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

Abstract

We present a physical characterization of MMJ100026.36+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 2mm blank-field map in the COSMOS field. Though identified in prior samples of DSFGs at 850μm-1.2mm with unknown redshift, the detection at 2mm 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 comprised of a pair of galaxies separated by 6kpc with corresponding star-formation rates of 590M⊙yr⁻¹ and 220M⊙yr⁻¹ total molecular hydrogen gas mass of (1.7±0.4)×10¹¹M⊙, dust mass of (1.3±0.3)×10⁹M⊙ and stellar mass of (3.2+1.0−1.5)×10⁹M⊙. The total halo mass, (3.3±0.8)×10¹²M⊙, is predicted to exceed >10¹⁵M⊙ by z = 0. The system is undergoing a merger-driven starburst which will increase the stellar mass of the system tenfold in τ_(depl) = 40−80Myr, converting its large molecular gas reservoir (gas fraction of 96^(+1)_(−2)%) 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 (<1Gyr).