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

Nature volume 617, pages 261–264 (2023)Cite this article

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Abstract

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 https://almascience.nao.ac.jp/aq/ (project code 2017.1.00621.S), the HST data archive at https://archive.stsci.edu/missions-and-data/hst (proposal IDs 9777, 13303 and 15085) and the European Southern Observatory Science Archive at http://archive.eso.org (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 https://casa.nrao.edu. The interferopy package that includes the findclumps source finding program is available at https://github.com/interferopy/interferopy. The VLT/XShooter spectral data were reduced using the PypeIt package available at https://github.com/pypeit/PypeIt. The Magellan FIRE spectral data were reduced using the FIREHOSE pipeline available at https://wikis.mit.edu/confluence/display/FIRE/FIRE+Data+Reduction and corrected using the xtellcor package available at http://irtfweb.ifa.hawaii.edu/~spex/index.html. The DrizzlePac software used for the HST data reduction is available at https://www.stsci.edu/scientific-community/software/drizzlepac.html. The reduction pipelines of the VLT instruments are available at https://www.eso.org/sci/software/pipelines/.

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Acknowledgements

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.

Author information

Authors and Affiliations

  1. Institute for Advanced Research, Nagoya University, Nagoya, Japan

    Daichi Kashino

  2. Department of Physics, Nagoya University, Nagoya, Japan

    Daichi Kashino

  3. Department of Physics, ETH Zürich, Zürich, Switzerland

    Simon J. Lilly, Ruari Mackenzie & Jorryt Matthee

  4. MIT Kavli Institute for Astrophysics and Space Research, Cambridge, MA, USA

    Robert A. Simcoe & Anna-Christina Eilers

  5. Department of Physics, North Carolina State University, Raleigh, NC, USA

    Rongmon Bordoloi

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Contributions

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.

Corresponding author

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
Full size table
Extended Data Table 2 Absorption-line measurements
Full size table

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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). https://doi.org/10.1038/s41586-023-05901-3

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