---
title: 'MAMBO-9: High-z Dusty Starburst Merger'
url: https://www.emergentmind.com/topics/mambo-9
type: topic
---

# MAMBO-9: High-z Dusty Starburst Merger

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 [1910.13331] [2508.06607].

## 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 [1910.13331].

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 \(160\,\mu{\rm m}\), Herschel/SPIRE \(250\)–\(500\,\mu{\rm m}\), 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 \(9.4\,{\rm arcmin}^2\) 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 \(850\,\mu{\rm m}\) and 1.1 mm samples; for MAMBO-9 the color \(S_{850\mu{\rm m}}/S_{2{\rm mm}}=8.3\pm0.9\) is much lower than the \(\sim 15\pm3\) expected for typical DSFGs at \(z\sim1\)–4, favoring a high-redshift interpretation [1910.13331].

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 \(z=5.850\pm0.001\), based on detection of \({}^{12}\mathrm{CO}(J=6\!\to\!5)\) and para-\(\mathrm{H_2O}(2_{11}\!\to\!2_{02})\). For the integrated system, the CO line has \(I_{\rm CO(6-5)}=0.48\pm0.03~{\rm Jy\,km\,s^{-1}}\), \(14.7\sigma\) significance, line width \(700\pm70~{\rm km\,s^{-1}}\), and \(L'_{\rm CO(6-5)}=(1.56\pm0.10)\times10^{10}~{\rm K\,km\,s^{-1}\,pc^2}\). The water line has \(I_{\rm H_2O}=0.09\pm0.02~{\rm Jy\,km\,s^{-1}}\), \(4.3\sigma\) significance, width \(900\pm200~{\rm km\,s^{-1}}\), and \(L'_{\rm H_2O(2_{11}-2_{02})}=(2.5\pm0.5)\times10^{9}~{\rm K\,km\,s^{-1}\,pc^2}\). The CO line is detected in both A and B, whereas the water line is detected only in A [1910.13331].

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 \(870\,\mu{\rm m}\) 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 [1910.13331].

The \(870\,\mu{\rm m}\) continuum shows both components to be extremely compact. Using \(uv\)-plane Sérsic fitting with fixed \(n=1\), the circularized half-light radii are \(R_e(A)=408\pm12\) pc and \(R_e(B)=660\pm60\) pc; the authors adopted \(R_e(A)=380\pm30\) pc and \(R_e(B)=760\pm130\) pc for later calculations. The deconvolved major-axis FWHM at \(870\,\mu{\rm m}\) is \(0\farcs15\pm0\farcs01\) for A and \(0\farcs30\pm0\farcs05\) for B, with axis ratios \(b/a=0.87\pm0.15\) and \(0.37\pm0.24\), respectively [1910.13331].

## 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 \(L_{\rm IR}=(4.0^{+0.9}_{-0.7})\times10^{12}~L_\odot\), \({\rm SFR}=590^{+140}_{-100}~M_\odot\,{\rm yr^{-1}}\), \(\lambda_{\rm peak}=87\pm7~\mu{\rm m}\), \(T_{\rm dust}=56.3^{+5.9}_{-5.7}~{\rm K}\), and \(\beta=1.95\pm0.11\). For component B, it gave \(L_{\rm IR}=(1.5^{+1.1}_{-0.5})\times10^{12}~L_\odot\), \({\rm SFR}=220^{+150}_{-70}~M_\odot\,{\rm yr^{-1}}\), \(\lambda_{\rm peak}=100^{+30}_{-80}~\mu{\rm m}\), \(T_{\rm dust}=29.7^{+8.5}_{-6.6}~{\rm K}\), and \(\beta=2.66^{+0.22}_{-0.34}\). For the integrated system, \(L_{\rm IR}=(6.3^{+1.1}_{-0.9})\times10^{12}~L_\odot\) and \({\rm SFR}=930^{+160}_{-130}~M_\odot\,{\rm yr^{-1}}\) [1910.13331].

The dust mass, derived from the 3 mm continuum with an explicit CMB correction, was estimated as \(M_{\rm dust}(A)=(1.3\pm0.3)\times10^9~M_\odot\), \(M_{\rm dust}(B)=(1.9^{+1.3}_{-0.8})\times10^8~M_\odot\), and \(M_{\rm dust}(A+B)=(1.6^{+0.4}_{-0.3})\times10^9~M_\odot\). Molecular gas masses inferred from the dust continuum were \(M_{\rm gas}(A)=(1.4\pm0.4)\times10^{11}~M_\odot\), \(M_{\rm gas}(B)=(1.2\pm0.5)\times10^{10}~M_\odot\), and \(M_{\rm gas}(A+B)=(1.7\pm0.4)\times10^{11}~M_\odot\). 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 [1910.13331].

