We present the analysis of over 3,000 red-$Herschel$ sources ($S_{ \mathrm{ 250\mu m}}<S_{ \mathrm{ 350\mu m}}<S_{ \mathrm{ 500\mu m} }$) 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 arcsecond; $\sigma_{ 1.3\mathrm{ mm}}\sim0.17$ 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 $\sim$ 1,500 $\mathrm{ M_{ \odot} yr^{ -1}}$ and molecular gas mass of $M_{ \mathrm{ gas}}\sim10^{ 11}$ $\mathrm{ M_{ \odot}}$. The median redshift of individual sources is $z\approx2.8$, while the likely lensed systems are at $z\approx3.3$, with redshift distributions extending to $z\sim6$. Our results suggest a common star-formation mode for extreme galaxies across cosmic time, likely triggered by close interactions or disk-instabilities, and with short depletion times consistent with the starburst-type population. Moreover, all galaxies with $S_{ \mathrm{ 1.3mm}}\geq13$ mJy are gravitationally amplified which, similar to the established $S_{ 500\mathrm{ \mu m}}>100$ mJy threshold, can be used as a simple criterion to identify gravitationally lensed galaxies.
Categories: astro-ph.GA