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Compact [C ii] emitters around a C iv absorption complex at redshift 5.7


The physical conditions of the circumgalactic medium are investigated by means of intervening absorption-line systems in the spectrum of background quasi-stellar objects (QSOs) out to the epoch of cosmic reionization1,2,3,4. A correlation between the ionization state of the absorbing gas and the nature of the nearby galaxies has been suggested by the sources detected in either Lyα or [C ii] 158 μm near to, respectively, highly ionized and neutral absorbers5,6. This is also probably linked to the global changes in the incidence of absorption systems of different types and the process of cosmic reionization7,8,9,10,11,12. Here we report the detection of two [C ii]-emitting galaxies at redshift z ≈ 5.7 that are associated with a complex, high-ionization C iv absorption system. These objects are part of an overdensity of galaxies and have compact sizes (<2.4 kpc) and narrow linewidths (full width at half maximum (FWHM) ≈ 62–64 km s−1). Hydrodynamic simulations predict that similar narrow [C ii] emission may arise from the heating of small (3 kpc) clumps of cold neutral medium or a compact photodissociation region13,14. The lack of counterparts in the rest-frame ultraviolet (UV) indicates severe obscuration of the sources that are exciting the [C ii] emission. These results may suggest a connection between the properties of the [C ii] emission, the rare overdensity of galaxies and the unusual high ionization state of the gas in this region.

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Fig. 1: ALMA observations of the two [C ii] sources.
Fig. 2: Emission and absorption spectra of the [C ii] sources and QSO J1030+0524.

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Data availability

The ALMA, HST and VLT datasets used in this work are publicly available, respectively, through the ALMA data archive at (project code 2017.1.00621.S), the HST data archive at (proposal IDs 9777, 13303 and 15085) and the European Southern Observatory Science Archive at (programme IDs 084.A-0360, 086.A-0162, 086.A-0574, 087.A-0607 and 095.A-0714).

Code availability

The ALMA data were processed using the CASA pipeline version 5.4, which is available at The interferopy package that includes the findclumps source finding program is available at The VLT/XShooter spectral data were reduced using the PypeIt package available at The Magellan FIRE spectral data were reduced using the FIREHOSE pipeline available at and corrected using the xtellcor package available at The DrizzlePac software used for the HST data reduction is available at The reduction pipelines of the VLT instruments are available at


  1. Simcoe, R. A., Sargent, W. L. W., Rauch, M. & Becker, G. Observations of chemically enriched QSO absorbers near z~2.3 galaxies: galaxy formation feedback signatures in the intergalactic medium. Astrophys. J. 637, 648–668 (2006).

    Article  ADS  CAS  Google Scholar 

  2. Prochaska, J. X. et al. The UCSD/Keck damped Lyα abundance database: a decade of high-resolution spectroscopy. Astrophys. J. Suppl. Ser. 171, 29–60 (2007).

    Article  ADS  CAS  Google Scholar 

  3. Fox, A. J., Ledoux, C., Petitjean, P. & Srianand, R. C IV absorption in damped and sub-damped Lyman-α systems. Correlations with metallicity and implications for galactic winds at z ≈ 2–3. Astron. Astrophys. 473, 791–803 (2007).

    Article  ADS  CAS  Google Scholar 

  4. Bordoloi, R. et al. Resolving the H I in damped Lyman α systems that power star formation. Nature 606, 59–63 (2022).

    Article  ADS  CAS  PubMed  Google Scholar 

  5. Díaz, C. G. et al. Faint LAEs near z > 4.7 C IV absorbers revealed by MUSE. Mon. Not. R. Astron. Soc. 502, 2645–2663 (2021).

    Article  ADS  Google Scholar 

  6. Wu, Y. et al. A [C II] 158 μm emitter associated with an O I absorber at the end of the reionization epoch. Nat. Astron. 5, 1110–1117 (2021).

    Article  ADS  Google Scholar 

  7. Becker, G. D., Sargent, W. L. W., Rauch, M. & Simcoe, R. A. Discovery of excess O I absorption toward the z = 6.42 QSO SDSS J1148+5251. Astrophys. J. 640, 69–80 (2006).

    Article  ADS  CAS  Google Scholar 

  8. Simcoe, R. A. et al. Constraints on the universal C IV mass density at z ~ 6 from early infrared spectra obtained with the Magellan FIRE spectrograph. Astrophys. J. 743, 21 (2011).

    Article  ADS  Google Scholar 

  9. Chen, S.-F. S. et al. Mg II absorption at 2 < z < 7 with Magellan/Fire. III. Full statistics of absorption toward 100 high-redshift QSOs. Astrophys. J. 850, 188 (2017).

    Article  ADS  Google Scholar 

  10. Codoreanu, A. et al. The CGM and IGM at z ~ 5: metal budget and physical connection. Mon. Not. R. Astron. Soc. 481, 4940–4959 (2018).

