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

Abstract

We present a physical characterization of MM J100026.36+021527.9 (a.k.a. “M ambo-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 μ m 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 12 CO( J  = 6→5) and p-H 2 O (2 1,1  → 2 0,2 ). M ambo-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) × 10 11 M ⊙ , dust mass of (1.3 ± 0.3) × 10 9 M ⊙ , and stellar mass of ( ) × 10 9 M ⊙ . The total halo mass, (3.3 ± 0.8) × 10 12 M ⊙ , is predicted to exceed 10 15 M ⊙ 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 ) into stars. M ambo-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 M ambo-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).