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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

Periodic Reporting for period 2 - 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: 2020-07-01 to 2021-12-31

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 cosmic reionization, 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 Intensity Mapping. Instead of detecting individual galaxies, this emerging 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 Gyr of the Universe. [CII] is one of the most valuable tracers of star formation. We will use the [CII] line emission as a tracer of cosmic density structure and give the first constraints on the power spectrum of dusty star-forming matter. Our experiment is also observing the CO intensity fluctuations arising from galaxies in the older Universe (with age >3 Gyr), giving the spatial distribution and abundance of molecular gas over a broad range of cosmic time.
We have successfully designed, built, tested, and finally installed the CONCERTO instrument on the APEX antenna in 2021, following an agreement with APEX partners. The spectrometer is based on state-of-the-art development of new arrays in the millimeter using kinetic inductance detectors. Spectra are obtained using a fast Martin-Puplett interferometer. We are now observing a few square degrees to offer a straightforward alternative for probing star formation and dust build-up in the early Universe. Finally, CONCERTO will set to music the various cosmic evolution probes: cross-correlation of the signals will be used to capture the topology of the end of reionization era.
Our novel technology targets an unexplored observable touching on some fundamental processes building the early Universe. In the flourishing of new ideas in the intensity mapping field, CONCERTO lies at the forefront.
Most of the activities has been dedicated to the design, fabrication, intensive testing, shipment and finally installation of the instrument at the APEX telescope. Installation was immediately followed by successful technical and scientific commissioning campaigns. Building, installing and operating such a complex instrument in a very short time was a great challenge.
The main achievements are:
1. We have designed and built the CONCERTO instrument in 2019-2020, according to our initial plans. We presented our first laboratory results on the instrument, together with scientific forecast on the [CII] survey, in “A wide field-of-view low-resolution spectrometer at APEX: Instrument design and scientific forecast”, published in the Astronomy & Astrophysics journal.
2. We successfully passed the APEX and ESO reviews (from the preliminary design up to the final acceptance) and started the installation at APEX by mounting all the pipes in the cabin of the telescope in February 2020.
3. We conducted intensive Laboratory characterisation campaigns of the complete instrument from November 2020 to February 2021.
4. In March 2021, we dismounted and shipped the experiment from the lab to the APEX telescope in Chile. The delivery, despite COVID restrictions, was successfully achieved.
5. We participated to the ESO, OSO and Chilean calls for proposal and were successfully granted 800 hours of observing time on APEX to conduct our [CII] survey (at the time of writing the results from the Chilean call are not yet known).
6. We have installed CONCERTO on site in April 2021, despite travel restrictions. Our mission was specially approved by CNRS, AMU and ESO.
7. CONCERTO was successfully interfaced with the APEX telescope and we achieved first light in April 2021.
8. We have performed our first remote observations (from France) in May 2021.
9. First tests on the camera on sky give sensitivity in line with expectation.

In parallel, we have developed the real time analysis software and we are developing the pipeline for data reduction. We have built new realistic simulations of the extragalactic sky that include both dust continuum and [CII] and CO spectral features (see the figure of the previous report). These simulations are unique and will serve as a reference for (sub)-millimeter intensity mapping experiments.
All this is in line with the proposal.

The first very difficult step of our project (design, construction, testing, installation, and commissioning of the instrument in a very short time) has been incredibly successful. As ESO's Director of Operations, A. Kaufer, wrote, “Let me take the opportunity to congratulate you and the team for the extraordinary achievement in installing and commissioning CONCERTO under the current circumstances. Even under normal circumstances I have not seen any instrument at APEX going on sky so successfully in such a short time!”.
We are now entering the data analysis phase. To date, we have 35.6 Tb of data, and we will continue to observe with CONCERTO from April to December 2022. The photometric mode data are well understood. For the spectroscopy, we have a systematic effect of which we have just found the origin. It is related to acoustic noise on the membrane of one of the two polarizers (noise induced by the air conditioning system of the cabin) and to the vibration of the membrane induced by the wind created by the movement of the moving mirror of the FTS. We have developed in laboratory in autumn 2021 an active and real time correction of these vibrations by propagating a correcting sound wave. This system has just been mounted on the instrument at APEX and is being tested. To date, the exact gain is not fully quantified, but it is
highly qualitative.
During the period, we also developed a model of the instrument and observations that contains: a simulated input sky (a planet, or the full [CII] survey sky simulation); the atmosphere representative of that of APEX, including the temporal variations measured especially during a dedicated campaign for CONCERTO ; the scanning strategies; the instrument noise; and the bandpass. The pipeline of data reduction is continuously improved.
Finally, we have built new realistic simulations of the extragalactic sky that include both dust continuum and [CII], CI and CO spectral features. These simulations are unique and serve as a reference for (sub)-millimeter intensity mapping experiments. We are using these simulations to prepare the analysis of the [CII] survey large program (component separation, modelling for interpretation).
CONCERTO is the only instrument in the world currently performing intensity mapping of the [CII] line. It represents a unique opportunity before new experiments will come through (e.g. on the CCAT-p telescope that has a first light planned in late 2023). The timeline for CONCERTO instrument development and installation was extremely challenging but we managed to be on schedule, despite COVID restrictions. We have started the first ever done survey of the [CII] line emission at large scale at high redshift. We expect our experiment to give a unique view of dusty star formation in the early Universe, but it must overcome several observational challenges, especially effects linked to the atmosphere, and contaminations from extragalactic foregrounds, as the signal we are searching for is extremely weak. We are preparing ourselves to tackle these challenges. Even if penetrating deep into the epoch of reionization is uncertain, mainly due to our ignorance regarding the expected signal amplitude, the endeavor is well worth the effort. After years of predictions (more than 120 papers on 3D intensity mapping), the time has come for measurements.
The Observing Program Office from ESO stated in 2019 that “CONCERTO will bring a new life to APEX, particularly in the ALMA world. As such, this is a very interesting instrument, and its installation should be supported”. However, ESO’s management decided to retire from APEX by December 2022 and thus will not support any more CONCERTO, that will have to stop observing in the first half of 2023 at the latest.
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