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 \pm 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 \mu m–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 \rightarrow 5 ) and p-H _ { 2 } O ( 2 _ { 1 , 1 } \rightarrow 2 _ { 0 , 2 } ) . Mambo–9 is comprised of a pair of galaxies separated by 6 kpc with corresponding star-formation rates of 590 M _ { \odot } yr ^ { -1 } and 220 M _ { \odot } yr ^ { -1 } , total molecular hydrogen gas mass of ( 1.7 \pm 0.4 ) \times 10 ^ { 11 } M _ { \odot } , dust mass of ( 1.3 \pm 0.3 ) \times 10 ^ { 9 } M _ { \odot } and stellar mass of ( 3.2 ^ { +1.0 } _ { -1.5 } ) \times 10 ^ { 9 } M _ { \odot } .
The total halo mass , ( 3.3 \pm 0.8 ) \times 10 ^ { 12 } M _ { \odot } , is predicted to exceed > 10 ^ { 15 } M _ { \odot } by z = 0 .
The system is undergoing a merger-driven starburst which will increase the stellar mass of the system tenfold in \tau _ { depl } = 40 - 80 Myr , 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 \lower 2.15 pt \hbox { $ \buildrel > \over { \sim } $ } 4 , the formation of the first massive galaxies , and the formation of interstellar dust at early times ( \buildrel < \over { \sim } 1 Gyr ) .