2019ApJ...887...55C

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 (2_1,1 - 2_0,2). MAMBO-9 is composed of a pair of galaxies separated by 6 kpc with corresponding star formation rates of 590 M_☉ yr^–1 and 220 M_☉ yr^–1, total molecular hydrogen gas mass of (1.7 ± 0.4) x 10¹¹M_☉, dust mass of (1.3 ± 0.3) x 10⁹M_☉, and stellar mass of (3.2_–1.5^+1.0) x 10⁹M_☉. The total halo mass, (3.3 ± 0.8) x 10¹²M_☉, is predicted to exceed 10¹⁵M_☉ 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).