ALMA follow-up of ~ 3,000 red-Herschel galaxies: the nature of extreme submillimeter galaxies

We present the analysis of over 3,000 red-$Herschel$ sources (S250μ m<S350μ m<S500μ m) using public data from the ALMA archive and the $Herschel$-ATLAS survey. This represents the largest sample of red-$Herschel$ sources with interferometric follow-up observations to date. The high ALMA angular resolution and sensitivity (θFWHM~1 arcsecond; σ1.3mm~0.17 mJy beam⁻¹) allow us to classify the sample into individual sources, multiple systems, and potential lenses and/or close mergers. Interestingly, even at this high angular resolution, 73 per cent of our detections are single systems, suggesting that most of these galaxies are isolated and/or post-merger galaxies. For the remaining detections, 20 per cent are classified as multiple systems, 5 per cent as lenses and/or mergers, and 2 per cent as low-z galaxies or Active Galactic Nuclei. Combining the $Herschel$/SPIRE and ALMA photometry, these galaxies are found to be extreme and massive systems with a median star formation rate of ~ 1,500 M_ yr⁻¹ and molecular gas mass of Mgas~10¹¹ M_. The median redshift of individual sources is z≈2.8, while the likely lensed systems are at z≈3.3, with redshift distributions extending to z~6. The inferred depletion times are consistent with the starburst-type population with a mild redshift evolution. Our results suggest a common star-formation mode for extreme galaxies across cosmic time, likely triggered by close interactions or disk-instabilities. Moreover, all galaxies with S1.3mm≥13 mJy are gravitationally amplified which, similar to the established S500 μ m>100 mJy threshold, can be used as a simple criterion to identify gravitationally lensed galaxies.