NIKA2 Cosmological Legacy Survey. First measurement of the confusion noise at the IRAM 30 m telescope

The NIKA2 Cosmological Legacy Survey (N2CLS) is a large programme using the NIKA2 dual-band camera on the IRAM 30,m telescope. Its goal is to improve our understanding of the physics of distant Dusty Star Forming Galaxies (DSFGs) by carrying out deep surveys of two fields, GOODS-North and COSMOS. This work is focussed on GOODS-North, which was observed for 78.2 hours, simultaneously at 1.2 and 2,mm, with a field of view of $\sim$240,arcmin$^2$. With such a deep integration, we were able to measure, for the first time, the confusion noise limits at the 30,m telescope using the best sampled $\sim 62$,arcmin$^2$ and masking sources with a flux greater than 0.54 or 0.17,mJy at 1.2 or 2,mm, respectively. We found a confusion noise of $139.1^{+ 15.9}{- 19.2}\pm11.9$,$μ$Jy/beam at 1.2,mm and $38.6^{+ 9.6}{- 13.1} \pm3.7$,$μ$Jy/beam at 2,mm (the first uncertainty is statistical, the second is the cosmic variance). In this region, this corresponds to half the instrumental noise. To derive these estimates, we devised a novel estimator, referred to as the cross variance, which also enabled us to estimate the correlated confusion noise between the two bands. Thus, we obtained a result of $49.6^{+ 15.9}_{- 24.8}\pm 6.4$,$μ$Jy/beam. These values are consistent with the state of the art Simulated Infrared Dusty Extragalactic Sky (SIDES) model.

💡 Research Summary

This paper presents the first direct measurement of the confusion noise limit at the IRAM 30-meter telescope, achieved as part of the NIKA2 Cosmological Legacy Survey (N2CLS). The primary goal of N2CLS is to advance the understanding of distant Dusty Star-Forming Galaxies (DSFGs) through deep millimeter-wave observations. This work focuses on the GOODS-North field, which was observed for a total of 78.2 hours simultaneously at 1.2 mm and 2 mm wavelengths using the NIKA2 dual-band camera, covering a field of view of approximately 240 arcmin².

Confusion noise, the fundamental sensitivity limit imposed by the blended emission of countless unresolved background sources within the telescope’s beam, sets the ultimate depth for single-dish surveys. Historically, it has been estimated either from models of galaxy number counts or empirically from the asymptotic behavior of map noise versus integration time. This study introduces a novel, robust estimator termed the “cross variance” to directly measure this noise component from the data itself.

The observational campaign spanned from October 2017 to March 2021. After quality selection, 668 individual scans (out of 749) were used in the analysis. The data reduction pipeline was significantly enhanced for this ultra-deep integration. Key improvements included a refined method for monitoring the resonance frequency shifts of the Kinetic Inductance Detectors (KIDs) to better remove atmospheric and low-frequency electronic noise. The data model was also sophisticated, separating the atmospheric signal into low- and high-frequency components, including its time derivative, and utilizing “off-resonance tones” as proxies for correlated electronic noise from different readout boxes.

The core of the analysis involved applying the new cross-variance estimator to the best-sampled central region of about 62 arcmin². Bright sources above flux thresholds of 0.54 mJy at 1.2 mm and 0.17 mJy at 2 mm were masked. The results provide the first empirical confusion noise values for the 30-meter telescope: 139.1 ^{+15.9}{-19.2} (stat.) ± 11.9 (cosmic var.) μJy/beam at 1.2 mm and 38.6 ^{+9.6}{-13.1} ± 3.7 μJy/beam at 2 mm. In this field, the confusion noise constitutes about half of the total noise, with instrumental noise making up the other half. Furthermore, the cross-variance technique enabled the first measurement of the correlated confusion noise between the two bands, yielding a value of 49.6 ^{+15.9}_{-24.8} ± 6.4 μJy/beam.

These measured values show excellent agreement with predictions from the state-of-the-art Simulated Infrared Dusty Extragalactic Sky (SIDES) model. This consistency validates both the observational methodology and the current theoretical understanding of the population and clustering of DSFGs that contribute to the cosmic infrared background.

In conclusion, this work successfully quantifies a fundamental limit for single-dish millimeter astronomy. The measured confusion noise levels provide crucial benchmarks for planning future deep-field observations with the IRAM 30-meter telescope and for designing complementary observing strategies with interferometers like ALMA. The development and successful application of the cross-variance estimator also offer a powerful new tool for characterizing background fluctuations in multi-wavelength deep surveys.