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An almost NIRCam-dark dusty star-forming galaxy at z=6.63

Longji Bing, Seb Oliver, Mengyuan Xiao, Guilaine Lagache, Sylvia Adscheid, Daizhong Liu, Benjamin Magnelli, Roberto Neri, Miroslava Dessauges-Zavadsky, Anton M. Koekemoer, Maximilien Franco, Shuowen Jin, Olivia R. Cooper, Andreas L. Faisst, Catilin M. Casey, Jeyhan S. Kartaltepe, Hollis Akins, Alexandre Beelen, David Elbaz, Steven Gillman, Santosh Harish, Arianna S. Long, Henry Joy McCracken, Pascal Oesch, Louise Paquereau, Nicolas Ponthieu, Jason Rhodes, Brant Robertson, David B. Sanders, Marko Shuntov, Stephen Wilkins

Abstract

We present AC-2168, an almost NIRCam-dark, millimetre-bright galaxy in the COSMOS field. The source was identified blindly in ALMA Band-4 continuum data and remains undetected in the COSMOS-Web DR1 NIRCam catalogue. We spectroscopically confirm a redshift of $z_{\rm spec}=6.631$ from [CII] 158 $μ$m and four tentatively detected CO lines in NOEMA and ALMA data. SED fitting to near-IR to millimetre photometry yields $\rm L_{IR}=1.6\times10^{12}\,L_\odot$, an SFR of $\rm 244\,M_\odot/yr$, heavy dust attenuation $\rm A_V=5.4$ mag, and a stellar mass $\rm M_\star=3.7\times10^{10}\,M_\odot$. From the millimetre continuum and [CII] emission, we infer a warm ISM with $\rm T_{\rm dust}=60K$, $\rm M_{dust}=3.0\times10^{8}\,M_\odot$ and $\rm M_{gas}=4.1\times10^{10}\,M_\odot$. AC-2168 has a gas fraction ($f_{\rm gas}=M_{\rm gas}/(M_\star+M_{\rm gas})$) of $\sim52\%$, a short depletion time of $\rm \sim170Myr$, a compact ($\rm \sim1kpc$) dust-continuum size, and an SFR consistent with the star-forming main sequence at its mass. These properties match expectations for progenitors of massive quiescent galaxies at the peak of their assembly, as implied by NIRSpec-based SFHs of $z\sim4-5$ systems. Using the blind detection, we estimate a space density of $\rm 7.8^{+18.0}_{-6.5}\times10^{-6}\,cMpc^{-3}$ for AC-2168-like NIRCam-dark galaxies at $z\sim6-7$, $\sim42\%$ of the abundance of massive quiescent galaxies at $z\sim4-5$. No overdensity of Ly$α$ emitters or Lyman-break galaxies is found nearby, suggesting AC-2168 does not lie in a prominent protocluster and highlighting the importance of unbiased blind surveys for this population.

An almost NIRCam-dark dusty star-forming galaxy at z=6.63

Abstract

We present AC-2168, an almost NIRCam-dark, millimetre-bright galaxy in the COSMOS field. The source was identified blindly in ALMA Band-4 continuum data and remains undetected in the COSMOS-Web DR1 NIRCam catalogue. We spectroscopically confirm a redshift of from [CII] 158 m and four tentatively detected CO lines in NOEMA and ALMA data. SED fitting to near-IR to millimetre photometry yields , an SFR of , heavy dust attenuation mag, and a stellar mass . From the millimetre continuum and [CII] emission, we infer a warm ISM with , and . AC-2168 has a gas fraction () of , a short depletion time of , a compact () dust-continuum size, and an SFR consistent with the star-forming main sequence at its mass. These properties match expectations for progenitors of massive quiescent galaxies at the peak of their assembly, as implied by NIRSpec-based SFHs of systems. Using the blind detection, we estimate a space density of for AC-2168-like NIRCam-dark galaxies at , of the abundance of massive quiescent galaxies at . No overdensity of Ly emitters or Lyman-break galaxies is found nearby, suggesting AC-2168 does not lie in a prominent protocluster and highlighting the importance of unbiased blind surveys for this population.

