Aims. Future astrophysics and cosmic microwave background space missions operating in the far-infrared to millimetre part of the spectrum will require very large arrays of ultra-sensitive detectors in combination with high multiplexing factors and efficient lownoise and low-power readout systems. We have developed a demonstrator system suitable for such applications. Methods. The system combines a 961 pixel imaging array based upon Microwave Kinetic Inductance Detectors (MKIDs) with a readout system capable of reading out all pixels simultaneously with only one readout cable pair and a single cryogenic amplifier. We evaluate, in a representative environment, the system performance in terms of sensitivity, dynamic range, optical efficiency, cosmic ray rejection, pixel-pixel crosstalk and overall yield at an observation centre frequency of 850 GHz and 20% fractional bandwidth. Results. The overall system has an excellent sensitivity, with an average detector sensitivity NEP det = 3 × 10 −19 W/ √ Hz measured using a thermal calibration source. At a loading power per pixel of 50 fW we demonstrate white, photon noise limited detector noise down to 300 mHz. The dynamic range would allow the detection of ∼1 Jy bright sources within the field of view without tuning the readout of the detectors. The expected dead time due to cosmic ray interactions, when operated in an L2 or a similar far-Earth orbit, is found to be <4%. Additionally, the achieved pixel yield is 83% and the crosstalk between the pixels is <−30 dB. Conclusions. This demonstrates that MKID technology can provide multiplexing ratios on the order of a 1000 with state-of-the-art single pixel performance, and that the technology is now mature enough to be considered for future space based observatories and experiments.

The New IRAM KID Array (NIKA) instrument is a dual-band imaging camera operating with kinetic inductance detectors (KID) cooled at 100 mK. NIKA is designed to observe the sky at wavelengths of 1.25 and 2.14 mm from the IRAM 30 m telescope at Pico Veleta with an estimated resolution of 13 arcsec and 18 arcsec, respectively. This work presents the performance of the NIKA camera prior to its opening to the astrophysical community as an IRAM common-user facility in early 2014. NIKA is a test bench for the final NIKA2 instrument to be installed at the end of 2015. The last NIKA observation campaigns on November 2012 and June 2013 have been used to evaluate this performance and to improve the control of systematic effects. We discuss here the dynamical tuning of the readout electronics to optimize the KID working point with respect to background changes and the new technique of atmospheric absorption correction. These modifications significantly improve the overall linearity, sensitivity, and absolute calibration performance of NIKA. This is proved on observations of point-like sources for which we obtain a best sensitivity (averaged over all valid detectors) of 40 and 14 mJy s 1/2 for optimal weather conditions for the 1.25 and 2.14 mm arrays, respectively. NIKA observations of well known extended sources (DR21 complex and the Horsehead nebula) are presented. This performance makes the NIKA camera a competitive astrophysical instrument.

The thermal Sunyaev-Zel'dovich (tSZ) effect is expected to provide a low scatter mass proxy for galaxy clusters since it is directly proportional to the cluster thermal energy. The tSZ observations have proven to be a powerful tool for detecting and studying them, but high angular resolution observations are now needed to push their investigation to a higher redshift. In this paper, we report high angular (<20 arcsec) resolution tSZ observations of the high-redshift cluster CL J1226.9+3332 (z = 0.89). It was imaged at 150 and 260 GHz using the NIKA camera at the IRAM 30-m telescope. The 150 GHz map shows that CL J1226.9+3332 is morphologically relaxed on large scales with evidence of a disturbed core, while the 260 GHz channel is used mostly to identify point source contamination. NIKA data are combined with those of Planck and X-ray from Chandra to infer the cluster's radial pressure, density, temperature, and entropy distributions. The total mass profile of the cluster is derived, and we find M 500 = 5.96 +1.02 −0.79 × 10 14 M within the radius R 500 = 930 +50 −43 kpc, at a 68% confidence level. (R 500 is the radius within which the average density is 500 times the critical density at the cluster's redshift.) NIKA is the prototype camera of NIKA2, a KIDs (kinetic inductance detectors) based instrument to be installed at the end of 2015. This work is, therefore, part of a pilot study aiming at optimizing tSZ NIKA2 large programs.

