ALMA follow-up of ~3000 red-Herschel galaxies: the nature of extreme submillimetre galaxies.

We present the analysis of over 3000 red- Herschel sources (⁠|$S_{\mathrm{250\, \mu m}}\lt S_{\mathrm{350\, \mu m}}\lt S_{\mathrm{500\, \mu m} }$|⁠) 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 (⁠|$\theta _{\rm FWHM}\sim$| 1 arcsec; |$\sigma _{1.3\mathrm{\, mm}}\sim 0.17$|  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 |$\sim$| 1500  |$\mathrm{M_{\odot }\, yr^{-1}}$| and molecular gas mass of |$M_{\mathrm{gas}}\sim 10^{11}$|   |$\mathrm{M_{\odot }}$|⁠. The median redshift of individual sources is |$z\approx 2.8$|⁠ , while the likely lensed systems are at |$z\approx 3.3$|⁠ , with redshift distributions extending to |$z\sim 6$|⁠. 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 |$S_{\mathrm{1.3mm}}\ge 13$|  mJy are gravitationally amplified which, similar to the established |$S_{500\mathrm{ \, \mu m}}\gt 100$|  mJy threshold, can be used as a simple criterion to identify gravitationally lensed galaxies. [ABSTRACT FROM AUTHOR]