Under the preferred low-stellar-mass solution adopted in that study, the total stellar mass was \(M_\star=(3.2^{+1.0}_{-1.5})\times10^9~M_\odot\), implying \(f_{\rm gas}=96^{+1}_{-2}\%\) and a gas-to-dust ratio of \(122^{+64}_{-43}\). The depletion times were \(\tau_{\rm depl}(A)=38^{+16}_{-12}~{\rm Myr}\) and \(\tau_{\rm depl}(B)=80^{+160}_{-40}~{\rm Myr}\), summarized as \(\tau_{\rm depl}=40\)–80 Myr for the system. The star-formation surface densities were \(\Sigma_{\rm SFR}(A)=640\pm170~M_\odot\,{\rm yr^{-1}\,kpc^{-2}}\) and \(\Sigma_{\rm SFR}(B)=60\pm35~M_\odot\,{\rm yr^{-1}\,kpc^{-2}}\), indicating that A in particular is a compact, intense starburst [1910.13331].

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 \(M_\star=(2.1\pm0.7)\times10^{11}~M_\odot\), a discrepancy of about \(60\times\) 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 [1910.13331].

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

High-resolution ALMA \([{\rm C}\,\textsc{ii}]\) \(158\,\mu{\rm m}\) observations and JWST/NIRCam+MIRI imaging substantially revised the internal picture of MAMBO-9. The new ALMA data, with \(0\farcs07\times0\farcs06\) resolution corresponding to about 400 pc, resolve compact dust cores, more diffuse \([{\rm C}\,\textsc{ii}]\)-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 \(A_V\sim1\) and contains a few \(\times10^9\,M_\odot\) in stars, about \(\sim7\%\) of the total stellar mass of the system [2508.06607].

Both galaxies show double-peaked \([{\rm C}\,\textsc{ii}]\) spectra and ordered velocity gradients, but also strong disturbances. The adopted inclinations are \(i=46\pm2^\circ\) for A and \(i=45\pm6^\circ\) for B. With \(V_{\rm rot,max}=570\) km s\(^{-1}\) for A and \(250\) km s\(^{-1}\) for B, the inferred dynamical masses are \(M_{\rm dyn,A}=(1.1\pm0.1)\times10^{11}\,M_\odot\) and \(M_{\rm dyn,B}=(2.0\pm0.5)\times10^{10}\,M_\odot\), 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 [2508.06607].

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 \(A_V>10\), while much of the observed rest-optical light and inferred H\(\alpha\) emission emerge from less obscured outskirts with \(A_V\sim1\)–5. For B, attenuation is lower overall but still substantial. The fiducial stellar masses from the resolved fits are \(M_{\star,A}=1.75^{+0.55}_{-0.42}\times10^{10}\,M_\odot\) and \(M_{\star,B}=6.9^{+2.4}_{-1.8}\times10^9\,M_\odot\), with mass-weighted extinctions \(A_{V,A}=9.9^{+0.4}_{-0.4}\) and \(A_{V,B}=5.2^{+0.5}_{-0.5}\). A NIRSpec PRISM spectrum of B gives \({\rm H}\alpha/{\rm H}\beta=8.3^{+1.2}_{-1.0}\), implying \(A_V=3.6^{+0.5}_{-0.4}\) for the nebular emission and an attenuation-corrected \({\rm SFR}=19.5^{+7.6}_{-5.4}\,M_\odot\,{\rm yr}^{-1}\), far below the FIR- and \([{\rm C}\,\textsc{ii}]\)-based rates. This demonstrates that the observed rest-optical line emission samples only the less obscured star-forming regions [2508.06607].

The new mass budget also altered the preferred gas-conversion factors. Using the requirement that \(M_\star+M_{\rm gas}+M_{\rm dust}\) should not exceed the dynamical mass, the inferred gas masses are \(M_{\rm gas,A}=9.1^{+3.6}_{-2.7}\times10^{10}\,M_\odot\) and \(M_{\rm gas,B}=1.3^{+1.0}_{-0.7}\times10^{10}\,M_\odot\), implying gas fractions \(f_{\rm gas,A}=83^{+2}_{-2}\%\) and \(f_{\rm gas,B}=64^{+8}_{-12}\%\). The required CO-to-H\(_2\) conversion factor is \(\alpha_{\rm CO}\sim1\)–2, with “roughly unity” favored, and the corresponding gas-to-dust ratio is about \(70\)–\(90\). A plausible implication is that the very high \(f_{\rm gas}=96^{+1}_{-2}\%\) estimate in the earlier analysis was driven primarily by the low OIR-only stellar-mass solution rather than by a fully constrained mass budget [2508.06607] [1910.13331].

## 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 \(\Delta z=0.03\) of MAMBO-9, at \(z=5.82\)–5.88, on a scale of roughly \(\sim25\) cMpc. The full structure appears to span most of the PRIMER-COSMOS field, with an extent of order \(\sim40\) cMpc and possible connections over line-of-sight distances approaching \(\sim100\) cMpc to neighboring \(z\sim6\) 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 [2508.06607].

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 \(10^{11}\,M_\odot\), the overdensity supports the interpretation of MAMBO-9 as a prototype of massive galaxy formation at \(z=5.85\) [2508.06607].

## 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 \(850\,\mu{\rm m}\)–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 \(z>4\), the formation of the first massive galaxies, and the buildup of interstellar dust at early times [1910.13331].

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 [2508.06607].

Source: https://www.emergentmind.com/topics/mambo-9