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Quasar radiation transforms the gas in a merging companion galaxy

Nature volume 641, pages 1137–1141 (2025)Cite this article

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Abstract

Quasars, powered by gas accretion onto supermassive black holes1,2, rank among the most energetic objects in the Universe3,4. Although they are thought to be ignited by galaxy mergers5,6,7,8,9,10,11 and affect the surrounding gas12,13,14,15, observational constraints on both processes remain scarce16,17,18. Here we describe a major merging system at redshift z ≈ 2.7 and demonstrate that radiation from the quasar in one galaxy directly alters the gas properties in the other galaxy. Our findings reveal that the galaxies, with centroids separated by only a few kiloparsecs and approaching each other at a speed of approximately 550 km s−1, are massive, are forming stars and contain a substantial molecular mass. Yet, dusty molecular gas seen in absorption against the quasar nucleus is highly excited and confined within cloudlets with densities of approximately 105 to 106 cm−3 and sizes of less than 0.02 pc, several orders of magnitude more compact than those observed in intervening (non-quasar) environments. This is also approximately 105 times smaller than currently resolvable through molecular-line emission at high redshifts. We infer that, wherever it is exposed to the quasar radiation, the molecular gas is disrupted, leaving behind surviving dense clouds too small to give birth to new stars. Our results not only underscore the role of major galaxy mergers in triggering quasar activity but also reveal localized negative feedback as a profound alteration of the internal gas structure, which probably hampers star formation.

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Fig. 1: Optical and millimetre observations of the quasar-host and companion-galaxy system.
Fig. 2: Absorption lines in the spectrum of the quasar J012555.11−012925.00.
Fig. 3: Spectral energy distribution of the companion galaxy and quasar host galaxy.
Fig. 4: H2 excitation diagram and physical conditions in H2-bearing medium.

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Data availability

The Subaru imaging data are available from the HSC Legacy Archive (https://hscla.mtk.nao.ac.jp/). VLT spectroscopic data were collected at the European Southern Observatory under ESO programme 105.203L (principal investigator P.N.) and are publicly available through the ESO science archive at https://archive.eso.org/cms.html. ALMA data collected under programme ID 2022.1.01792.S (principal investigator S.L.) are publicly available from the ALMA science archive (https://almascience.eso.org/aq/).

Code availability

The following software was used: Astropy86, Bagpipes48, Galfit43, Matplotlib87, NumPy88, Photutils89, SciPy90, SOFIA46,47, Emcee45, spectro (https://github.com/balashev/spectro) and Meudon PDR (https://pdr.obspm.fr/). Requests for materials should be addressed to S.B.

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Acknowledgements

S.B. is supported by RSF grant no. 23-12-00166 and thanks IAP for hospitality, as this was where part of this work was done. The research leading to these results received support from the French Agence Nationale de la Recherche (ANR grant no. 17-CE31-0011-01/project HIH2, principal investigator P.N.). N.G. acknowledges the NRAO for generous financial support for the sabbatical visit to Socorro during which a part of this work was done.

Author information

Author notes

  1. These authors contributed equally: Sergei Balashev, Pasquier Noterdaeme

Authors and Affiliations

  1. Department of Theoretical Astrophysics, Ioffe Institute, Saint Petersburg, Russia

    Sergei Balashev

  2. Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095, Paris, France

    Pasquier Noterdaeme, Patrick Petitjean & Alain Omont

  3. French-Chilean Laboratory for Astronomy, IRL 3386, CNRS and Universidad de Chile, Santiago, Chile

    Pasquier Noterdaeme & Jens-Kristian Krogager

  4. Inter-University Centre for Astronomy and Astrophysics, Pune, India

    Neeraj Gupta & Raghunathan Srianand

  5. Centre de Recherche Astrophysique de Lyon, UMR 5574, Université Lyon I, ENS de Lyon, CNRS, Lyon, France

    Jens-Kristian Krogager

  6. Collège de France, PSL University, Sorbonne University, CNRS, LERMA, Observatoire de Paris, Paris, France

    Françoise Combes

  7. Departamento de Astronomía, Universidad de Chile, Santiago, Chile

    Sebastián López & Rodrigo Cuellar

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Contributions

S.B. and P.N. led the analysis and wrote the manuscript. S.B. modelled the physical conditions. P.N. designed the optical observations. N.G. performed the analysis of the ALMA data. J.-K.K. reduced the X-shooter data. The post-processing was done by R.C. F.C., S.L. and A.O. contributed to the ALMA observing proposal. P.P. and R.S. contributed to the VLT proposal. All authors contributed to the text and aided in interpreting the results.

Corresponding authors

Correspondence to Sergei Balashev or Pasquier Noterdaeme.

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The authors declare no competing interests.

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Nature thanks Max Gronke, Tanya Urrutia and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Hyper Suprime-Cam deep images of the field around J 012555.11−012925.00.

a, e, the original image. b, f, quasar’s point spread function (PSF) subtracted image. c, g, model of additional sources. d, h, residuals. The top and bottom panels correspond to g and r filters, respectively. The white cross and circle indicate the position of the quasar and the FWHM of the point spread function, respectively.