    Article  ADS  CAS  Google Scholar 

  11. Cooper, T. J. et al. Heavy element absorption systems at 5.0 < z < 6.8: metal-poor neutral gas and a diminishing signature of highly ionized circumgalactic matter. Astrophys. J. 882, 77 (2019).

    Article  ADS  CAS  Google Scholar 

  12. Becker, G. D. et al. The evolution of O I over 3.2 < z < 6.5: reionization of the circumgalactic medium. Astrophys. J. 883, 163 (2019).

    Article  ADS  CAS  Google Scholar 

  13. Vallini, L., Gallerani, S., Ferrara, A. & Baek, S. Far-infrared line emission from high-redshift galaxies. Mon. Not. R. Astron. Soc. 433, 1567–1572 (2013).

    Article  ADS  CAS  Google Scholar 

  14. Vallini, L., Gallerani, S., Ferrara, A., Pallottini, A. & Yue, B. On the [CII]-SFR relation in high redshift galaxies. Astrophys. J. 813, 36 (2015).

    Article  ADS  Google Scholar 

  15. Dutta, R. et al. Metal-enriched halo gas across galaxy overdensities over the last 10 billion years. Mon. Not. R. Astron. Soc. 508, 4573–4599 (2021).

    Article  ADS  CAS  Google Scholar 

  16. Toshikawa, J. et al. Discovery of protoclusters at z ~ 3.7 and 4.9: embedded in primordial superclusters. Astrophys. J. 888, 89 (2020).

    Article  ADS  CAS  Google Scholar 

  17. Venemans, B. P. et al. Kiloparsec-scale ALMA imaging of [C II] and dust continuum emission of 27 quasar host galaxies at z ~ 6. Astrophys. J. 904, 130 (2020).

    Article  ADS  CAS  Google Scholar 

  18. Zana, T. et al. Enhanced star formation in z ~ 6 quasar companions. Mon. Not. R. Astron. Soc. 513, 2118–2135 (2022).

    Article  ADS  CAS  Google Scholar 

  19. Aravena, M. et al. The ALMA Spectroscopic Survey in the Hubble Ultra Deep Field: search for [C II] line and dust emission in 6 < z < 8 galaxies. Astrophys. J. 833, 71 (2016).

    Article  ADS  Google Scholar 

  20. Neeleman, M. et al. [C II] 158-μm emission from the host galaxies of damped Lyman-alpha systems. Science 355, 1285–1288 (2017).

    Article  ADS  CAS  PubMed  Google Scholar 

  21. Fujimoto, S. et al. The ALPINE–ALMA [C II] survey: size of individual star-forming galaxies at z = 4–6 and their extended halo structure. Astrophys. J. 900, 1 (2020).

    Article  ADS  CAS  Google Scholar 

  22. Fudamoto, Y. et al. The ALMA REBELS survey: average [C II] 158 μm sizes of star-forming galaxies from z ~ 7 to z ~ 4. Astrophys. J. 934, 144 (2022).

    Article  ADS  Google Scholar 

  23. Olsen, K. P. et al. Simulator of Galaxy Millimeter/Submillimeter Emission (SíGAME): The [C II]-SFR relationship of massive z = 2 main sequence galaxies. Astrophys. J. 814, 76 (2015).

    Article  ADS  Google Scholar 

  24. De Looze, I. et al. The applicability of far-infrared fine-structure lines as star formation rate tracers over wide ranges of metallicities and galaxy types. Astron. Astrophys. 568, A62 (2014).

    Article  Google Scholar 

  25. Schaerer, D. et al. The ALPINE-ALMA [C II] survey. Little to no evolution in the [C II]-SFR relation over the last 13 Gyr. Astron. Astrophys. 643, A3 (2020).

    Article  CAS  Google Scholar 

  26. Carniani, S. et al. Kiloparsec-scale gaseous clumps and star formation at z = 5–7. Mon. Not. R. Astron. Soc. 478, 1170–1184 (2018).

    Article  ADS  CAS  Google Scholar 

  27. Le Fèvre, O. et al. The ALPINE-ALMA [CII] survey. Survey strategy, observations, and sample properties of 118 star-forming galaxies at 4 < z < 6. Astron. Astrophys. 643, A1 (2020).

    Article  Google Scholar 

  28. Fudamoto, Y. et al. The ALPINE-ALMA [CII] survey. Dust attenuation properties and obscured star formation at z ~ 4.4–5.8. Astron. Astrophys. 643, A4 (2020).

    Article  CAS  Google Scholar 

  29. Pentericci, L. et al. Tracing the reionization epoch with ALMA: [C II] emission in z ~ 7 galaxies. Astrophys. J. Lett. 829, L11 (2016).

