Intensity mapping of the atomic carbon CII line: the promise of a new observational probe of dusty star-formation in post-reionization and reionization epoch

Project Information

CONCERTO

Grant agreement ID: 788212

DOI

10.3030/788212

Project closed

EC signature date 28 June 2018

Start date 1 January 2019

End date 30 June 2025

Funded under

EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 3 499 942,24

EU contribution

€ 3 499 942,24

3 499 942,24

Coordinated by

UNIVERSITE D'AIX MARSEILLE
France

Periodic Reporting for period 4 - CONCERTO (Intensity mapping of the atomic carbon CII line: the promise of a new observational probe of dusty star-formation in post-reionization and reionization epoch)

Reporting period: 2023-07-01 to 2025-06-30

CONCERTO aims to answer the questions of whether dusty star-formation contributes to early galaxy evolution, and whether dusty galaxies play an important role in shaping the end of the cosmic reionization period, which is a critical transition in the life of the Universe. To reach that goal, we have constructed a spectrometer to map in 3D the intensity due to line emission, a technique known as Line Intensity Mapping (LIM). Instead of detecting individual galaxies, this technique measures signal fluctuations produced by the combined emission of the galaxy population on large regions of the sky in a wide redshift band, and thus increases sensitivity to faint sources.
Capitalizing on a recent technology breakthrough, our intensity mapping experiment is measuring the 3D fluctuations of the [CII] line in the first Gyrs of the Universe. [CII] is one of the most valuable tracers of star formation. Our experiment is also observing the CO intensity fluctuations arising from galaxies in the older Universe (with age >3 Gyr), possibly giving the spatial distribution and abundance of molecular gas over a broad range of cosmic time.

We successfully designed, built, tested, and installed the CONCERTO instrument on the APEX antenna in 2021, following an agreement with the APEX partners. The spectrometer is based on state-of-the-art millimeter-wave detector arrays using kinetic inductance detectors, with spectra obtained through a fast Martin–Puplett interferometer. After a commissioning period, we observed 1.4 square degree in the COSMOS field to provide a new avenue for probing star formation and dust build-up in the early Universe through [CII] LIM. In addition, the CONCERTO instrument was opened to external proposals from the community. The last science observing run ended in December 2022, and the instrument was dismounted from the telescope in May 2023. Following this intense period, the team shifted its efforts to data reduction and analysis, tackling the formidable challenge of processing an unprecedented dataset of 870 terabytes.

Our novel technology targeted an unexplored observable touching on some fundamental processes building the early Universe. As the only experiment to date to perform a [C II] LIM survey of this kind, our pioneering results are establishing a foundation for forthcoming studies.

Finally, CONCERTO addressed one of humanity’s oldest questions: how galaxies and stars formed in the first billion years of the Universe. By probing dusty star formation, it shed light on the origins of the structures that led to planets. The project also drove technological innovation through advanced detector arrays, fast interferometric spectrometers, and methods to process unprecedented volumes of data. These breakthroughs had potential applications beyond astronomy, including in Earth observation and big-data analytics. CONCERTO also trained and inspired a new generation of scientists and engineers, and its achievements will continue to benefit science and society well beyond the project’s lifetime.

In the first period (January 2019 – May 2021), we focused on designing, building, testing, shipping, and installing the CONCERTO instrument at APEX, followed by successful commissioning. Achieving this under COVID-19 restrictions was a major challenge. Key milestones:
- Instrument design and construction
- Passing APEX and ESO reviews and mounting telescope cabin pipes in February 2020.
- Laboratory characterization campaigns from November 2020 to February 2021.
- Development of real-time analysis software and initial data reduction pipeline.
- Shipping to APEX in March 2021 and on-site installation in April 2021.
- Securing 800 hours for the [C II] survey.
- First light in April 2021 and first remote observations in May 2021.
- Initial on-sky tests confirmed expected camera sensitivity.

Scientific observations began with the first [C II] survey scan on July 14, 2021, and included other targets such as galaxy clusters, Galactic star-forming regions, old stars, and the Small Magellanic Cloud. Photometric mode data was well understood from the start; spectroscopy required months to diagnose a systematic caused by acoustic and mirror-induced vibrations on the polariser. A real-time active correction using a propagating sound wave, installed in February 2022, proved highly effective.

ESO and OSO’s withdrawal from APEX in December 2022 increased team workload and caused some observations to be made with degraded instrument performance. The last run was in December 2022, and the instrument was removed in May 2023 after a final upgrade that proved unsuccessful. The team has since focused on ongoing data reduction and analysis. Over 19 scientific runs, 870 terabytes of data were acquired, and significant progress has been made in processing this unprecedented dataset.

In parallel, we developed the SIDES extragalactic sky simulations, including dust continuum, [C II], CI, and CO spectral features. SIDES supports (sub-)millimeter intensity mapping experiments, power spectrum analysis, component separation, and modeling. Using SIDES, we retrieved the CO spectral line energy distribution via cross-correlations and discovered a new foreground affecting kinetic Sunyaev–Zeldovich effect measurements. Both SIDES products and code have been publicly released.

We also developed a detailed instrument and observation model that accurately reproduces real data, along with a forward model of interferogram timelines including atmospheric noise, reference sources, and straylight. This supports precise parameter determination and continuous pipeline improvement.

All results are published in peer-reviewed journals (full list: https://mission.lam.fr/concerto/pages/publications.html) and regularly presented at international conferences.

CONCERTO is currently the only instrument in the world to have performed intensity mapping of the [CII] line. It represented a unique opportunity before upcoming experiments, such as those planned on the CCAT-p telescope with first light using EoRSpec in 2027. The timeline for the development and installation of the CONCERTO instrument was extremely challenging, but we managed to stay on schedule despite COVID-19 restrictions. We have conducted the first large-scale survey of [CII] line emission at high redshift. Our experiment is expected to provide a unique view of dusty star formation in the early Universe, though it must overcome several observational challenges, particularly those related to atmospheric effects and contamination from extragalactic foregrounds, as the signal is extremely faint. We have carefully prepared to address these challenges. Even though penetrating deeply into the epoch of reionization remains uncertain, mainly due to limited knowledge of the expected signal amplitude, the endeavor is well worth the effort. After years of predictions, the time has come for a first measurement.

The Observing Program Office from ESO stated in 2019 that “CONCERTO will bring new life to APEX, particularly in the ALMA world. As such, this is a very interesting instrument, and its installation should be supported.” ESO (as well as OSO) subsequently decided to retire from APEX by December 2022, which led to the instrument’s earlier-than-planned removal. We extend our immense thanks to the APEX and ESO support teams, whose commitment, skill, and constant assistance made the success of CONCERTO possible.

CONCERTO installed in the C-Cabin of APEX (Credit: Alessandro Monfardini, CONCERTO collaboration)

Data cube Simulation: simulated map at 310 GHz w/ dust continuum,[CII],distant galaxies CO2 emission

The “chassis” of CONCERTO being lifted to the C-cabin of the APEX telescope (Credit: Martino calvo,

Aka the Cat’s Paw Nebula.One of the first-light images taken by the CONCERTO instrument on the APEX

CONCERTO instrument building (06/2020): chassis,cryostat,electronics,foldable mirror,interferometer

APEX staff’s warm farewell to CONCERTO, featured on their website

Periodic Reporting for period 4 - CONCERTO (Intensity mapping of the atomic carbon CII line: the promise of a new observational probe of dusty star-formation in post-reionization and reionization epoch)

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