NIKA2 Camera Research Articles

NIKA2 observations of starless cores in Taurus and Perseus

Dusty starless cores play an important role in regulating the initial phases of the formation of stars and planets. In their interiors, dust grains coagulate and ice mantles form, thereby changing the millimeter emissivities and hence the ability to cool. We mapped four regions with more than a dozen cores in the nearby Galactic filaments of Taurus and Perseus using the NIKA2 camera at the IRAM 30-meter telescope. Combining the 1mm to 2mm flux ratio maps with dust temperature maps from Herschel allowed to create maps of the dust emissivity index β1,2 at resolutions of 2430 and 5600 a.u. in Taurus and Perseus, respectively. Here, we study the variation with total column densities and environment. β1,2 values at the core centers (Av =12 – 19 mag) vary significantly between ~ 1.1 and 2.3. Several cores show a strong rise of β1,2 from the outskirts at ~ 4 mag to the peaks of optical extinctions, consistent with the predictions of grain models and the gradual build-up of ice mantles on coagulated grains in the dense interiors of starless cores.

Constraining millimeter dust emission in nearby galaxies with NIKA2: The case of NGC2146 and NGC2976

This study presents the first millimeter continuum mapping observations of two nearby galaxies, the starburst spiral galaxy NGC2146 and the dwarf galaxy NGC2976, at 1.15 mm and 2 mm using the NIKA2 camera on the IRAM 30m telescope, as part of the Guaranteed Time Large Project IMEGIN. These observations provide robust resolved information about the physical properties of dust in nearby galaxies by constraining their FIR-radio SED in the millimeter domain. After subtracting the contribution from the CO line emission, the SEDs are modeled spatially using a Bayesian approach. Maps of dust mass surface density, temperature, emissivity index, and thermal radio component of the galaxies are presented, allowing for a study of the relations between the dust properties and star formation activity (using observations at 24μm as a tracer). We report that dust temperature is correlated with star formation rate in both galaxies. The effect of star formation activity on dust temperature is stronger in NGC2976, an indication of the thinner interstellar medium of dwarf galaxies. Moreover, an anti-correlation trend is reported between the dust emissivity index and temperature in both galaxies.

Exploring the interstellar medium of NGC 891 at millimeter wavelengths using the NIKA2 camera

In the framework of the IMEGIN Large Program, we used the NIKA2 camera on the IRAM 30-m telescope to observe the edge-on galaxy NGC 891 at 1.15 mm and 2 mm and at a FWHM of 11.1” and 17.6”, respectively. Multiwavelength data enriched with the new NIKA2 observations fitted by the HerBIE SED code (coupled with the THEMIS dust model) were used to constrain the physical properties of the ISM. Emission originating from the diffuse dust disk is detected at all wavelengths from mid-IR to mm. while mid-lR observations reveal warm dust emission from compact H II regions. Indications of mm excess emission have also been found in the outer parts of the galactic disk. Furthermore, our SED fitting analysis constrained the mass fraction of the small (< 15 Å) dust grains. We found that small grains constitute 9.5% of the total dust mass in the galactic plane, but this fraction increases up to ~ 20% at large distances (|z| > 3 kpc) from the galactic plane.

Polarization angle accuracy for future CMB experiments

The Cosmic Microwave Background (CMB) radiation offers a unique window into the early Universe, facilitating precise examinations of fundamental cosmological theories. However, the quest for detecting B-modes in the CMB, predicted by theoretical models of inflation, faces substantial challenges in terms of calibration and foreground modeling. The COSMOCal (COsmic Survey of Millimeter wavelengths Objects for CMB experiments Calibration) project aims at enhancing the accuracy of the absolute calibration of the polarization angle ψ of current and future CMB experiments. The concept includes the build of a very well known artificial source emitting in the frequency range [20-350] GHz that would act as an absolute calibrator for several polarization facilities on Earth. A feasibility study to place the artificial source in geostationary orbit, in the far field for all the telescopes on Earth, is ongoing. In the meanwhile ongoing hardware work is dedicated to build a prototype to test the technology, the precision and the stability of the polarization recovering in the 1 mm band (220-300 GHz). High-resolution experiments as the NIKA2 camera at the IRAM 30m telescope will be deployed for such use. Once carefully calibrated (Δψ < 0.1◦) it will be used to observe astrophysical sources such as the Crab nebula, which is the best candidate in the sky for the absolute calibration of CMB experiments.