    Article  ADS  Google Scholar 

  30. Bradač, M. et al. ALMA [C II] 158 μm detection of a redshift 7 lensed galaxy behind RX J1347.1–1145. Astrophys. J. Lett. 836, L2 (2017).

    Article  ADS  Google Scholar 

  31. Behrens, C., Pallottini, A., Ferrara, A., Gallerani, S. & Vallini, L. Dusty galaxies in the Epoch of Reionization: simulations. Mon. Not. R. Astron. Soc. 477, 552–565 (2018).

    Article  ADS  CAS  Google Scholar 

  32. Planck Collaboration et al. Planck 2018 results. VI. Cosmological parameters. Astron. Astrophys. 641, A6 (2020).

    Article  Google Scholar 

  33. Alam, S. et al. The eleventh and twelfth data releases of the Sloan Digital Sky Survey: final data from SDSS-III. Astrophys. J. Supp. 219, 12 (2015).

    Article  ADS  Google Scholar 

  34. McMullin, J. P., Waters, B., Schiebel, D., Young, W. & Golap, K. in Astronomical Data Analysis Software and Systems XVI ASP Conference Series Vol. 376 (eds Shaw, R. A., Hill, F. & Bell, D. J.) 127 (Astronomical Society of the Pacific, 2007).

  35. Walter, F. et al. ALMA spectroscopic survey in the Hubble Ultra Deep Field: survey description. Astrophys. J. 833, 67 (2016).

    Article  ADS  Google Scholar 

  36. Boogaard, L., Meyer, R. A. & Novak, M. Interferopy: analysing datacubes from radio-to-submm observations. Zenodo (2021).

  37. Popping, G. et al. Sub-mm emission line deep fields: CO and [C II] luminosity functions out to z = 6. Mon. Not. R. Astron. Soc. 461, 93–110 (2016).

    Article  ADS  CAS  Google Scholar 

  38. Ferrara, A. et al. The ALMA REBELS Survey. Epoch of Reionization giants: properties of dusty galaxies at z ≈ 7. Mon. Not. R. Astron. Soc. 512, 58–72 (2022).

    Article  ADS  CAS  Google Scholar 

  39. Béthermin, M. et al. The ALPINE-ALMA [CII] survey: data processing, catalogs, and statistical source properties. Astron. Astrophys. 643, A2 (2020).

    Article  Google Scholar 

  40. Madau, P. & Dickinson, M. Cosmic star-formation history. Annu. Rev. Astron. Astrophys. 52, 415–486 (2014).

    Article  ADS  Google Scholar 

  41. Chabrier, G. Galactic stellar and substellar initial mass function. Publ. Astron. Soc. Pac. 115, 763–795 (2003).

    Article  ADS  Google Scholar 

  42. Vernet, J. et al. X-shooter, the new wide band intermediate resolution spectrograph at the ESO Very Large Telescope. Astron. Astrophys. 536, A105 (2011).

    Article  Google Scholar 

  43. Simcoe, R. A. et al. FIRE: a facility class near-infrared echelle spectrometer for the Magellan telescopes. Publ. Astron. Soc. Pac. 125, 270 (2013).

    Article  ADS  Google Scholar 

  44. Prochaska, J. et al. PypeIt: the Python spectroscopic data reduction pipeline. J. Open Source Softw. 5, 2308 (2020).

    Article  ADS  Google Scholar 

  45. Gagné, J., Lambrides, E., Faherty, J. K. & Simcoe, R. FireHose_v2: Firehose v2.0. Zenodo (2015).

  46. Vacca, W. D., Cushing, M. C. & Rayner, J. T. A method of correcting near-infrared spectra for telluric absorption. Publ. Astron. Soc. Pac. 115, 389–409 (2003).

    Article  ADS  Google Scholar 

  47. Cushing, M. C., Vacca, W. D. & Rayner, J. T. Spextool: a spectral extraction package for SpeX, a 0.8–5.5 micron cross-dispersed spectrograph. Publ. Astron. Soc. Pac. 116, 362–376 (2004).

    Article  ADS  Google Scholar 

  48. Foreman-Mackey, D., Hogg, D. W., Lang, D. & Goodman, J. emcee: the MCMC hammer. Publ. Astron. Soc. Pac. 125, 306–312 (2013).

    Article  ADS  Google Scholar 

  49. Hoffmann, S. L. et al. The DrizzlePac Handbook, Version 2.0. Space Telescope Science Institute (2021).

  50. Weilbacher, P. M. et al. The data processing pipeline for the MUSE instrument. Astron. Astrophys. 641, A28 (2020).

    Article  Google Scholar 

  51. Borisova, E. et al. Ubiquitous giant Lyα nebulae around the brightest quasars at z ~ 3.5 revealed with MUSE. Astrophys. J. 831, 39 (2016).

