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

ABSTRACT We present the analysis of over 3000 red-Herschel sources (S₂₅₀\, ₌ S₃₅₀\, ₌ S₅₀₀\, ₌) 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 (₅ₖ₇₌1 arcsec; ₁. ₃\, ₌₌ 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 \, ₘₑ^-₁ and molecular gas mass of M₆₀ₒ 10^11 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. ₃₌₌ 13 mJy are gravitationally amplified which, similar to the established S₅₀₀ \, ₌ 100 mJy threshold, can be used as a simple criterion to identify gravitationally lensed galaxies.