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Ionized gas extends over 40 kpc in an odd radio circle host galaxy

Nature volume 625, pages 459–462 (2024)Cite this article

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Abstract

A new class of extragalactic astronomical sources discovered in 2021, named odd radio circles (ORCs)1, are large rings of faint, diffuse radio continuum emission spanning approximately 1 arcminute on the sky. Galaxies at the centres of several ORCs have photometric redshifts of z ≃ 0.3–0.6, implying physical scales of several 100 kpc in diameter for the radio emission, the origin of which is unknown. Here we report spectroscopic data on an ORC including strong [O ii] emission tracing ionized gas in the central galaxy of ORC4 at z = 0.4512. The physical extent of the [O ii] emission is approximately 40 kpc in diameter, larger than expected for a typical early-type galaxy2 but an order of magnitude smaller than the large-scale radio continuum emission. We detect an approximately 200 km s−1 velocity gradient across the [O ii] nebula, as well as a high velocity dispersion of approximately 180 km s−1. The [O ii] equivalent width (approximately 50 Å) is extremely high for a quiescent galaxy. The morphology, kinematics and strength of the [O ii] emission are consistent with the infall of shock ionized gas near the galaxy, following a larger, outward-moving shock. Both the extended optical and radio emission, although observed on very different scales, may therefore result from the same dramatic event.

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Fig. 1: Optical stellar continuum and [O ii] emission of ORC4.
Fig. 2: Radial surface brightness and EW profile of the [O ii] emission.
Fig. 3: Kinematics of the [O ii] emission.
Fig. 4: Simulation of starburst-driven wind.

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Data availability

The raw data generated at the Keck Observatory are available from the Keck Observatory Archive (koa.ipac.caltech.edu) following the standard 18-month proprietary period after the date of observation. The reduced KCWI spectral data cube and the results of the [O ii] emission-line fits are available on Zenodo at https://doi.org/10.5281/zenodo.8377942.

Code availability

The code used is available upon request from the first author.

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Acknowledgements

We thank F. Brighenti for useful feedback on an earlier draft of the paper, and we thank R. Norris and H. Intema for sharing the Giant Meterwave Radio Telescope discovery image of ORC4. A.L.C. acknowledges support from the Ingrid and Joseph W. Hibben chair at UC San Diego. C.L. thanks C. Cimino IV for computing resources and assistance. The data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The observatory was made possible by the financial support of the W. M. Keck Foundation. We wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Maunakea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain.

Author information

Authors and Affiliations

  1. Department of Astronomy and Astrophysics, University of California, La Jolla, CA, USA

    Alison L. Coil & Serena Perrotta

  2. Department of Physics, Rhodes College, Memphis, TN, USA

    David S. N. Rupke

  3. Space Telescope Science Institute, Baltimore, MD, USA

    Cassandra Lochhaas

  4. Department of Astronomy, University of Wisconsin-Madison, Madison, WI, USA

    Christy A. Tremonti

  5. Department of Physics and Astronomy, Bates College, Lewison, ME, USA

    Aleks Diamond-Stanic

  6. Center for Computational Astrophysics, Flatiron Institute, New York, NY, USA

    Drummond Fielding

  7. Centre for Astrophysics Research, University of Hertfordshire, Hatfield, UK

    James E. Geach

  8. Department of Physics and Astronomy, Dartmouth College, Hanover, NH, USA

    Ryan C. Hickox & Kelly E. Whalen

  9. Department of Physics and Astronomy, Siena College, Loudonville, NY, USA

    John Moustakas

  10. Department of Physics and Astronomy, University of Kansas, Lawrence, KS, USA

    Gregory H. Rudnick

  11. Department of Astronomy, University of Florida, Gainesville, FL, USA

    Paul Sell

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Contributions

D.S.N.R. and A.L.C. conceived the observations, following C.A.T.’s suggestion that ORCs may resemble the galaxies that we have been studying. A.L.C. obtained the observing time. S.P. performed the KCWI observations and led the data reduction. A.L.C. performed all the data analysis, with input from D.S.N.R. C.L. ran the starburst-wind model simulation and K.W. performed the SED fitting. A.L.C. wrote the manuscript, with input from all coauthors. The figures were created by S.P., A.L.C., K.W., C.L. and R.H.

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Correspondence to Alison L. Coil.

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Extended data figures and tables

Extended Data Fig. 1 Spatially integrated spectrum of the gas nebula and host galaxy stellar continuum emission in ORC4.

a, Strong [O II] 3726, 3729 Å and weak Mg II 2796, 2803 Å and [Ne III] 3869 Å emission are observed in a spatially integrated spectrum of ORC4 (spanning the inner 26x26 kpc), at wavelengths that correspond to z = 0.4512; weak stellar continuum is also detected. The solid black line shows the observed spectrum, the pink dotted line is the 1σ error spectrum, and the orange dashed vertical lines indicate the observed wavelengths of Mg II, [O II], and [Ne III] at the spectroscopic redshift of the source. b, Gaussian fits to the Mg II and [O II] emission doublets and the [Ne III] singlet in the spatially-integrated spectrum. Blue and cyan lines show fits to the individual emission lines in each doublet.

Extended Data Fig. 2 Spectral energy distribution (SED) fit to optical SDSS and near infrared WISE photometry of ORC4.

a, The observed photometry of ORC4 is shown as green circles with 1σ error bars, while orange circles show the photometry implied from the best fit stellar population model, including an AGN contribution, which is shown in blue. Flux values are given in mJy and observed frame wavelengths in μm. b, The derived distributions from the SED fit for the stellar mass, stellar age, star formation history parameters, and AGN contribution for ORC4, as well as the covariance between the parameters.

Extended Data Fig. 3 Comparison of ORC4 radio continuum and [O II] line luminosity to radio AGN.

The 1.4 GHz radio continuum luminosity and [O II] emission line luminosity of ORC4, shown with a red circle, compared to the radio-loud AGN sample of ref. 49, shown with grey points. The [O II] luminosity of ORC4 is two orders of magnitude higher than the median value for an AGN with the same 1.4 GHz radio continuum luminosity.

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Coil, A.L., Perrotta, S., Rupke, D.S.N. et al. Ionized gas extends over 40 kpc in an odd radio circle host galaxy. Nature 625, 459–462 (2024). https://doi.org/10.1038/s41586-023-06752-8

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