Article  ADS  Google Scholar 

  30. Dong, X.-B. et al. Eddington ratio governs the equivalent width of Mg II emission line in active galactic nuclei. Astrophys. J. 703, L1–L5 (2009).

    Article  ADS  Google Scholar 

  31. Korista, K., Baldwin, J. & Ferland, G. Quasars as cosmological probes: the ionizing continuum, gas metallicity, and the Wλ-L relation. Astrophys. J. 507, 24–30 (1998).

    Article  ADS  Google Scholar 

  32. Kang, W.-Y., Wang, J.-X., Cai, Z.-Y. & Ren, W.-K. More variable quasars have stronger emission lines. Astrophys. J. 911, 148 (2021).

    Article  ADS  Google Scholar 

  33. Kelly, B. C., Bechtold, J. & Siemiginowska, A. Are the variations in quasar optical flux driven by thermal fluctuations? Astrophys. J. 698, 895–910 (2009).

    Article  ADS  Google Scholar 

  34. Dexter, J. & Agol, E. Quasar accretion disks are strongly inhomogeneous. Astrophys. J. 727, L24 (2011).

    Article  ADS  Google Scholar 

  35. Cai, Z.-Y. et al. Simulating the timescale-dependent color variation in quasars with a revised inhomogeneous disk model. Astrophys. J. 826, 7 (2016).

    Article  ADS  Google Scholar 

  36. Cai, Z.-Y. et al. EUCLIA—exploring the UV/Optical continuum lag in active galactic nuclei. I. A model without light echoing. Astrophys. J. 855, 117 (2018).

    Article  ADS  Google Scholar 

  37. Czerny, B. & Hryniewicz, K. The origin of the broad line region in active galactic nuclei. Astron. Astrophys. 525, L8 (2011).

    Article  ADS  Google Scholar 

  38. MacLeod, C. L. et al. Modeling the time variability of SDSS stripe 82 quasars as a damped random walk. Astrophys. J. 721, 1014–1033 (2010).

    Article  ADS  Google Scholar 

  39. Faucher-Giguère, C.-A. A cosmic UV/X-ray background model update. Mon. Not. R. Astron. Soc. 493, 1614–1632 (2020).

    Article  ADS  Google Scholar 

  40. Vanden Berk, D. E. et al. Composite quasar spectra from the Sloan Digital Sky Survey. Astron. J. 122, 549–564 (2001).

    Article  ADS  Google Scholar 

  41. Scott, J. E. et al. A composite extreme-ultraviolet QSO spectrum from FUSE. Astrophys. J. 615, 135–149 (2004).

    Article  ADS  Google Scholar 

  42. Novikov, I. D. & Thorne, K. S. in Black Holes (Les Astres Occlus) 343–450 (Gordon and Breach, 1973).

  43. Laor, A. & Davis, S. W. Line-driven winds and the UV turnover in AGN accretion discs. Mon. Not. R. Astron. Soc. 438, 3024–3038 (2014).

    Article  ADS  Google Scholar 

  44. Murray, N., Chiang, J., Grossman, S. A. & Voit, G. M. Accretion disk winds from active galactic nuclei. Astrophys. J. 451, 498 (1995).

    Article  ADS  Google Scholar 

  45. Proga, D., Stone, J. M. & Kallman, T. R. Dynamics of line-driven disk winds in active galactic nuclei. Astrophys. J. 543, 686–696 (2000).

    Article  ADS  Google Scholar 

  46. King, A. R. Black hole outflows. Mon. Not. R. Astron. Soc. 402, 1516–1522 (2010).

    Article  ADS  Google Scholar 

  47. Slone, O. & Netzer, H. The effects of disc winds on the spectrum and black hole growth rate of active galactic nuclei. Mon. Not. R. Astron. Soc. 426, 656–664 (2012).

    Article  ADS  Google Scholar 

  48. Czerny, B. & Elvis, M. Constraints on quasar accretion disks from the optical/ultraviolet/soft X-ray big bump. Astrophys. J. 321, 305 (1987).

    Article  ADS  Google Scholar 

  49. Jiang, Y.-F. & Blaes, O. Opacity-driven convection and variability in accretion disks around supermassive black holes. Astrophys. J. 900, 25 (2020).

    Article  ADS  Google Scholar 

  50. Gong, Y. et al. Cosmology from the Chinese Space Station Optical Survey (CSS-OS). Astrophys. J. 883, 203 (2019).

    Article  ADS  Google Scholar 

  51. Kulkarni, S. R. et al. Science with the Ultraviolet Explorer (UVEX). Preprint at https://arxiv.org/abs/2111.15608 (2021).

  52. Yuan, F. & Narayan, R. Hot accretion flows around black holes. Annu. Rev. Astron. Astrophys. 52, 529–588 (2014).

    Article  ADS  Google Scholar 

  53. Predehl, P. et al. The eROSITA X-ray telescope on SRG. Astron. Astrophys. 647, A1 (2021).

    Article  Google Scholar 

  54. Morrissey, P. et al. The on-orbit performance of the Galaxy Evolution Explorer. Astrophys. J. 619, L7–L10 (2005).

    Article  ADS  Google Scholar 

  55. Bianchi, L., Shiao, B. & Thilker, D. Revised Catalog of GALEX Ultraviolet Sources. I. The All-Sky Survey: GUVcat_AIS. Astrophys. J. Suppl. Ser. 230, 24 (2017).

    Article  ADS  Google Scholar 

  56. Bianchi, L. The Galaxy Evolution Explorer (GALEX). Its legacy of UV surveys, and science highlights. Astrophys. Space Sci. 354, 103–112 (2014).

