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 $z=5.850\pm 0.001$. This is the highest redshift unlensed DSFG (and fourth most distant overall) found to-date, and is the first source identified in a new 2mm blank-field map in the COSMOS field. Though identified in prior samples of DSFGs at 850$\hat{I}¼$m-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 ${}^{12}$CO($J=$6$\to$5) and p-H${}_2$O(2${}_{1,1}\to$2${}_{0,2}$). MAMBO-9 is comprised of a pair of galaxies separated by 6kpc with corresponding star-formation rates of 590M${}_{\odot }$yr${}^{-1}$ and 220M${}_{\odot }$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${}_{-1.5}^{+1.0}$)$\times 10^9$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 $\ddot{I}{„}_{\mathrm{depl}}=40-80$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 ($<$1Gyr).

24 pages, 8 figures, accepted for publication in ApJ