ALMA follow-up of ∼ 3000 red-Herschel galaxies: The nature of extreme submillimeter galaxies

We present the analysis of over 3000 red-Herschel sources (S250\, μ m< S350\, μ m< S500\, μ 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 (θ FWHM~1 arcsec; σ 1.3\, mm~ 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 ~1500 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. Our results suggest a common star formation mode for extreme galaxies across cosmic time, likely triggered by close interactions or disc-instabilities, and with short depletion times consistent with the starburst-type population. 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.