MAMBO-9: High-z Dusty Starburst Merger

• MAMBO-9 is a high-redshift dusty star-forming galaxy resolved into two interacting components, evidencing merger-driven starburst activity just 1 Gyr after the Big Bang.
• ALMA and JWST observations reveal compact dust cores, intense CO and H2O emissions, and substantial interstellar obscuration that drive rapid star formation.
• Situated in a significant galaxy overdensity at z≈5.85, MAMBO-9 serves as a prototype for understanding early massive galaxy assembly and interstellar-medium enrichment.

MAMBO-9 is the dusty star-forming galaxy $\mathrm{MMJ100026.36+021527.9}$ in the COSMOS field, spectroscopically confirmed at $z=5.850\pm0.001$. It was initially identified in millimeter surveys but remained without a secure redshift for roughly a decade because it lacked clear optical-to-far-infrared counterparts. Subsequent ALMA observations resolved it into two interacting, optically dark components, MAMBO-9-A and MAMBO-9-B, establishing it as a merger-driven, heavily obscured system less than $1$ Gyr after the Big Bang. MAMBO-9 has become a reference case for the study of unlensed high-redshift dusty star formation, rapid interstellar-medium enrichment, and massive-galaxy assembly in an overdense environment (Casey et al., 2019, Akins et al., 8 Aug 2025).

1. Identification, nomenclature, and discovery history

MAMBO-9 is known under several survey-specific designations. In the 1.2 mm MAMBO survey it appeared as “ID9”; at 1.1 mm it was cataloged as “AzTEC/C148”; in SCUBA-2 imaging it appeared as “850.43”; and in later SCUBA-2/COSMOS work it was designated “COS.0059.” In ALMA-resolved imaging, its two subcomponents are named MAMBO-9-A and MAMBO-9-B, with A the brighter northern source and B the fainter southern source. Its first published 1.2 mm detection had flux density $S_{1.2{\rm mm}}=4.9\pm0.9$ mJy, later corroborated by $S_{1.1{\rm mm}}=4.6\pm1.2$ mJy and by SCUBA-2 $850\,\mu{\rm m}$ measurements of $5.55\pm1.11$ and $5.84\pm0.87$ mJy in successive catalogs (Casey et al., 2019).

Despite this consistent submillimeter and millimeter detection history, MAMBO-9 had no secure redshift for about a decade. It was not clearly detected in Spitzer/MIPS $24\,\mu{\rm m}$, Herschel/PACS $100$ or $z=5.850\pm0.001$0, Herschel/SPIRE $z=5.850\pm0.001$1–$z=5.850\pm0.001$2, and it was absent from the deep COSMOS optical/NIR catalog of Laigle et al. The decisive step was its appearance as the brightest source in the first $z=5.850\pm0.001$3 of a new ALMA 2 mm blank-field map in COSMOS. The 2 mm selection was important because it suppresses the lower-redshift DSFG population that dominates $z=5.850\pm0.001$4 and 1.1 mm samples; for MAMBO-9 the color $z=5.850\pm0.001$5 is much lower than the $z=5.850\pm0.001$6 expected for typical DSFGs at $z=5.850\pm0.001$7–4, favoring a high-redshift interpretation (Casey et al., 2019).

A historical point is that the 2009 COSMOS environmental study of MAMBO sources did not discuss MAMBO-9 or COSBO-9 as an individual source. That work analyzed COSMOS MAMBO sources in aggregate and source-by-source discussed COSBO-1, 3, 6, and 16, but not MAMBO-9; accordingly, it provided no direct sky position, counterpart designation, redshift, flux, or environmental result specifically for MAMBO-9 (0911.4297).

2. Spectroscopic confirmation and resolved morphology

The secure redshift determination rests on ALMA Band 3 spectroscopy. The final adopted redshift is $z=5.850\pm0.001$8, based on detection of $z=5.850\pm0.001$9 and para-$1$0. For the integrated system, the CO line has $1$1, $1$2 significance, line width $1$3, and $1$4. The water line has $1$5, $1$6 significance, width $1$7, and $1$8. The CO line is detected in both A and B, whereas the water line is detected only in A (Casey et al., 2019).

ALMA resolved MAMBO-9 into a close pair of dusty galaxies separated by roughly 6 kpc. Component A dominates the continuum in every ALMA band and is brighter in CO; component B is clearly detected at $1$9 and in CO, but only marginally at 1.3 and 3 mm and not significantly at 2 mm. The different millimeter colors of A and B were interpreted as physical rather than instrumental, with A inferred to be warmer and optically thicker than B (Casey et al., 2019).

