Physical Characterization of an Unlensed, Dusty Star-forming Galaxy at z = 5.85

Caitlin M. Casey, Jorge A. Zavala, Manuel Aravena, Matthieu Béthermin, Karina I. Caputi, Jaclyn B. Champagne, David L. Clements, Elisabete Da Cunha, Patrick Drew, Steven L. Finkelstein, Christopher C. Hayward, Jeyhan S. Kartaltepe, Kirsten Knudsen, Anton M. Koekemoer, Georgios E. Magdis, Allison Man, Sinclaire M. Manning, Nick Z. Scoville, Kartik Sheth, Justin SpilkerJohannes Staguhn, Margherita Talia, Yoshiaki Taniguchi, Sune Toft, Ezequiel Treister, Min Yun

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49 Citations (Web of Science)


We present a physical characterization of MM J100026.36+021527.9 (a.k.a. "Mambo-9"), a dusty star-forming galaxy (DSFG) at z = 5.850 ± 0.001. This is the highest-redshift unlensed DSFG (and fourth most distant overall) found to date and is the first source identified in a new 2 mm blank-field map in the COSMOS field. Though identified in prior samples of DSFGs at 850 μm to 1.2 mm with unknown redshift, the detection at 2 mm prompted further follow-up as it indicated a much higher probability that the source was likely to sit at z > 4. Deep observations from the Atacama Large Millimeter and submillimeter Array (ALMA) presented here confirm the redshift through the secure detection of 12CO(J = 6→5) and p-H2O (21,1 → 20,2). Mambo-9 is composed of a pair of galaxies separated by 6 kpc with corresponding star formation rates of 590 M o˙ yr-1 and 220 M o˙ yr-1, total molecular hydrogen gas mass of (1.7 ± 0.4) × 1011 M o˙, dust mass of (1.3 ± 0.3) × 109 M o˙, and stellar mass of (3.2-1.5 +1.0) × 109 M o˙. The total halo mass, (3.3 ± 0.8) × 1012 M o˙, is predicted to exceed 1015 M o˙ by z = 0. The system is undergoing a merger-driven starburst that will increase the stellar mass of the system tenfold in τ depl = 40-80 Myr, converting its large molecular gas reservoir (gas fraction of 96-2 +1) into stars. Mambo-9 evaded firm spectroscopic identification for a decade, following a pattern that has emerged for some of the highest-redshift DSFGs found. And yet, the systematic identification of unlensed DSFGs like Mambo-9 is key to measuring the global contribution of obscured star formation to the star formation rate density at z ⪆ 4, the formation of the first massive galaxies, and the formation of interstellar dust at early times (≲1 Gyr).

Original languageEnglish
Article number55
JournalThe Astrophysical Journal
Issue number1
Publication statusPublished - 10 Dec 2019


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