    Article  ADS  Google Scholar 

  57. Schlegel, D. J., Finkbeiner, D. P. & Davis, M. Maps of dust infrared emission for use in estimation of reddening and cosmic microwave background radiation foregrounds. Astrophys. J. 500, 525–553 (1998).

    Article  ADS  Google Scholar 

  58. Schlafly, E. F. & Finkbeiner, D. P. Measuring reddening with Sloan Digital Sky Survey stellar spectra and recalibrating SFD. Astrophys. J. 737, 103 (2011).

    Article  ADS  Google Scholar 

  59. Fitzpatrick, E. L. Correcting for the effects of interstellar extinction. Publ. Astron. Soc. Pac. 111, 63–75 (1999).

    Article  ADS  Google Scholar 

  60. Yuan, H. B., Liu, X. W. & Xiang, M. S. Empirical extinction coefficients for the GALEX, SDSS, 2MASS and WISE passbands. Mon. Not. R. Astron. Soc. 430, 2188–2199 (2013).

    Article  ADS  Google Scholar 

  61. Stoughton, C. et al. Sloan Digital Sky Survey: early data release. Astron. J. 123, 485–548 (2002).

    Article  ADS  Google Scholar 

  62. Feigelson, E. D. & Nelson, P. I. Statistical methods for astronomical data with upper limits. I. Univariate distributions. Astrophys. J. 293, 192–206 (1985).

    Article  ADS  MathSciNet  Google Scholar 

  63. Haardt, F. & Madau, P. Radiative transfer in a clumpy Universe. IV. New synthesis models of the cosmic UV/X-ray background. Astrophys. J. 746, 125 (2012).

    Article  ADS  Google Scholar 

  64. Paresce, F., McKee, C. F. & Bowyer, S. Galactic and extragalactic contributions to the far-ultraviolet background. Astrophys. J. 240, 387–400 (1980).

    Article  ADS  Google Scholar 

  65. Møller, P. & Jakobsen, P. The Lyman continuum opacity at high redshifts: through the Lyman forest and beyond the Lyman valley. Astron. Astrophys. 228, 299–309 (1990).

    ADS  Google Scholar 

  66. Meiksin, A. & Madau, P. On the photoionization of the intergalactic medium by quasars at high redshift. Astrophys. J. 412, 34 (1993).

    Article  ADS  Google Scholar 

  67. Madau, P. & Haardt, F. He II absorption and the sawtooth spectrum of the cosmic far-UV background. Astrophys. J. 693, L100–L103 (2009).

    Article  ADS  Google Scholar 

  68. Draine, B. T. Physics of the Interstellar and Intergalactic Medium (Princeton Univ. Press, 2011).

  69. Wiese, W. L. & Fuhr, J. R. Accurate atomic transition probabilities for hydrogen, helium, and lithium. J. Phys. Chem. Ref. Data 38, 565–720 (2009).

    Article  ADS  Google Scholar 

  70. Puchwein, E., Haardt, F., Haehnelt, M. G. & Madau, P. Consistent modelling of the meta-galactic UV background and the thermal/ionization history of the intergalactic medium. Mon. Not. R. Astron. Soc. 485, 47–68 (2019).

    Article  ADS  Google Scholar 

Download references

Acknowledgements

We thank W. Zheng for pointing out the important effect of Lyman limit systems on the broadband GALEX photometry, and T.-G. Wang, G. De Zotti and L. Danese for valuable comments. Z-Y.C. thanks F.-F. Zhu for his help with using the GALEX database and J.-L. Kang for help with the survival analysis. This work is supported by National Key R&D Program of China No. 2022YFF0503402 and the National Science Foundation of China (grant nos. 12033006, 11890693, 12192221 and 12373016). Z.-Y.C. acknowledges support from the USTC Research Funds of the Double First-Class Initiative under grant no. YD2030002009, science research grants from the China Manned Space Project under grant no. CMS-CSST-2021-A06 and the Cyrus Chun Ying Tang Foundations.

Author information

Authors and Affiliations

  1. CAS Key Laboratory for Research in Galaxies and Cosmology, Department of Astronomy, University of Science and Technology of China, Hefei, China

    Zhen-Yi Cai & Jun-Xian Wang

  2. School of Astronomy and Space Science, University of Science and Technology of China, Hefei, China

    Zhen-Yi Cai & Jun-Xian Wang

Authors
  1. Zhen-Yi Cai
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Jun-Xian Wang
    View author publications

    You can also search for this author in PubMed Google Scholar

Contributions

After the idea of sample incompleteness was pointed out by J.-X.W. at the beginning of 2019, Z.-Y.C. gradually analysed the public data, performed Monte Carlo simulations and prepared the paper. Both authors discussed and revised the paper.

Corresponding authors

Correspondence to Zhen-Yi Cai or Jun-Xian Wang.

Ethics declarations

Competing interests

The authors declare no competing interests.

Peer review

Peer review information

Nature Astronomy thanks the anonymous reviewers for their contribution to the peer review of this work.

Additional information

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Supplementary information

Supplementary Information

Supplementary Discussions and Figs. 1–10.

Rights and permissions

Springer Nature or its licensor (e.g. a society or other partner) holds exclusive rights to this article under a publishing agreement with the author(s) or other rightsholder(s); author self-archiving of the accepted manuscript version of this article is solely governed by the terms of such publishing agreement and applicable law.

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Cai, ZY., Wang, JX. A universal average spectral energy distribution for quasars from the optical to the extreme ultraviolet. Nat Astron 7, 1506–1516 (2023). https://doi.org/10.1038/s41550-023-02088-5

Download citation

  • Received:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1038/s41550-023-02088-5

Search

Advanced search

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