The NIKA2 Sunyaev-Zeldovich Large Program

The NIKA2 camera operating at the IRAM 30-m telescope excels in high-angular resolution mapping of the thermal Sunyaev-Zel’dovich effect towards galaxy clusters at intermediate and high-redshift. As part of the NIKA2 guaranteed-time, the SZ Large Program (LPSZ) aims at tSZ-mapping a representative sample of SZ-selected galaxy clusters in the catalogues of the Planck satellite and of the Atacama Cosmology Telescope, and also observed in X-ray with XMM-Newton or Chandra. Having completed observations in January 2023, we present tSZ maps of 38 clusters spanning the targeted mass (3 < M500/1014M⊙ < 10) and redshift (0.5 < z < 0.9) range. The first in-depth studies of individual clusters highlight the potential of combining tSZ and X-ray observations at similar angular resolution for precised mass measurements under the hydrostatic assumption MHSE. These were milestones for the development of a standard data analysis pipeline to go from NIKA2 raw data to the thermodynamic properties of galaxy clusters for the upcoming LPSZ data release. Final products will include measurements of the mean pressure profile of unprecedented quality and MHSE-observable scaling relation using a distinctive SZ-selected sample, which will be key for ultimately improving the accuracy of cluster-based cosmology.

The stratification of ISM properties in the edge-on galaxy NGC 891 revealed by NIKA2

Context. As the millimeter wavelength range remains a largely unexplored spectral region for galaxies, the IMEGIN large program aims to map the millimeter continuum emission of 22 nearby galaxies at 1.15 and 2 mm. Aims. Using the high-resolution maps produced by the NIKA2 camera, we explore the existence of very cold dust and take possible contamination by free–free and synchrotron emission into account. We study the IR-to-radio emission coming from different regions along the galactic plane and at large vertical distances. Methods. New observations of NGC 891, using the NIKA2 camera on the IRAM 30 m telescope, along with a suite of observations at other wavelengths were used to perform a multiwavelength study of the spectral energy distribution in the interstellar medium in this galaxy. This analysis was performed globally and locally, using the advanced hierarchical Bayesian fitting code, HerBIE, coupled with the THEMIS dust model. Results. Our dust modeling is able to reproduce the near-IR to millimeter emission of NGC 891, with the exception of an excess at a level of 25% obtained by the NIKA2 observations in the outermost parts of the disk. The radio continuum and thermal dust emission are distributed differently in the disk and galaxy halo. Different dusty environments are also revealed by a multiwavelength investigation of the emission features. Our detailed decomposition at millimeter and centimeter wavelengths shows that emission at 1 mm is purely originated by dust. Radio components become progressively important with increasing wavelengths. Finally, we find that emission arising from small dust grains accounts for ∼9.5% of the total dust mass, reaching up to 20% at large galactic latitudes. Shock waves in the outflows that shatter the dust grains might explain this higher fraction of small grains in the halo. Conclusions. NIKA2 observations have proven essential for a complete characterization of the interstellar medium in NGC 891. They have been critical to separate the dust, free–free, and synchrotron emission in the various emitting regions within the galaxy.

