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

Abstract

ABSTRACT 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.