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The NIKA2 cosmological legacy survey at 2 mm: catalogs, colors, redshift distributions, and implications for deep surveys

M. Béthermin, G. Lagache, C. Carvajal-Bohorquez, R. Adam, P. Ade, H. Ajeddig, S. Amarantidis, P. André, H. Aussel, A. Beelen, A. Benoît, S. Berta, L. J. Bing, A. Bongiovanni, J. Bounmy, O. Bourrion, M. Calvo, A. Catalano, D. Chérouvrier, M. De Petris, F. -X. Désert, S. Doyle, E. F. C. Driessen, G. Ejlali, A. Ferragamo, A. Gomez, J. Goupy, C. Hanser, S. Katsioli, F. Kéruzoré, C. Kramer, B. Ladjelate, S. Leclercq, J. -F. Lestrade, J. F. Macías-Pérez, S. C. Madden, A. Maury, F. Mayet, A. Monfardini, A. Moyer-Anin, M. Muñoz-Echeverría, I. Myserlis, A. Paliwal, L. Perotto, G. Pisano, N. Ponthieu, V. Revéret, A. J. Rigby, A. Ritacco, H. Roussel, F. Ruppin, M. Sánchez-Portal, S. Savorgnano, K. Schuster, A. Sievers, C. Tucker, R. Zylka

TL;DR

This study leverages the unprecedented 2 mm N2CLS data from the NIKA2 instrument, in combination with the SIDES semi-empirical sky simulations and end-to-end data processing, to quantify the colors and redshift distributions of 2 mm-selected dusty star-forming galaxies. The authors find a mean color of $S_{ m 2,mm}/S_{ m 1.2,mm}\approx 0.22$ with notable dispersion largely driven by observational effects, and mean redshifts around $z\approx 3.2$–$3.6$ depending on the field, with GOODS-N showing a known overdensity at $z\sim5.2$. The results show no evidence for a population of exotic 2 mm–only dusty galaxies, and the 2 mm selection primarily misses $z\sim2$ dusty sources, explaining the higher mean redshift of 2 mm samples compared to 1.2 mm. Through comparisons with Ex-MORA and SIDES, the paper concludes that 1.2 mm surveys remain the most efficient for building large samples of DSFGs across cosmic noon to reionization, while highlighting the complementary role of 2 mm data and the importance of combining single-dish surveys with interferometric follow-up for optimal survey strategies and JWST synergy.

Abstract

Millimeter galaxy surveys are particularly effective in detecting dusty star-forming galaxies at high redshift. While such observations are typically conducted at ~1mm, some studies suggest that 2mm may be better suited for selecting sources at even higher redshifts. We use the unprecedented 2mm data from the N2CLS, together with the SIDES simulation, to study and interpret the statistical properties of 2mm-selected galaxies. We use the N2CLS robust sample at 2mm, which contains 25 sources in the deep GOODS-N field and 90 sources in the wide COSMOS. The sources are matched with the N2CLS 1.2mm sources, the ancillary 850um sources, and redshift catalogs to study the colors and redshift distributions. We also produce end-to-end simulations based on SIDES and the observed N2CLS detector timelines to interpret the data. We find a mean S2/S1.2 color of 0.222$\pm$0.008 with a standard deviation of 0.070$\pm$0.010. We measure a mean redshift of $3.6\pm0.3$ in GOODS-N, which is marginally higher than expectations from SIDES ($2.9\pm0.2$) because of an overdensity at $z\sim5.2$, and $3.2\pm0.2$ in COSMOS, which agrees with the $3.2\pm0.2$ predicted by SIDES. We also show that the observed S2/S1.2 colors exhibit a weak dependence with redshift but a large dispersion, which limits its efficiency to select high-z sources. Finally, we studied the nine 2mm sources not detected at 1.2mm, and found that two of them are radiogalaxies, one is a z~2 galaxy, and the remaining six are compatible with the expected number of spurious detections. The N2CLS survey shows no evidence for any exotic 2mm-only galaxy population. Using SIDES, we show that 2mm samples have a higher mean redshift compared to 1.2mm because they miss z~2 dusty galaxies. Finally, we discuss the efficiency of single-dish and interferometric blind surveys to build samples of high-z dusty galaxies.

The NIKA2 cosmological legacy survey at 2 mm: catalogs, colors, redshift distributions, and implications for deep surveys

TL;DR

This study leverages the unprecedented 2 mm N2CLS data from the NIKA2 instrument, in combination with the SIDES semi-empirical sky simulations and end-to-end data processing, to quantify the colors and redshift distributions of 2 mm-selected dusty star-forming galaxies. The authors find a mean color of with notable dispersion largely driven by observational effects, and mean redshifts around – depending on the field, with GOODS-N showing a known overdensity at . The results show no evidence for a population of exotic 2 mm–only dusty galaxies, and the 2 mm selection primarily misses dusty sources, explaining the higher mean redshift of 2 mm samples compared to 1.2 mm. Through comparisons with Ex-MORA and SIDES, the paper concludes that 1.2 mm surveys remain the most efficient for building large samples of DSFGs across cosmic noon to reionization, while highlighting the complementary role of 2 mm data and the importance of combining single-dish surveys with interferometric follow-up for optimal survey strategies and JWST synergy.