NIKA2 Cosmological Legacy Survey

Context. Finding and characterizing the heavily obscured galaxies with extreme star formation up to very high redshift is key for constraining the formation of the most massive galaxies in the early Universe. It has been shown that these obscured galaxies are major contributors to the accumulation of stellar mass to z ~ 4. At higher redshift, and despite recent progress, the contribution of dust-obscured galaxies remains poorly known. Aims. Deep surveys in the millimeter domain are necessary in order to probe the dust-obscured galaxies at high redshift. We conducted a large observing program at 1.2 and 2 mm with the NIKA2 camera installed on the IRAM 30m telescope. This NIKA2 Cosmological Legacy Survey (N2CLS) covers two emblematic fields: GOODS-N and COSMOS. We introduce the N2CLS survey and present new 1.2 and 2 mm number counts measurements based on the tiered N2CLS observations (from October 2017 to May 2021) covering 1169 arcmin2. Methods. After a careful data reduction and source extraction, we develop an end-to-end simulation that combines an input sky model with the instrument noise and data reduction pipeline artifacts. This simulation is used to compute the sample purity, flux boosting, pipeline transfer function, completeness, and effective area of the survey (taking into account the non-homogeneous sky coverage). For the input sky model, we used the 117 square degree SIDES simulations, which include galaxy clustering. Our formalism allows us to correct the source number counts to obtain galaxy number counts, the difference between the two being due to resolution effects caused by the blending of several galaxies inside the large beam of single-dish instruments. Results. The N2CLS-May2021 survey is already the deepest and largest ever made at 1.2 and 2 mm. It reaches an average 1σ- noise level of 0.17 and 0.048 mJy on GOODS-N over 159 arcmin2, and 0.46 and 0.14 mJy on COSMOS over 1010 arcmin2, at 1.2 and 2 mm, respectively. For a purity threshold of 80%, we detect 120 and 67 sources in GOODS-N and 195 and 76 sources in COSMOS at 1.2 and 2 mm, respectively. At 1.2 mm, the number counts measurement probes consistently 1.5 orders of magnitude in flux density, covering the full flux density range from previous single-dish surveys and going a factor of 2 deeper into the sub-mJy regime. Our measurement connects the bright single-dish to the deep interferometric number counts. At 2 mm, our measurement matches the depth of the deepest interferometric number counts and extends a factor of 2 above the brightest constraints. After correcting for resolution effects, our results reconcile the single-dish and interferometric number counts, which can be further accurately compared with model predictions. Conclusions. While the observation in GOODS-N have already reached the target depth, we expect the final N2CLS survey to be 1.5 times deeper for COSMOS. Thanks to its volume-complete flux selection, the final N2CLS sample will be an ideal reference for conducting a full characterization of dust-obscured galaxies at high redshift.

Candidate cosmic filament in the GJ526 field, mapped with the NIKA2 camera

Distinctive large-scale structures have been identified in the spatial distribution of optical galaxies up to redshift z ∼ 1. In the more distant universe, the relationship between the dust-obscured population of star-forming galaxies observed at millimetre wavelengths and the network of cosmic filaments of dark matter apparent in all cosmological hydrodynamical simulations is still under study. Using the NIKA2 dual-band millimetre camera, we mapped a field of ∼90 arcmin2 in the direction of the star GJ526 simultaneously in its 1.15-mm and 2.0-mm continuum wavebands to investigate the nature of the quasi-alignment of five sources found ten years earlier with the MAMBO camera at 1.2 mm. We find that these sources are not clumps of a circumstellar debris disc around this star as initially hypothesized. Rather, they must be dust-obscured star-forming galaxies, or sub-millimetre galaxies (SMGs), in the distant background. The new NIKA2 map at 1.15 mm reveals a total of seven SMGs distributed in projection on the sky along a filament-like structure crossing the whole observed field. Furthermore, we show that the NIKA2 and supplemental Herschel photometric data are compatible with a model of the spectral energy distributions (SEDs) of these sources when a common redshift of 2.5 and typical values of the dust parameters for SMGs are adopted. Hence, we speculate that these SMGs might be located in a filament of the distant ‘cosmic web’. The length of this candidate cosmic filament crossing the whole map is at least 4 cMpc (comoving), and the separations between sources are between 0.25 cMpc and 1.25 cMpc at this redshift, in line with expectations from cosmological simulations. Nonetheless, further observations to determine the precise spectroscopic redshifts of these sources are required to definitively support this hypothesis of SMGs embedded in a cosmic filament of dark matter.

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

The Crab nebula is a supernova remnant exhibiting a highly polarized synchrotron radiation at radio and millimeter wavelengths. It is the brightest source in the microwave sky with an extension of 7 by 5 arcminutes and commonly used as a standard candle for any experiment which aims at measuring the polarization of the sky. Though its spectral energy distribution has been well characterized in total intensity, polarization data are still lacking at millimetre wavelengths. We report in this paper high resolution (18 arcsec FWHM) observations of the Crab nebula in total intensity and linear polarization at 150 GHz with the NIKA camera. NIKA, operated at the IRAM 30 m telescope from 2012 to 2015, is a camera made of Lumped Element Kinetic Inductance Detectors (LEKIDs) observing the sky at 150 and 260 GHz. From these observations we are able to reconstruct the spatial distribution of the polarization degree and angle of the Crab nebula, which is found to be compatible with previous observations at lower and higher frequencies. Averaging across the source and using other existing data sets we find that the Crab nebula polarization angle is consistent with being constant over a wide range of frequencies with a value of -87.7$^\circ$ +- 0.3 in Galactic coordinates. We also present the first estimation of the Crab nebula spectral energy distribution polarized flux in a wide frequency range: 30-353 GHz. Assuming a single power law emission model we find that the polarization spectral index $\beta_{pol}$ = - 0.347 +- 0.026 is compatible with the intensity spectral index $\beta$ = - 0.323 +- 0.001.

