ALMA follow-up of $\sim$ 3,000 red-Herschel galaxies: the nature of extreme submillimeter galaxies

Abstract

We present the analysis of over 3,000 red-$Herschel$ sources ($S_{250\mu \mathrm{m}}<S_{350\mu \mathrm{m}}<S_{500\mu \mathrm{m}}$) 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 }_{\mathrm{FWHM}}\sim$1 arcsecond; ${\sigma }_{1.3\mathrm{mm}}\sim 0.17$ 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 $\sim$ 1,500 ${\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ 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 disk-instabilities, and with short depletion times consistent with the starburst-type population. Moreover, all galaxies with $S_{1.3mm}\ge 13$ mJy are gravitationally amplified which, similar to the established $S_{500\mu \mathrm{m}}>100$ mJy threshold, can be used as a simple criterion to identify gravitationally lensed galaxies.

15 pages, 12 figures. Accepted for publication in MNRAS