A. Bracco

The NIKA2 large-field-of-view millimetre continuum camera for the 30 m IRAM telescope

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. CMB - cosmic microwave background and high-redshift galaxies). General purpose, large-field-of-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-ofview of 6.5 arcminutes 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 (KID), 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 ad-hoc 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-meter IRAM (Institut of Millimetric Radio Astronomy) 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 (FWHM) angular resolutions of around 11 and 17.5 arc-seconds at respectively 260 and 150 GHz. The noise equivalent flux densities (NEFD) 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.

Statistical simulations of the dust foreground to cosmic microwave background polarization

The characterization of the dust polarization foreground to the cosmic microwave background (CMB) is a necessary step toward the detection of the B-mode signal associated with primordial gravitational waves. We present a method to simulate maps of polarized dust emission on the sphere that is similar to the approach used for CMB anisotropies. This method builds on the understanding of Galactic polarization stemming from the analysis of Planck data. It relates the dust polarization sky to the structure of the Galactic magnetic field and its coupling with interstellar matter and turbulence. The Galactic magnetic field is modeled as a superposition of a mean uniform field and a Gaussian random (turbulent) component with a power-law power spectrum of exponent αM. The integration along the line of sight carried out to compute Stokes maps is approximated by a sum over a small number of emitting layers with different realizations of the random component of the magnetic field. The model parameters are constrained to fit the power spectra of dust polarization EE, BB, and TE measured using Planck data. We find that the slopes of the E and B power spectra of dust polarization are matched for α_M = −2.5, an exponent close to that measured for total dust intensity but larger than the Kolmogorov exponent − 11/3. The model allows us to compute multiple realizations of the Stokes Q and U maps for different realizations of the random component of the magnetic field, and to quantify the variance of dust polarization spectra for any given sky area outside of the Galactic plane. The simulations reproduce the scaling relation between the dust polarization power and the mean total dust intensity including the observed dispersion around the mean relation. We also propose a method to carry out multifrequency simulations, including the decorrelation measured recently by Planck, using a given covariance matrix of the polarization maps. These simulations are well suited to optimize component separation methods and to quantify the confidence with which the dust and CMB B-modes can be separated in present and future experiments. We also provide an astrophysical perspective on our phenomenological modeling of the dust polarization spectra.

Modelling and simulation of large-scale polarized dust emission over the southern Galactic cap using the GASS Hi data

The Planck survey has quantified polarized Galactic foregrounds and established that they are a main limiting factor in the quest for the cosmic microwave background (CMB) B-mode signal induced by primordial gravitational waves during cosmic inflation. The necessity of achieving an accurate separation of the Galactic foregrounds therefore binds the search for the signal from cosmic inflation to our understanding of the magnetized interstellar medium (ISM). The two most relevant observational results coming out of Planck data analysis are the line of sight depolarization due to the fluctuations of the Galactic magnetic field orientation and the alignment of the dust filamentary structures with the magnetic field at high Galactic latitude. Furthermore, Planck and HI emission data in combination indicate that most of the dust filamentary structures are present in the cold neutral medium. The goal of this paper is to test whether together these salient observational results can account fully for the statistical properties of the dust polarization over a selected low column density portion within the southern Galactic cap (

Probing changes of dust properties along a chain of solar-type prestellar and protostellar cores in Taurus with NIKA

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NIKA 150 GHz polarization observations of the Crab nebula and its spectral energy distribution

). To do that, we construct a dust model incorporating HI column density maps as tracers of the dust intensity structures and a phenomenological description of the Galactic magnetic field. Adjusting the parameters of the dust model, we are able to reproduce the Planck dust observations at 353 GHz in the selected region comprising 34% of the sky in the southern Galactic cap. The realistic simulations of the polarized dust emission enabled by such a dust model are useful for testing the accuracy of component separation methods, non-Gaussianity studies, and constraining the level of decorrelation with frequency.

Statistics of the polarized submillimetre emission maps from thermal dust in the turbulent, magnetized, diffuse ISM

The characterization of dust properties in the interstellar medium (ISM) is key for star formation. Mass estimates are crucial to determine gravitational collapse conditions for the birth of new stellar objects in molecular clouds. However, most of these estimates rely on dust models that need further observational constraints from clouds to prestellar and protostellar cores. We present results of a study of dust emissivity changes based on mm-continuum data obtained with the NIKA camera at the IRAM-30m telescope. Observing dust emission at 1.15 mm and 2 mm allows us to constrain the dust emissivity index (

Statistical simulations of the dust foreground to cosmic microwave background polarization (Corrigendum)

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A NIKA view of two star-forming infrared dark clouds: Dust emissivity variations and mass concentration

) in the Rayleigh-Jeans tail of the dust spectral energy distribution (SED) far from its peak emission, where the contribution of other parameters (i.e. dust temperature) is important. Focusing on the Taurus molecular cloud, a low-mass star-forming regions in the Gould Belt, we analyze the emission properties of several distinct objects in the B213 filament: three prestellar cores, two Class-0/I protostellar cores and one Class-II object. By means of the ratio of the two NIKA channel-maps, we show that in the Rayleigh-Jeans approximation the dust emissivity index varies among the objects. For one prestellar and two protostellar cores, we produce a robust study using Herschel data to constrain the dust temperature of the sources. By using the Abel transform inversion technique we get accurate radial

Magnetic field morphology in nearby molecular clouds as revealed by starlight and submillimetre polarization

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