The $S_{1.2{\rm mm}}=4.9\pm0.9$0 continuum shows both components to be extremely compact. Using $S_{1.2{\rm mm}}=4.9\pm0.9$1-plane Sérsic fitting with fixed $S_{1.2{\rm mm}}=4.9\pm0.9$2, the circularized half-light radii are $S_{1.2{\rm mm}}=4.9\pm0.9$3 pc and $S_{1.2{\rm mm}}=4.9\pm0.9$4 pc; the authors adopted $S_{1.2{\rm mm}}=4.9\pm0.9$5 pc and $S_{1.2{\rm mm}}=4.9\pm0.9$6 pc for later calculations. The deconvolved major-axis FWHM at $S_{1.2{\rm mm}}=4.9\pm0.9$7 is $S_{1.2{\rm mm}}=4.9\pm0.9$8 for A and $S_{1.2{\rm mm}}=4.9\pm0.9$9 for B, with axis ratios $S_{1.1{\rm mm}}=4.6\pm1.2$0 and $S_{1.1{\rm mm}}=4.6\pm1.2$1, respectively (Casey et al., 2019).

3. Dust, gas, star formation, and the early physical picture

The 2019 physical characterization modeled MAMBO-9 as a massive, gas-rich starburst. For component A, the far-infrared fit gave $S_{1.1{\rm mm}}=4.6\pm1.2$2, $S_{1.1{\rm mm}}=4.6\pm1.2$3, $S_{1.1{\rm mm}}=4.6\pm1.2$4, $S_{1.1{\rm mm}}=4.6\pm1.2$5, and $S_{1.1{\rm mm}}=4.6\pm1.2$6. For component B, it gave $S_{1.1{\rm mm}}=4.6\pm1.2$7, $S_{1.1{\rm mm}}=4.6\pm1.2$8, $S_{1.1{\rm mm}}=4.6\pm1.2$9, $850\,\mu{\rm m}$0, and $850\,\mu{\rm m}$1. For the integrated system, $850\,\mu{\rm m}$2 and $850\,\mu{\rm m}$3 (Casey et al., 2019).

The dust mass, derived from the 3 mm continuum with an explicit CMB correction, was estimated as $850\,\mu{\rm m}$4, $850\,\mu{\rm m}$5, and $850\,\mu{\rm m}$6. Molecular gas masses inferred from the dust continuum were $850\,\mu{\rm m}$7, $850\,\mu{\rm m}$8, and $850\,\mu{\rm m}$9. CO-based estimates were higher but more uncertain; the dust-based total gas mass was adopted because it gave a more plausible mass budget and gas-to-dust ratio (Casey et al., 2019).

Under the preferred low-stellar-mass solution adopted in that study, the total stellar mass was $5.55\pm1.11$0, implying $5.55\pm1.11$1 and a gas-to-dust ratio of $5.55\pm1.11$2. The depletion times were $5.55\pm1.11$3 and $5.55\pm1.11$4, summarized as $5.55\pm1.11$5–80 Myr for the system. The star-formation surface densities were $5.55\pm1.11$6 and $5.55\pm1.11$7, indicating that A in particular is a compact, intense starburst (Casey et al., 2019).

A central caveat already present in the 2019 analysis was the stellar-mass uncertainty. An energy-balance MAGPHYS fit to the joint A+B SED instead yielded $5.55\pm1.11$8, a discrepancy of about $5.55\pm1.11$9 relative to the OIR-only estimate. The authors therefore emphasized that the stellar mass was formally unconstrained and that downstream quantities such as gas fraction and halo mass were correspondingly uncertain (Casey et al., 2019).

4. Resolved kinematics, obscuration structure, and revised mass budget

High-resolution ALMA $5.84\pm0.87$0 $5.84\pm0.87$1 observations and JWST/NIRCam+MIRI imaging substantially revised the internal picture of MAMBO-9. The new ALMA data, with $5.84\pm0.87$2 resolution corresponding to about 400 pc, resolve compact dust cores, more diffuse $5.84\pm0.87$3-emitting gas, and velocity gradients across both galaxies. JWST reveals a continuous bridge of moderately dust-obscured material between A and B, strongly supporting an ongoing interaction. The bridge has $5.84\pm0.87$4 and contains a few $5.84\pm0.87$5 in stars, about $5.84\pm0.87$6 of the total stellar mass of the system (Akins et al., 8 Aug 2025).

Both galaxies show double-peaked $5.84\pm0.87$7 spectra and ordered velocity gradients, but also strong disturbances. The adopted inclinations are $5.84\pm0.87$8 for A and $5.84\pm0.87$9 for B. With $24\,\mu{\rm m}$0 km s$24\,\mu{\rm m}$1 for A and $24\,\mu{\rm m}$2 km s$24\,\mu{\rm m}$3 for B, the inferred dynamical masses are $24\,\mu{\rm m}$4 and $24\,\mu{\rm m}$5, implying a relative mass ratio of roughly 1:5. The kinematics are consistent with both rotation and strong tidal interaction, and the preferred interpretation is a minor merger that has already undergone a close encounter (Akins et al., 8 Aug 2025).