PSZ2 G091: A massive double cluster atz~ 0.822 observed by the NIKA2 camera

PSZ2 G091.83+26.11 is a massive galaxy cluster withM500= 7:43 × 1014M⊙atz= 0:822. This object exhibits a complex morphology with a clear bimodality observed in X-rays. However, it was detected and analysed in thePlancksample as a single, spherical cluster following a universal profile [1]. This model can lead to miscalculations of thermodynamical quantities, like the pressure profile. As future multiwavelength cluster experiments will detect more and more objects at higher redshifts (where we expect the fraction of merging objects to be higher), it is crucial to quantify this systematic effect. In this work, we use high-resolution observations of PSZ2 G091.83+26.11 by the NIKA2 camera to integrate the morphological characteristics of the cluster in our modelling. This is achieved by fitting a two-halo model to the SZ image and then by reconstruction of the resulting projected pressure profile. We then compare these results with the spherical assumption.

Foreword: Observing the mm Universe with the NIKA2 camera

Foreword: Observing the mm Universe with the NIKA2 camera

Multi-probe analysis of the galaxy cluster CL J1226.9+3332: Hydrostatic mass and hydrostatic-to-lensing bias

We present a multi-probe analysis of the well-known galaxy cluster CL J1226.9+3332 as a proof of concept for multi-wavelength studies within the framework of the NIKA2 Sunyaev-Zel’dovich Large Program (LPSZ). CL J1226.9+3332 is a massive and high redshift (z = 0.888) cluster that has already been observed at several wavelengths. A joint analysis of the thermal SZ (tSZ) effect at millimeter wavelength with the NIKA2 camera and in X-ray with theXMM-Newtonsatellite permits the reconstruction of the cluster’s thermodynamical properties and mass assuming hydrostatic equilibrium. We test the robustness of our mass estimates against different definitions of the data analysis transfer function. Using convergence maps reconstructed from the data of the CLASH program we obtain estimates of the lensing mass, which we compare to the estimated hydrostatic mass. This allows us to measure the hydrostatic-to-lensing mass bias and the associated systematic effects related to the NIKA2 measurement. We obtainM500HSE= (7:65 ± 1:03) × 1014M⊙andM500lens= (7:35 ± 0:65) × 1014M⊙, which implies a HSE-to-lensing bias consistent with 0 within 20%.

Exploring the millimetre emission in nearby galaxies: Analysis of the edge-on galaxy NGC 891

New observations of the edge-on galaxy NGC 891, at 1.15 and 2mm obtained with the IRAM 30-m telescope and the NIKA2 camera, within the framework of the IMEGIN (Interpreting the Millimetre Emission of Galaxies with IRAM and NIKA2) Large Program, are presented in this work. By using multiwavelength maps (from the mid-IR to the cm wavelengths) we perform SED fitting in order to extract the physical properties of the galaxy on both global and local (~kpc) scales. For the interpretation of the observations we make use of a state-of-the-art SED fitting code, HerBIE (HiERarchical Bayesian Inference for dust Emission). The observations indicate a galaxy morphology, at mm wavelengths, similar to that of the cold dust emission traced by submm observations and to that of the molecular gas. The contribution of the radio emission at the NIKA2 bands is very small (negligible at 1.15 mm and ~ 10% at 2 mm) while it dominates the total energy budget at longer wavelengths (beyond 5 mm). On local scales, the distribution of the free-free emission resembles that of the dust thermal emission while the distribution of the synchrotron emission shows a deficiency along the major axis of the disc of the galaxy.