    Article  ADS  Google Scholar 

  52. Cantalupo, S. et al. The large- and small-scale properties of the intergalactic gas in the Slug Ly α nebula revealed by MUSE He II emission observations. Mon. Not. R. Astron. Soc. 483, 5188–5204 (2019).

    Article  ADS  CAS  Google Scholar 

  53. Bacon, R. et al. The MUSE 3D view of the Hubble Deep Field South. Astron. Astrophys. 575, A75 (2015).

    Article  Google Scholar 

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This work is based on the ALMA observation under ADS/JAO.ALMA#2011.0.01234.S. ALMA is a partnership of the ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by the ESO, AUI/NRAO and NAOJ. This research makes use of observations made with the NASA/ESA HST obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. This paper includes data gathered with the 6.5-m Magellan telescopes located at Las Campanas Observatory, Chile. Data analysis was in part carried out on the Multi-wavelength Data Analysis System operated by the Astronomy Data Center, NAOJ. We acknowledge support from JSPS KAKENHI grant number JP21K13956.

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Authors and Affiliations



D.K., S.J.L., R.B. and R.A.S. discussed and planned the ALMA programme and the HST observations in the F775W band. R.A.S. obtained the HST observation in the F160W band. A.-C.E. and R.A.S. analysed the XShooter and FIRE spectroscopic data of QSO J1030+0524. R.M. reduced and calibrated the MUSE data. S.J.L., J.M., R.A.S., R.M. and A.-C.E. contributed to the discussion of the presented results and the preparation of the manuscript. D.K. led the team, being principal investigator of the ALMA programme, analysed the ALMA and ancillary data, wrote the main text and the Methods section, and produced all figures and the table in the article.

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Correspondence to Daichi Kashino.

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Extended data figures and tables

Extended Data Fig. 1 Antenna response (primary beam) map of the mosaic ALMA observation.

The response is normalized to one at the field centre and the survey field is defined as the area of about 23″ in radius at which the relative response is above 20%. The primary beam size of the seven individual pointings (r = 15″) that compose the mosaic observation is shown by the dotted-line circles with pointing numbers from 1 to 7. The positions of the two [C ii] sources ([C ii]1030A and B) are marked by red stars.

Extended Data Fig. 2 Fidelity in our line search.

Upper panels, number of positive (blue) and negative (that is, noise; red) peaks detected in the four ALMA datacubes (one column for each). The frequency range of each SPW is denoted at the top. Lower panels, the fidelity at given S/N with the model fit as an error function (solid red line).

Extended Data Fig. 3 Reliability check of the [C ii] detection.

For each of [C ii]1030A (left) and B (right), the spectra and the [C ii] moment-0 maps (2″ × 2″) constructed in the reliability tests are compared with those from the full dataset that are shown in the top row. The second and third rows show the results under only XX or YY polarization. The fourth and fifth rows show the results for the first and second half exposure time intervals. The sixth and seventh rows show the results from processing each of the two individual ALMA pointings that cover the source position. In the spectral plots, the collapsed channel window is shown by dotted vertical lines. In the [C ii] intensity maps, the solid (dashed) contours mark positive (negative) steps of 1σ root mean square starting at 2σ (−2σ). All of these products from the partial dataset exhibit substantial signals that coincide both spatially and spectrally to those seen in the full dataset.

Extended Data Fig. 4 Continuum maps.

Each image cuts out the 3″ × 3″ region centred at the position of [C ii]1030A (left panel) and B (right panel). The rest-frame 1,900 GHz continuum intensity is shown by the colour scale. The synthesized continuum beam size is indicated by the white circle in the lower-left corner.

Extended Data Fig. 5 C iv and Si iv absorption systems at zabs ≈ 5.7.

Each panel corresponds to the transition labelled at the upper left. The black line shows the observed spectrum of QSO J1030+0524, whereas the light-brown line denotes the standard deviation of the flux. The blue lines indicate 50 random Voigt profiles sampled from the posterior distribution. The origin of the velocity scale was chosen to correspond to the strongest absorption system at zabs = 5.7246, which is marked along with the two other systems (zabs = 5.7411 and 5.7443) with the dashed blue lines. The vertical red lines indicate the relative velocities of [C ii]1030A and B.

Extended Data Fig. 6 HST broadband images.

Each image cuts out the 3″ × 3″ region centred at the position of [C ii]1030A (upper row) and B (lower row). From left to right, these images are F775W, F850LP and F160W, respectively, which sample the rest-frame 1,150 Å, 1,350 Å and 2,300 Å. No notable rest-frame UV counterpart is detected at the positions of the [C ii] emission.

Extended Data Table 1 Expected number counts of CO sources
Extended Data Table 2 Absorption-line measurements

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Kashino, D., Lilly, S.J., Simcoe, R.A. et al. Compact [C ii] emitters around a C iv absorption complex at redshift 5.7. Nature 617, 261–264 (2023).

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