ALMA reveals a dust-obscured galaxy merger at cosmic noon

Context. Galaxy mergers play a critical role in galaxy evolution. They alter the size, morphology, dynamics, and composition of galaxies. Galaxy mergers have so far mostly been identified through visual inspection of their rest-frame optical and near-IR (NIR) emission. Dust can obscure this emission, however, resulting in the misclassification of mergers as single galaxies and in an incorrect interpretation of their baryonic properties. Aims. Having serendipitously discovered a dust-obscured galaxy merger at z = 1.17, we aim to determine the baryonic properties of the two merging galaxies, including the star formation rate (SFR) and the stellar, molecular gas and dust masses. Methods. Using Band 3 and 6 observations from the Atacama Large Millimeter and submillimeter Array (ALMA) and ancillary data, we studied the morphology of this previously misclassified merger. We deblended the emission, derived the gas masses from CO observations, and modeled the spectral energy distributions to determine the properties of each galaxy. Using the rare combination of ALMA CO(2–1), CO(5–4) and dust-continuum (rest-frame 520 μm) observations, we provide insight into the gas and dust content and into the properties of the interstellar medium of each merger component. Results. We find that only one of the two galaxies is highly obscured by dust, but both are massive (> 1010.5 M⊙) and highly star forming (SFR = 60 − 900 M⊙/yr), have a moderate-to-short depletion time (tdepl < 0.7 Gyr) and a high gas fraction (fgas ≥ 1). Conclusions. These properties can be interpreted as the positive impact of the merger. With this serendipitous discovery, we highlight the power of (sub)millimeter observations to identify and characterise the individual components of obscured galaxy mergers.