Overdensity of SubMillimiter Galaxies in the GJ526 Field mapped with the NIKA2 Camera

Using the NIKA2 dual band millimeter camera installed on the IRAM30m telescope, we have mapped a relatively large field (~ 70 arcmin2) in the direction of the star GJ526 to investigate the nature of the sources found with the MAMBO camera at 1.2 mm ten years earlier. We have found that they must be dust-obscured galaxies (SMGs) in the background beyond the star. The new NIKA2 map at 1.15 mm reveals additional sources and, in fact, an overdensity of SMGs predominantly distributed along a filament-like structure in projection on the sky across the whole observed field. We speculate this might be a cosmic filament at high redshift as revealed in cosmological hydrodynamical simulations. Measurement of spectroscopic redshifts of the SMGs in the candidate filament is required now for a definitive confirmation of the nature of the structure.

The Three Hundred–NIKA2 Sunyaev–Zeldovich Large Program twin samples: Synthetic clusters to support real observations

The simulation database of The Three Hundred Project has been used to pick synthetic clusters of galaxies with properties close to the observational targets of the NIKA2 camera Sunyaev–Zeldovich (SZ) Large Program. Cross–matching of cluster parameters such as mass and redshift of the cluster in the two databases has been implemented to generate the so–called twin samples for the Large Program. This SZ Large Program is observing a selection of galaxy clusters at intermediate and high redshift (0:5 < z < 0:9), covering one order of magnitude in mass. These are SZ–selected clusters from the Planck and Atacama Cosmology Telescope catalogs, wherein the selection is based on their integrated Compton parameter values, Y500: the value of the parameter within the characteristics radius R500. The Three Hundred hydrodynamical simulations provide us with hundreds of clusters satisfying these redshift, mass, and Y500 requirements. In addition to the standard post-processing analysis of the simulation, mock observational maps are available mimicking X–ray, optical, gravitational lensing, radio, and SZ observations of galaxy clusters. The primary goal of employing the twin samples is to compare different cluster mass proxies from synthetic X–ray, SZ effect and optical maps (via the velocity dispersion of member galaxies and lensing κ-maps) of the clusters. Eventually, scaling laws between different mass proxies and the cluster mass will be cross–correlated to reduce the scatter on the inferred mass and the mass bias will be related to various physical parameters.

The XXL Survey

High-mass clusters at low redshifts have been intensively studied at various wavelengths. However, while more distant objects at lower masses constitute the bulk population of future surveys, their physical state remain poorly explored to date. In this paper, we present resolved observations of the Sunyaev-Zel’dovich (SZ) effect, obtained with the NIKA2 camera, towards the cluster of galaxies XLSSC 102, a relatively low-mass system (M500 ∼ 2 × 1014 M⊙) atz = 0.97 detected from the XXL survey. We combine NIKA2 SZ data,XMM-NewtonX-ray data, and Megacam optical data to explore, respectively, the spatial distribution of the gas electron pressure, the gas density, and the galaxies themselves. We find significant offsets between the X-ray peak, the SZ peak, the brightest cluster galaxy, and the peak of galaxy density. Additionally, the galaxy distribution and the gas present elongated morphologies. This is interpreted as the sign of a recent major merging event, which induced a local boost of the gas pressure towards the north of XLSSC 102 and stripped the gas out of the galaxy group. The NIKA2 data are also combined with XXL data to construct the thermodynamic profiles of XLSSC 102, obtaining relatively tight constraints up to about ∼r500, and revealing properties that are typical of disturbed systems. We also explore the impact of the cluster centre definition and the implication of local pressure substructure on the recovered profiles. Finally, we derive the global properties of XLSSC 102 and compare them to those of high-mass-and-low-redshift systems, finding no strong evidence for non-standard evolution. We also use scaling relations to obtain alternative mass estimates from our profiles. The variation between these different mass estimates reflects the difficulty to accurately measure the mass of low-mass clusters atz ∼ 1, especially with low signal-to-noise ratio data and for a disturbed system. However, it also highlights the strength of resolved SZ observations alone and in combination with survey-like X-ray data. This is promising for the study of high redshift clusters from the combination of eROSITA and high resolution SZ instruments and will complement the new generation of optical surveys from facilities such as LSST andEuclid.