Extended Data Fig. 2 Spatial profile of the 2D X-shooter spectrum of J 012555.11−012925.00.

The data (black lines with error bars) is shown here collapsed into three a, b, c, wavelength regions (indicated in the top right corner). The spatial profile is modeled by the sum (green) of two Moffat profiles, corresponding to the quasar (blue) and the companion galaxy (red). The error bars indicate the standard deviation of the collapsed flux, while the colored stripes represent the 0.683 credible interval of the models.

Extended Data Fig. 3 Spectral regions around C II 1,334 Å and Lyα lines at z = 2.622.

a–c, C II 1,334 Å, line. d– f, Lyα line. The strong C II and Lyα absorption suppress light from the quasar’s central engine, acting like a coronagraph which reveals the ~ 10 times fainter emission from the host galaxy as residual emission in the absorption trough. a, d, the portions of the 1D X-shooter spectrum extracted along the quasar trace, with cyan and blue dashed lines indicating the zero flux level and the average line flux residual, respectively. The error bars represent the standard deviation of the measured flux. b, e, the 2D X-shooter spectrum (smoothed for illustration purposes) of the same spectral regions. c, f, the spatial profiles integrated along the dispersion axis, with the blue histogram corresponding to regions within the saturated lines (black dashed boxes in b, e), while the red filled histogram correspond to the continuum emission. This faint emission clearly extends beyond the point spread function, in agreement with an origin in an extended object. In e, the blob at λ ~ 4,430 − 4,465 Å corresponds to the extended Lyα emission seen in the bottom of the extremely strong Lyα absorption line. The red dotted and blue dashed lines denote the centroids of the ALMA CO emission of the companion and quasar host galaxies, respectively in both spatial and velocity space, with the stripe indicating their velocity spread.

Extended Data Fig. 4 Line and continuum ALMA images of the field around J 012555.11−012925.00.

a, b, c, The integrated emission of CO(3-2) and C I(2-1) lines, together with CO(7-6)/(3-2) line ratios, respectively, centered on the companion galaxy. d, e, ALMA band 6 and 3 wideband continuum images, respectively. The gray contours indicate the CO(7-6) emission shown in Fig. 1. The green hatch ellipses in each panel represent the synthesized beam.

Extended Data Fig. 5 A full view of the X-shooter spectrum of J 012555.11−012925.00.

The black, purple, green lines show respectively the observed spectrum, the quasar composite spectrum from62 and the same composite reddened by an SMC-like extinction law with AV = 0.2 at z = 2.66. The blue solid line depicts the continuum component of the quasar spectrum (i.e. removing emission lines), while the blue dashed line corresponds to this continuum component reddened by the same extinction. The red spectrum is the sum of the reddened continuum and the unreddened emission lines and clearly reproduces much better the emission lines redwards of Ly-α. This indicates that the continuum light from the quasar accretion disc passes through dusty gas, while light from the emission line regions is almost unaffected, implying that dust is confined in much smaller regions. b, c indicate zoom-in on the spectral region around C IV and C III] emission lines. The position of metal absorption lines associated with the proximate DLA are marked by grey ticks and labels.

Extended Data Fig. 6 Fit to the line profiles of H2 lines associated with the proximate absorber at z = 2.662 towards J 012555.11−012925.00.

The black lines represent the observed normalized spectrum, with the error bars representing the standard deviation. The green, violet, and red stripes indicate the profiles of H2 components A, B, and the total profile, respectively. Black and green dashed horizontal lines at the bottom of each panel show the zero flux level and partial coverage, respectively. Blue step-like lines at the top of each panel display the residuals, with horizontal lines representing − 2σ, 0, and 2σ levels. Red horizontal segments identify the band (Lyman/Werner, vibrational level of upper and lower state) as well as main rotational levels (numbers above ticks, only shown for component A for clarity). Note that lines from low rotational levels in the L2-0 band (around 3950 Å) are blended with the strong Lyα line from intervening H I at z = 2.25, identified through its associated metal transitions.

Extended Data Fig. 7 Fit to the C I absorption line profiles associated with the proximate absorber at z ≈ 2.662 towards J 012555.11−012925.00.

The graphical elements are the same as in Extended Data Fig. 6 with profiles of additional components shown by gray lines. Each profile contains absorption from three fine-structure levels of the ground state.

Extended Data Fig. 8 Possible detection of CO absorption lines at z = 2.662 in the proximate absorber towards J 012555.11−012925.00.

a-f, The fit to individual CO bands. The black line show the observed spectrum, while the green, blue and red lines represent the two individual components and total line profiles. g, The stack of the observed spectrum (black) and total profile model (red).

Extended Data Table 1 Details of ALMA spectral line cubes and detected line emission
Full size table
Extended Data Table 2 Spectral energy distribution fitting of the companion galaxy and the quasar host galaxy using bagpipes
Full size table

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Balashev, S., Noterdaeme, P., Gupta, N. et al. Quasar radiation transforms the gas in a merging companion galaxy. Nature 641, 1137–1141 (2025). https://doi.org/10.1038/s41586-025-08966-4

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