Paper Structure

This paper contains 13 sections, 1 equation, 6 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Source Identification, Observation and Data reduction
  3. Identification of the source
  4. Archival and New Millimeter Observations
  5. Ancillary data
  6. Results
  7. Redshift of AC-2168
  8. SED modeling, Stellar mass and SFR
  9. Gas mass and depletion time
  10. Anchoring NIRCam-Dark Galaxies in the Cosmic Evolution of Massive Galaxies
  11. NIRCam-dark galaxies as the possible progenitor of earliest quiescent galaxies
  12. Diverse environment of NIRCam-dark galaxies
  13. Conclusions

Figures (6)

  • Figure 1: Near-IR to millimetre images of 6"$\times$6" size centred on AC-2168. (a)-(d): JWST NIRCam images from the COSMOS-Web survey. The aperture and annulus used in NIRCam photometry are outlined by orange and purple dotted circles, respectively (e)-(h): (sub) millimetre continuum data from ALMA archival data (870$\mu$m, 2.0mm and 3.0mm) and NOEMA DDT observation (1.2mm). The beam sizes are shown as yellow ellipses in the bottom right of the corresponding panels. The aperture used to extract ALMA Band 3+4 spectra is outlined by the yellow dotted circle. (i): ALMA 2mm continuum and NOEMA [CII] S/N contour overlaid on the pseudo-colour image generated by NIRCam F150W (blue), F277W (green) and F444W (red) data. The contour levels of the 2 mm continuum (yellow) and [CII] (pink) correspond to S/N=5 and 10 and S/N=3 and 5, respectively. Their beam shapes are shown as a yellow and pink ellipse in the bottom right.
  • Figure 2: Data confirming the spectroscopic redshift of AC-2168. (a): The probability distribution of photometric redshift of AC-2168 from CIGALE fitting compared to the $z_{\rm spec}$ from ALMA and NOEMA spectra. (b): NOEMA 1.2mm spectrum of AC-2168 centered on [CII]. (c): Stacked CO spectrum of AC-2168 based on ALMA Band 3 and Band 4 data. (d)-(g): ALMA Band 3 and Band 4 spectra centred on four individual CO lines within their coverage at the $z_{\rm spec}$ of AC-2168.
  • Figure 3: Left: the best-fit CIGALE full-wavelength model (black) and the best-fit modified blackbody model (blue) of AC-2168 SED compared to the SEDs of ULIRG Arp220, optically dark ALESS DSFGs and REBELS-12-2, an IRAC-dark DSFG in Fudamoto+21 at z=7.35. The flux and 2$\sigma$ flux upper limits of AC-2168 are marked in circles and triangles, respectively. The SEDs of literature sources are shifted to z=6.631 and normalised to AC-2168 at 1.2 mm. Right: the best-fit SFR and M$_*$ of AC-2168 compared to literature samples of high-z DSFGs Capak+11Riechers+13Marrone+18Williams+19Casey+19Jin+19Riechers+20Fudamoto+21Sun+24Xiao+24Sun+25, massive galaxies Dayal+22Schouws+22, and the star-formation main sequence at z=7 Schreiber+15Popesso+23.
  • Figure 4: Comparison between the $M_*$ and SFR of AC-2168 and other massive star-forming/starburst galaxies in literature with the inferred mass assembly history (left) and SFH (right) of GS-9209, a prototype of the earliest massive QGs at z=4.66. The shaded regions mark the 1-$\sigma$ uncertainty of the mass assembly history and SFH of GS-9209 based on medium-resolution NIRSpec observations from Carnall+24.
  • Figure 5: Our measurement of the number density of NIRCam-dark galaxies in comparison with the z=3-5 spectroscopically Carnall+23Valentino+23Baker+25Zhang+25 or photometrically selected Long+24adelaVega+25 massive QGs, z$\sim$6-7 dusty galaxies from REBELS Barrufet+23a and exMORA Long+24b, as well as the NIRCam-dark galaxies in ASPIRE Sun+25 under their "light" and "heavy" models.
  • ...and 1 more figures