Calibration and performance of the NIKA2 camera at the IRAM 30-m Telescope

Context.NIKA2 is a dual-band millimetre continuum camera of 2 900 kinetic inductance detectors, operating at 150 and 260 GHz, installed at the IRAM 30-m telescope in Spain. Open to the scientific community since October 2017, NIKA2 will provide key observations for the next decade to address a wide range of open questions in astrophysics and cosmology.Aims.Our aim is to present the calibration method and the performance assessment of NIKA2 after one year of observation.Methods.We used a large data set acquired between January 2017 and February 2018 including observations of primary and secondary calibrators and faint sources that span the whole range of observing elevations and atmospheric conditions encountered by the IRAM 30-m telescope. This allowed us to test the stability of the performance parameters against time evolution and observing conditions. We describe a standard calibration method, referred to as the “Baseline” method, to translate raw data into flux density measurements. This includes the determination of the detector positions in the sky, the selection of the detectors, the measurement of the beam pattern, the estimation of the atmospheric opacity, the calibration of absolute flux density scale, the flat fielding, and the photometry. We assessed the robustness of the performance results using the Baseline method against systematic effects by comparing results using alternative methods.Results.We report an instantaneous field of view of 6.5′ in diameter, filled with an average fraction of 84%, and 90% of valid detectors at 150 and 260 GHz, respectively. The beam pattern is characterised by a FWHM of 17.6″ ± 0.1″ and 11.1″ ± 0.2″, and a main-beam efficiency of 47%±3%, and 64%±3% at 150 and 260 GHz, respectively. The point-source rms calibration uncertainties are about 3% at 150 GHz and 6% at 260 GHz. This demonstrates the accuracy of the methods that we deployed to correct for atmospheric attenuation. The absolute calibration uncertainties are of 5%, and the systematic calibration uncertainties evaluated at the IRAM 30-m reference Winter observing conditions are below 1% in both channels. The noise equivalent flux density at 150 and 260 GHz are of 9 ± 1 mJy s1/2and 30 ± 3 mJy s1/2. This state-of-the-art performance confers NIKA2 with mapping speeds of 1388 ± 174 and 111 ± 11 arcmin2mJy−2h−1at 150 and 260 GHz.Conclusions.With these unique capabilities of fast dual-band mapping at high (better that 18″) angular resolution, NIKA2 is providing an unprecedented view of the millimetre Universe.

Cluster cosmology with the NIKA2 SZ Large Program

The main limiting factor of cosmological analyses based on thermal Sunyaev-Zel’dovich (SZ) cluster statistics comes from the bias and systematic uncertainties that affect the estimates of the mass of galaxy clusters. High-angular resolution SZ observations at high redshift are needed to study a potential redshift or morphology dependence of both the mean pressure profile and of the mass-observable scaling relation used in SZ cosmological analyses. The NIKA2 camera is a new generation continuum instrument installed at the IRAM 30-m telescope. With a large field of view, a high angular resolution and a high-sensitivity, the NIKA2 camera has unique SZ mapping capabilities. In this paper, we present the NIKA2 SZ large program, aiming at observing a large sample of clusters at redshifts between 0.5 and 0.9, and the characterization of the first cluster oberved with NIKA2.

Analysis of Galactic molecular cloud polarization maps: a review of the methods

The Davis-Chandrasekhar-Fermi (DCF) method using the Angular Dispersion Function (ADF), the Histogram of Relative Orientations (HROs) and the Polarization-Intensity Gradient Relation (P-IGR) are the most common tools used to analyse maps of linearly polarized emission by thermal dust grains at submilliter wavelengths in molecular clouds and star-forming regions. A short review of these methods is given. The combination of these methods will provide valuable tools to shed light on the impact of the magnetic fields on the formation and evolution of subparsec scale hub-filaments that will be mapped with the NIKA2 camera and future experiments.

Debris disks around stars in the NIKA2 era

The new NIKA2 camera at the IRAM 30m radiotelescope was used to observe three known debris disks in order to constrain the SED of their dust emission in the millimeter wavelength domain. We have found that the spectral index between the two NIKA2 bands (1mm and 2mm) is consistent with the Rayleigh-Jeans regime (λ-2), unlike the steeper spectra (λ-3) measured in the submillimeter-wavelength domain for two of the three disks - around the stars Vega and HD107146. We provide a succesful proof of concept to model this spectral inversion in using two populations of dust grains, those smaller and those larger than a grain radius a0 of 0.5mm. This is obtained in breaking the slope of the size distribution and the functional form of the absorption coefficient of the standard model. The third disk - around the star HR8799 - does not exhibit this spectral inversion but is also the youngest.