Context. Millimetre-wave continuum astronomy is today an indispensable tool for both general astrophysics studies (e.g. star formation, nearby galaxies) and cosmology (e.g. cosmic microwave background and high-redshift galaxies). General purpose, large-fieldof-view instruments are needed to map the sky at intermediate angular scales not accessible by the high-resolution interferometers (e.g. ALMA in Chile, NOEMA in the French Alps) and by the coarse angular resolution space-borne or ground-based surveys (e.g. Planck, ACT, SPT). These instruments have to be installed at the focal plane of the largest single-dish telescopes, which are placed at high altitude on selected dry observing sites. In this context, we have constructed and deployed a three-thousand-pixel dual-band (150 GHz and 260 GHz, respectively 2 mm and 1.15 mm wavelengths) camera to image an instantaneous circular field-of-view of 6.5 arcmin in diameter, and configurable to map the linear polarisation at 260 GHz. Aims. First, we are providing a detailed description of this instrument, named NIKA2 (New IRAM KID Arrays 2), in particular focussing on the cryogenics, optics, focal plane arrays based on Kinetic Inductance Detectors, and the readout electronics. The focal planes and part of the optics are cooled down to the nominal 150 mK operating temperature by means of an adhoc dilution refrigerator. Secondly, we are presenting the performance measured on the sky during the commissioning runs that took place between October 2015 and April 2017 at the 30-m IRAM telescope at Pico Veleta, near Granada (Spain). Methods. We have targeted a number of astronomical sources. Starting from beam-maps on primary and secondary calibrators we have then gone to extended sources and faint objects. Both internal (electronic) and on-the-sky calibrations are applied. The general methods are described in the present paper. Results. NIKA2 has been successfully deployed and commissioned, performing in-line with expectations. In particular, NIKA2 exhibits full width at half maximum angular resolutions of around 11 and 17.5 arcsec at respectively 260 and 150 GHz. The noise equivalent flux densities are, at these two respective frequencies, 33 ± 2 and 8 ± 1 mJy s 1/2 . A first successful science verification run was achieved in April 2017. The instrument is currently offered to the astronomy community and will remain available for at least the following ten years.Key words. instrumentation: detectors -instrumentation: photometers -instrumentation: polarimeters -submillimeter: ISMsubmillimeter: galaxies -cosmic background radiation Article published by EDP Sciences A115, page 1 of 15 A&A 609, A115 (2018)

Magnetic fields, which play a major role in a large number of astrophysical processes can be traced via observations of dust polarization. In particular, Planck low-resolution observations of dust polarization have demonstrated that Galactic filamentary structures, where star formation takes place, are associated to well organized magnetic fields. A better understanding of this process requires detailed observations of galactic dust polarization on scales of 0.01 to 0.1 pc. Such high-resolution polarization observations can be carried out at the IRAM 30 m telescope using the recently installed NIKA2 camera, which features two frequency bands at 260 and 150 GHz (respectively 1.15 and 2.05 mm), the 260 GHz band being polarization sensitive. NIKA2 so far in commissioning phase, has its focal plane filled with ∼3300 detectors to cover a Field of View (FoV) of 6.5 arcmin diameter. The NIKA camera, which consisted of two arrays of 132 and 224 Lumped Element Kinetic Inductance Detectors (LEKIDs) and a FWHM (Full-Width-Half-Maximum) of 12 and 18.2 arcsec at 1.15 and 2.05 mm respectively, has been operated at the IRAM 30 m telescope from 2012 to 2015 as a test-bench for NIKA2. NIKA was equipped of a room temperature polarization system (a half wave plate (HWP) and a grid polarizer facing the NIKA cryostat window). The fast and continuous rotation of the HWP permits the quasi simultaneous reconstruction of the three Stokes parameters, I, Q, and U at 150 and 260 GHz. This paper presents the first polarization measurements with KIDs and reports the polarization performance of the NIKA camera and the pertinence of the choice of the polarization setup in the perspective of NIKA2. We describe the polarized data reduction pipeline, specifically developed for this project and how the continuous rotation of the HWP permits to shift the polarized signal far from any low frequency noise. We also present the dedicated algorithm developed to correct systematic leakage effects. We report results on compact and extended sources obtained during the February 2015 technical campaign. These results demonstrate a good understanding of polarization systematics and state-of-the-art performance in terms of photometry, polarization degree and polarization angle reconstruction.

