The Identification of Two JWST/NIRCam-Dark Starburst Galaxies at $z=6.6$ with ALMA
Fengwu Sun, Jinyi Yang, Feige Wang, Daniel J. Eisenstein, Roberto Decarli, Xiaohui Fan, George H. Rieke, Eduardo Ba~nados, Sarah E. I. Bosman, Zheng Cai, Jaclyn B. Champagne, Luis Colina, Francesco D’Eugenio, Yoshinobu Fudamoto, Mingyu Li, Xiaojing Lin, Weizhe Liu, Jianwei Lyu, Chiara Mazzucchelli, Xiangyu Jin, Hyunsung D. Jun, Yunjing Wu, Huanian Zhang
arXiv:2506.06418v1 Announce Type: new
Abstract: We analyze two dusty star-forming galaxies at $z=6.6$. These galaxies are selected from the ASPIRE survey, a JWST Cycle-1 medium and ALMA Cycle-9 large program targeting 25 quasars and their environments at $zsimeq6.5 – 6.8$. These galaxies are identified as companions to UV-luminous quasars and robustly detected in ALMA continuum and [C II] emission, yet they are extraordinarily faint at the NIRCam wavelengths (down to $>28.0$ AB mag in the F356W band). They are more obscured than galaxies like Arp220, and thus we refer to them as “NIRCam-dark” starburst galaxies (star formation rate $simeq 80 – 250,mathrm{M}_{odot},mathrm{yr}^{-1}$). Such galaxies are typically missed by (sub)-millimeter blank-field surveys. From the star-formation history (SFH), we show that the NIRCam-dark galaxies are viable progenitors of massive quiescent galaxies at $zgtrsim4$ and descendants of UV-luminous galaxies at $z>10$. Although it is hard to constrain their number density from a quasar survey, we conclude that NIRCam-dark galaxies can be as abundant as $nsim10^{-5.5}$ Mpc$^{-3}$ assuming a light halo occupation model. If true, this would equal to $sim$30% of the number densities of both the quiescent galaxies at $zgtrsim4$ and UV-luminous galaxies at $z>10$. We further predict that analogs at $zsim8$ should exist according to the SFH of early massive quiescent galaxies. However, they may fall below the current detection limits of wide JWST and ALMA surveys, thus remaining “JWST-dark”. To fully trace the evolution of massive galaxies and dust-obscured cosmic star formation at $zgtrsim8$, wide-field JWST/NIRCam imaging and slitless spectroscopic surveys of early protoclusters are essential.arXiv:2506.06418v1 Announce Type: new
Abstract: We analyze two dusty star-forming galaxies at $z=6.6$. These galaxies are selected from the ASPIRE survey, a JWST Cycle-1 medium and ALMA Cycle-9 large program targeting 25 quasars and their environments at $zsimeq6.5 – 6.8$. These galaxies are identified as companions to UV-luminous quasars and robustly detected in ALMA continuum and [C II] emission, yet they are extraordinarily faint at the NIRCam wavelengths (down to $>28.0$ AB mag in the F356W band). They are more obscured than galaxies like Arp220, and thus we refer to them as “NIRCam-dark” starburst galaxies (star formation rate $simeq 80 – 250,mathrm{M}_{odot},mathrm{yr}^{-1}$). Such galaxies are typically missed by (sub)-millimeter blank-field surveys. From the star-formation history (SFH), we show that the NIRCam-dark galaxies are viable progenitors of massive quiescent galaxies at $zgtrsim4$ and descendants of UV-luminous galaxies at $z>10$. Although it is hard to constrain their number density from a quasar survey, we conclude that NIRCam-dark galaxies can be as abundant as $nsim10^{-5.5}$ Mpc$^{-3}$ assuming a light halo occupation model. If true, this would equal to $sim$30% of the number densities of both the quiescent galaxies at $zgtrsim4$ and UV-luminous galaxies at $z>10$. We further predict that analogs at $zsim8$ should exist according to the SFH of early massive quiescent galaxies. However, they may fall below the current detection limits of wide JWST and ALMA surveys, thus remaining “JWST-dark”. To fully trace the evolution of massive galaxies and dust-obscured cosmic star formation at $zgtrsim8$, wide-field JWST/NIRCam imaging and slitless spectroscopic surveys of early protoclusters are essential.

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