The resolved SED analysis changed the view of the stellar mass and obscuration geometry. In MAMBO-9-A, the bulk of recent star formation is concentrated in an extremely obscured core with $24\,\mu{\rm m}$6, while much of the observed rest-optical light and inferred H$24\,\mu{\rm m}$7 emission emerge from less obscured outskirts with $24\,\mu{\rm m}$8–5. For B, attenuation is lower overall but still substantial. The fiducial stellar masses from the resolved fits are $24\,\mu{\rm m}$9 and $100$0, with mass-weighted extinctions $100$1 and $100$2. A NIRSpec PRISM spectrum of B gives $100$3, implying $100$4 for the nebular emission and an attenuation-corrected $100$5, far below the FIR- and $100$6-based rates. This demonstrates that the observed rest-optical line emission samples only the less obscured star-forming regions (Akins et al., 8 Aug 2025).

The new mass budget also altered the preferred gas-conversion factors. Using the requirement that $100$7 should not exceed the dynamical mass, the inferred gas masses are $100$8 and $100$9, implying gas fractions $z=5.850\pm0.001$00 and $z=5.850\pm0.001$01. The required CO-to-H$z=5.850\pm0.001$02 conversion factor is $z=5.850\pm0.001$03–2, with “roughly unity” favored, and the corresponding gas-to-dust ratio is about $z=5.850\pm0.001$04–$z=5.850\pm0.001$05. A plausible implication is that the very high $z=5.850\pm0.001$06 estimate in the earlier analysis was driven primarily by the low OIR-only stellar-mass solution rather than by a fully constrained mass budget (Akins et al., 8 Aug 2025, Casey et al., 2019).

5. Large-scale environment and evolutionary interpretation

The environmental interpretation of MAMBO-9 changed markedly once source-specific spectroscopy became available. The 2025 JWST+ALMA study identified a significant overdensity in the PRIMER-COSMOS field around the system: 39 galaxies are spectroscopically confirmed within $z=5.850\pm0.001$07 of MAMBO-9, at $z=5.850\pm0.001$08–5.88, on a scale of roughly $z=5.850\pm0.001$09 cMpc. The full structure appears to span most of the PRIMER-COSMOS field, with an extent of order $z=5.850\pm0.001$10 cMpc and possible connections over line-of-sight distances approaching $z=5.850\pm0.001$11 cMpc to neighboring $z=5.850\pm0.001$12 structures. Within that framework, MAMBO-9 is interpreted as a massive galaxy growing in an assembling large-scale structure and as a likely progenitor of a future brightest cluster galaxy (Akins et al., 8 Aug 2025).

This conclusion contrasts with the earlier state of the literature. The 2009 BzK-based analysis of COSMOS MAMBO environments found significant compact overdensities only around COSBO-1, 3, 6, and 16, and MAMBO-9 was not mentioned in the narrative, tables, or source-by-source discussion. On that basis, no direct evidence was then presented that MAMBO-9 lay in one of the compact galaxy overdensities identified in that study (0911.4297).

The later source-specific result is therefore not a refinement of a pre-existing MAMBO-9 environmental claim, but rather the first direct environmental characterization of the source in the provided record. In combination with its total baryonic mass of order $z=5.850\pm0.001$13, the overdensity supports the interpretation of MAMBO-9 as a prototype of massive galaxy formation at $z=5.850\pm0.001$14 (Akins et al., 8 Aug 2025).

6. Significance for high-redshift dusty-galaxy studies

At the time of its 2019 characterization, MAMBO-9 was described as the highest-redshift unlensed DSFG then known and the fourth most distant DSFG overall. Its observational history illustrated a broader selection effect: very high-redshift unlensed DSFGs can remain “hidden in plain sight” in $z=5.850\pm0.001$15–1.2 mm catalogs because their optical/NIR and far-infrared counterparts are faint or absent, candidate spectral lines may be ambiguous, and secure confirmation can require multiple ALMA tunings and substantial integration time. MAMBO-9 was presented explicitly as evidence that systematic identification of unlensed DSFGs is essential for measuring the obscured contribution to the star-formation-rate density at $z=5.850\pm0.001$16, the formation of the first massive galaxies, and the buildup of interstellar dust at early times (Casey et al., 2019).

The later JWST+ALMA view sharpened that significance. MAMBO-9 is not merely a high-redshift continuum source but a resolved, interacting pair in which the dominant star formation is deeply buried, the visible rest-optical galaxy does not trace the principal star-forming sites, the ISM is already chemically mature, and the system resides in a spectroscopically confirmed overdensity. This suggests that MAMBO-9 is best understood not only as an extreme starburst, but as a concrete instance of rapid, dust-obscured, merger-driven assembly of a very massive galaxy near the end of reionization (Akins et al., 8 Aug 2025).