NIKA2 is a cryogenic millimeter camera, operating at 150 and 260 GHz and featuring 2900 KIDs cooled to 100 mK, which is installed at the IRAM 30-meter telescope. It was built by an international collaboration of which the LPSC is a founding member. The COSMO-ML team played a major role in the development and construction of the NIKA2 camera. It was responsible for commissioning the experiment, calibrating it and characterizing its performance. The NIKA2 collaboration has a guaranteed observing time (1300 hours over 4 years), which is the largest ever allocated by IRAM to a single collaboration. It has been divided into five observing programs corresponding to five scientific objectives, with in particular the SZ Large Program (LPSZ hereafter) for which the COSMO-ML team is responsible. We have important responsibilities within the collaboration: project scientist, editorial board manager, LPSZ manager, etc. The NIKA2 camera is also used to measure polarization at 260 GHz, a field in which the team has also played a key role. The involvement of LPSC's technical services was significant and decisive for the success of this project. A Memorandum of Understanding (MoU) has been signed between IRAM and the institutes involved (including IN2P3). It stipulates that the NIKA2 collaboration will provide support for the operation of NIKA2 until 2028 (10 years at the telescope). We are also participating as PI and co-PI in open-time observations on various subjects (galaxy clusters in SZ, pulsars).
NIKA2 was preceded by the NIKA camera, also built and designed by the NIKA2 collaboration with a major contribution from LPSC. Its field of view was smaller (1.8 arcmin) due to the reduced number of detectors (356). It was used at IRAM's 30-meter telescope from 2012 to 2015. In this context, we observed a sample of six clusters in SZ and also made high-quality polarization observations.
The SZ Large Program (LPSZ) is an international collaboration running a multi-probe (X-ray, SZ, optical) and multi-instrument (XMM-Newton, NIKA2, GTC) program aimed at optimizing the scientific exploitation of past (Planck, ACT and SPT) and future (CMB-S4, Simons Observatory) SZ cosmological surveys. The LPSZ benefits from 300 observation hours granted by IRAM. This international research program involves 10 participating laboratories (LPSC, IP2I Lyon, INSU, CEA, Rome, Madrid, Tenerife). The scientific objective is to mass-calibrate a representative sample of 38 galaxy clusters selected in SZ (Planck and ACT catalogs) at medium-to-high redshift (0.5 <z <0.9) and covering an order of magnitude in mass. These observations enable a comprehensive study of cluster morphology and evolution. In addition, these data are combined with X-ray data from the XMM-Newton satellite to study radial thermodynamic profiles (density, pressure, mass, temperature, entropy). These are essential for a complete understanding of the observable-mass relationship of galaxy clusters and their mean pressure profile. These two deliverables will ultimately enable large-scale cluster surveys to be exploited to constrain cosmology. The observation of these clusters involved 29 observing campaigns (lasting from 1 to 2 weeks), requiring a high level of involvement from the team for telescope observations and calibration.
To date, all the clusters have been observed. Initial publications have shown the importance of the perturbation state and the presence of point sources in estimating cluster mass. Cosmological publications (scaling relations, mean pressure profiles and applications) are scheduled for late 2024.
The COSMO-ML team has achieved several important results. With NIKA, we carried out the first SZ mapping with a KID-based camera [R. Adam et al., A&A 2014]. The observation of the MACS cluster J0717.5 + 3745 enabled us to publish two major results: the first model-independent mapping of the kinetic Sunyaev-Zel'dovich effect [R. Adam et al., A&A 2017a] as well as the first temperature map obtained by combining X-ray and SZ data [R. Adam et al., A&A 2017b]. In [F. Ruppin et al., A&A 2017], we proposed a new procedure for non-parametric deprojection of the electronic pressure of the intracluster medium, from the core of the cluster to its peripheral regions. The full data set from this pilot sample has been made public. With NIKA2, the first studies represented important milestones in the estimation of systematic effects and the preparation of cosmological results. In [F. Ruppin et al., A&A 2018], we studied the impact of substructures on mass estimation. In particular, the publication [F. Kéruzoré et al. A&A 2020] presented the method developed to propagate uncertainties related to point source contamination to pressure profiles. In [A. Ferragamo et al., A&A 2022, M. Muñoz-Echeverría et al., A&A 2023], we studied the impact of systematic effects on mass reconstruction in the case of galaxy clusters CLJ1227 and PSZ2G144. Comparison with lensing data enabled us to estimate the hydrostatic bias. The article [F. Kéruzoré et al., OJA 2023] accompanies the public release of the PANCO2 code for evaluating pressure profiles of the intracluster medium from SZ observations. This is a major step forward towards the complete analysis of the LPSZ and the study of associated systematics. The code has been designed to be extremely fast, yet versatile enough to be used by other collaborations.
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We have also set up a hydrodynamic simulation activity. As part of the international The300 collaboration, we have proposed to create a synthetic replica of the SZ Large Program cluster sample. This will enable us to study the biases and systematic effects induced by cluster morphology. In a pilot study (Music simulation), we showed F. Ruppin et al., A&A 2019] the impact of perturbations of the intracluster medium on the mean pressure profile. In [G. Gianfagna et al., MNRAS 2021], we studied the hydrostatic bias in the synthetic twin sample of the LPSZ. More recently, the publication [M. Muñoz-Echeverría et al., A&A 2024] showed the importance of modeling uncertainties and intrinsic dispersion that should be taken into account when using mass estimates for scaling relationships. We confirm that the orientation of the clusters and the radius intervals considered for the fit have a significant impact on the mass bias.
The COSMO-ML team also played a key role in the development of polarization with the NIKA and NIKA2 cameras. We took charge of the design and construction of the polarization modulator based on a rotating half-wave plate. In addition, the polarized data processing pipeline was developed in collaboration with the IPAG team. This enabled us to obtain the first scientific results in polarization with arrays of KIDs [A. Ritacco et al., A&A 2017, 2018]. Recently, the team has also been heavily involved in the polarization commissioning of NIKA2, which has also enabled us to obtain the first scientific results [A. Ritacco et al., EPJ 2022]. Polarization observations for the Grand Programme have just begun at IRAM. This activity is part of the polarization calibration of future CMB experiments.
The strategy of multi-institute synergy between INP, INSU, IN2P3 and CEA, which underpinned the success of NIKA and NIKA2, is widely recognized. We have obtained 2 very substantial ANR grants: "NIKA" (ANR-12-BS05-0007), dedicated to the construction of the NIKA2 instrument, and NIKA2Sky (ANR-15-CE31-0017), for which the LPSC is the project leader and which was awarded to support scientific exploitation. The team's strong involvement in NIKA and NIKA2 has led to the publication of 35 papers, 19 of them with a COSMO-ML member as first author, and has found a definite echo in the worldwide community. The COSMO-ML team has been the framework for 9 theses on NIKA2 (50 to 100%), 4 of which are in progress.
The future of scientific activity around NIKA2 lies on the one hand in the short term (end 2024) with the publication of scientific results from major programs and in particular cosmological results from the LPSZ, on the other hand in the medium term with polarization observations and publications, and in the longer term (2028) within the framework of the MoU.