The prototype of the NIKA2 camera, NIKA, is a dual-band instrument operating at the IRAM 30-m telescope, which can observe the sky simultaneously at 150 and 260 GHz. One of the main goals of NIKA (and NIKA2) is to measure the pressure distribution in galaxy clusters at high angular resolution using the thermal Sunyaev-Zel'dovich (tSZ) effect. Such observations have already proved to be an excellent probe of cluster pressure distributions even at intermediate and high redshifts. However, an important fraction of clusters host sub-millimeter and/or radio point sources, which can significantly affect the reconstructed signal. Here we report on <20 arcsec angular resolution observations at 150 and 260 GHz of the cluster MACS J1423.8+2404, which hosts both radio and submillimeter point sources. We examine the morphological distribution of the tSZ signal and compare it to other datasets. The NIKA data are combined with Herschel satellite data to study the spectral energy distribution (SED) of the sub-millimeter point source contaminants. We then perform a joint reconstruction of the intracluster medium (ICM) electronic pressure and density by combining NIKA, Planck, XMM-Newton, and Chandra data, focusing on the impact of the radio and sub-millimeter sources on the reconstructed pressure profile. We find that large-scale pressure distribution is unaffected by the point sources because of the resolved nature of the NIKA observations. The reconstructed pressure in the inner region is slightly higher when the contribution of point sources are removed. We show that it is not possible to set strong constraints on the central pressure distribution without accurately removing these contaminants. The comparison with X-ray only data shows good agreement for the pressure, temperature, and entropy profiles, which all indicate that MACS J1423.8+2404 is a dynamically relaxed cool core system. The present observations illustrate the possibility of measuring these quantities with a relatively small integration time, even at high redshift and without X-ray spectroscopy. This work is part of a pilot study aiming at optimizing tSZ observations with the future NIKA2 camera.

NIKA2 (New IRAM KID Array 2) is a camera dedicated to millimeter wave astronomy based upon kilopixel arrays of Kinetic Inductance Detectors 1 (KID). The pathfinder instrument, NIKA 2 , has already shown state-of-the-art detector performance. NIKA2 builds upon this experience but goes one step further, increasing the total pixel count by a factor ∼10 while maintaining the same per pixel performance. For the next decade, this camera will be the resident photometric instrument of the Institut de Radio Astronomie Millimetrique (IRAM) 30m telescope in Sierra Nevada (Spain). In this paper we give an overview of the main components of NIKA2, and describe the achieved detector performance. The camera has been permanently installed at the IRAM 30m telescope in October 2015. It will be made accessible to the scientific community at the end of 2016, after a one-year commissioning period. When this happens, NIKA2 will become a fundamental tool for astronomers worldwide.

Context. Clusters of galaxies provide valuable information on the evolution of the Universe and large scale structures. Recent cluster observations via the thermal Sunyaev-Zel'dovich (tSZ) effect have proven to be a powerful tool to detect and study them. In this context, high resolution tSZ observations (∼tens of arcsec) are of particular interest to probe intermediate and high redshift clusters. Aims. Observations of the tSZ effect will be carried out with the millimeter dual-band NIKA2 camera, based on kinetic inductance detectors (KIDs) to be installed at the IRAM 30-m telescope in 2015. To demonstrate the potential of such an instrument, we present tSZ observations with the NIKA camera prototype, consisting of two arrays of 132 and 224 detectors that observe at 140 and 240 GHz with a 18.5 and 12.5 arcsec angular resolution, respectively. Methods. The cluster RX J1347.5-1145 was observed simultaneously at 140 and 240 GHz. We used a spectral decorrelation technique to remove the atmospheric noise and obtain a map of the cluster at 140 GHz. The efficiency of this procedure has been characterized through realistic simulations of the observations. Results. The observed 140 GHz map presents a decrement at the cluster position consistent with the tSZ nature of the signal. We used this map to study the pressure distribution of the cluster by fitting a gNFW model to the data. Subtracting this model from the map, we confirm that RX J1347.5-1145 is an ongoing merger, which confirms and complements previous tSZ and X-ray observations. Conclusions. For the first time, we demonstrate the tSZ capability of KID based instruments. The NIKA2 camera with ∼5000 detectors and a 6.5 arcmin field of view will be well-suited for in-depth studies of the intra cluster medium in intermediate to high redshifts, which enables the characterization of recently detected clusters by the Planck satellite.