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ABSTRACT We present Large Millimeter Telescope (LMT)/AzTEC 1.1 mm observations of ∼100 luminous high-redshift dusty star-forming galaxy candidates from the $$\sim 600\,$$ sq.deg Herschel-ATLAS survey, selected on the basis of their SPIRE red far-infrared colours and with $$S_{500\, \mu \rm m}=35-80$$ mJy. With an effective $$\theta _{\rm FWHM}\approx 9.5\,$$arcsec angular resolution, our observations reveal that at least 9 per cent of the targets break into multiple systems with signal-to-noise ratio ≥4 members. The fraction of multiple systems increases to ∼23 per cent (or more) if some non-detected targets are considered multiples, as suggested by the data. Combining the new AzTEC and deblended Herschel photometry, we derive photometric redshifts, infrared luminosities, and star formation rates. While the median redshifts of the multiple and single systems are similar (zmed ≈ 3.6), the redshift distribution of the latter is skewed towards higher redshifts. Of the AzTEC sources, ∼85 per cent lie at zphot > 3 while ∼33 per cent are at zphot > 4. This corresponds to a lower limit on the space density of ultrared sources at 4 < z < 6 of $$\sim 3\times 10^{-7}\, \textrm {Mpc}^{-3}$$ with a contribution to the obscured star formation of $$\gtrsim 8\times 10^{-4}\, \textrm {M}_\odot \, \textrm {yr}^{-1} \, \textrm {Mpc}^{-3}$$. Some of the multiple systems have members with photometric redshifts consistent among them suggesting possible physical associations. Given their angular separations, these systems are most likely galaxy over-densities and/or early-stage pre-coalescence mergers. Finally, we present 3 mm LMT/RSR spectroscopic redshifts of six red-Herschel galaxies at zspec = 3.85−6.03, two of them (at z ∼ 4.7) representing new redshift confirmations. Here, we release the AzTEC and deblended Herschel photometry as well as catalogues of the most promising interacting systems and z > 4 galaxies. 

We present images obtained with LABOCA on the APEX telescope of a sample of 22 galaxies selected via their red Herschel SPIRE 250-, 350- and $$500\textrm{-}\mu\textrm{m}$$ colors. We aim to see if these luminous, rare and distant galaxies are signposting dense regions in the early Universe. Our $$870\textrm{-}\mu\textrm{m}$$ survey covers an area of $$\approx0.8\,\textrm{deg}^2$$ down to an average r.m.s. of $$3.9\,\textrm{mJy beam}^{-1}$$, with our five deepest maps going $$\approx2\times$$ deeper still. We catalog 86 DSFGs around our 'signposts', detected above a significance of $$3.5\sigma$$. This implies a $$100\pm30\%$$ over-density of $$S_{870}>8.5\,\textrm{mJy}$$ DSFGs, excluding our signposts, when comparing our number counts to those in 'blank fields'. Thus, we are $$99.93\%$$ confident that our signposts are pinpointing over-dense regions in the Universe, and $$\approx95\%$$ confident that these regions are over-dense by a factor of at least $$\ge1.5\times$$. Using template SEDs and SPIRE/LABOCA photometry we derive a median photometric redshift of $$z=3.2\pm0.2$$ for our signposts, with an interquartile range of $$z=2.8\textrm{-}3.6$$. We constrain the DSFGs likely responsible for this over-density to within $$|\Delta z|\le0.65$$ of their respective signposts. These 'associated' DSFGs are radially distributed within $$1.6\pm0.5\,\textrm{Mpc}$$ of their signposts, have median SFRs of $$\approx(1.0\pm0.2)\times10^3\,M_{\odot}\,\textrm{yr}^{-1}$$ (for a Salpeter stellar IMF) and median gas reservoirs of $$\sim1.7\times10^{11}\,M_{\odot}$$. These candidate proto-clusters have average total SFRs of at least $$\approx (2.3\pm0.5)\times10^3\,M_{\odot}\,\textrm{yr}^{-1}$$ and space densities of $$\sim9\times10^{-7}\,\textrm{Mpc}^{-3}$$, consistent with the idea that their constituents may evolve to become massive ETGs in the centers of the rich galaxy clusters we see today.