Abstract

Millimeter galaxy surveys are particularly effective in detecting dusty star-forming galaxies at high redshift. While such observations are typically conducted at ~1mm, some studies suggest that 2mm may be better suited for selecting sources at even higher redshifts. We use the unprecedented 2mm data from the N2CLS, together with the SIDES simulation, to study and interpret the statistical properties of 2mm-selected galaxies. We use the N2CLS robust sample at 2mm, which contains 25 sources in the deep GOODS-N field and 90 sources in the wide COSMOS. The sources are matched with the N2CLS 1.2mm sources, the ancillary 850um sources, and redshift catalogs to study the colors and redshift distributions. We also produce end-to-end simulations based on SIDES and the observed N2CLS detector timelines to interpret the data. We find a mean S2/S1.2 color of 0.2220.008 with a standard deviation of 0.0700.010. We measure a mean redshift of in GOODS-N, which is marginally higher than expectations from SIDES () because of an overdensity at , and in COSMOS, which agrees with the predicted by SIDES. We also show that the observed S2/S1.2 colors exhibit a weak dependence with redshift but a large dispersion, which limits its efficiency to select high-z sources. Finally, we studied the nine 2mm sources not detected at 1.2mm, and found that two of them are radiogalaxies, one is a z~2 galaxy, and the remaining six are compatible with the expected number of spurious detections. The N2CLS survey shows no evidence for any exotic 2mm-only galaxy population. Using SIDES, we show that 2mm samples have a higher mean redshift compared to 1.2mm because they miss z~2 dusty galaxies. Finally, we discuss the efficiency of single-dish and interferometric blind surveys to build samples of high-z dusty galaxies.

Paper Structure

This paper contains 22 sections, 1 equation, 8 figures, 4 tables.

Table of Contents

  1. Introduction
  2. Data
  3. The NIKA2 cosmological legacy survey
  4. The GOODS-N catalog
  5. The COSMOS catalog
  6. The SIDES simulations
  7. End-to-end N2CLS simulation based on SIDES
  8. Colors and redshifts of the 2 mm sources
  9. 2 mm versus 1.2 mm colors
  10. Redshift distribution
  11. Colors versus redshift
  12. Colors versus redshift in the N2CLS
  13. Colors versus redshift in SIDES
  14. 2 mm sources without 1.2 mm counterparts
  15. Comparison with the Ex-MORA survey
  16. ...and 7 more sections

Figures (8)

  • Figure 1: Distribution of the 2 to 1.2 mm color for the true N2CLS COSMOS catalog (red filled histogram, only S/N$\ge4.6$ sources at both wavelengths and various simulated catalogs based on SIDES. The solid black histogram corresponds to the simulated SIDES galaxy catalog after applying a flux cut similar to COSMOS data (see Sect. \ref{['sect:colors']}). The yellow histogram is based on a similar selection, but applied to the blob catalog extracted from the noiseless simulated map. The grey histogram is the distribution obtained using the E2E simulation described in Sect. \ref{['sect:e2e']}. The error bars on the N2CLS histogram are computed assuming a Poisson law, and those from the E2E are the standard deviation between the various simulated fields. The histograms are normalized to have a unity area ($\int n(c)\, dc = 1$, where $c$ is the color). The lower limits on the color of 1.2 mm non-detections are shown as right-pointing triangles. Their y-axis position is arbitrary and was chosen to reduce overcrowding in the figure.
  • Figure 2: Redshift distribution of the GOODS-N high-reliability 2 mm sources. The red histogram is the distribution measured in the N2CLS. When sources are multiple, the redshift of the main component is used (see Sect. \ref{['sect:2mmonly']}). The uncertainties correspond to a Poisson law. The grey histogram is derived from the SIDES galaxy catalog after applying a flux cut similar to the GOODS-N field (see Sect. \ref{['sect:Nz']}). The uncertainties correspond to the field-to-field variance of a 15.9$\times$10 arcmin$^2$ field. The yellow histogram is based on a similar selection, but applied to the SIDES blob catalog and using the redshift of the brightest galaxy in the beam. The histograms are normalized to have a unity area ($\int n(z)\, dz = 1$).
  • Figure 3: Same as Fig. \ref{['fig:Nz_GOODSN']}, but for the COSMOS field. The blue dashed histogram represents the Ex-MORA survey Long2024.
  • Figure 4: Color as a function of the redshift in the COSMOS (top) and GOODS-N (bottom) fields. The left plots show the internal NIKA2 color (S$_{\rm 2\,mm}$/S$_{\rm 1.2\,mm}$) and the right ones show the ratio between the N2CLS 2 mm and the SCUBA2 850 $\mu$m fluxes from S2COSMOS Simpson2017 and SUPER GOODS Cowie2017. The filled blue circle are the N2CLS sources with spectroscopic redshifts, while the open red circles corresponds to photometric redshifts. The grey dots are from the SIDES 2 deg$^2$ simulated catalog after applying a 2 mm flux selection similar to SIDES (see Sect. \ref{['sect:Nz']}). The yellow solid line corresponds to the median of the SIDES blob catalog, using the redshift of the brightest galaxy in the beam, and the colored area correspond to the 1 $\sigma$ confidence region. The dashed line is the mean color as a function of redshift from SIDES 2 deg$^2$ without applying any selection. We also show the sensitivity ratio ($\sigma_{\rm 2\,mm} / \sigma_{\rm 1.2\,mm}$ where $\sigma_{\rm 2\,mm}$ and $\sigma_{\rm 1.2\,mm}$ are the RMS of the noise at 2 mm and 1.2 mm, respectively) for ALMA (dotted line) and N2CLS (dot-dashed line) ; see the discussion in Sect. \ref{['sect:disc_obs']}. The right-side panel of each plot show the color distribution of sources with a photometric redshift (red), a spectroscopic redshift (blue), and no redshift (green).
  • Figure 5: Comparison between the exMORA (x axis, Long2024) and N2CLS (y axis) 2 mm flux. The N2CLS sources with a single counterpart are in blue and the multiple sources are in red. The black dashed line indicates the one-to-one relation and the dotted line includes the various corrections (bandpass, source blending, and source sizes) discussed in Sect. \ref{['sect:MORA']}.
  • ...and 3 more figures