The ALMA data were obtained in Cycle 3 through proposal 2015.1.00540.S (PI: R. A. A. Bowler). We observed the six brightest LBGs at 6.5  $z$  7.5 presented in Bowler et al. (2012, 2014). The galaxies are all within the COSMOS field (Scoville et al. 2007a),1 and were initially selected in the near-IR YJHKs data based on the UltraVISTA2 survey data release 2 (McCracken et al. 2012). The coordinates of the six galaxies, which defined each ALMA pointing, and their photometric redshifts are shown in Table 1. The galaxies were selected via SED fitting and are secure $z$ > 6.5 objects as demonstrated by the sharp spectral break observed in the photometric data at ${\simeq } 1\, {\mu} {\rm m}$ (Fig. A1). The strength of the break ($z - Y \gt 2\, {\rm mag}$) observed in the sample cannot be reproduced by the common low-redshift or brown-dwarf contaminants (e.g. Ouchi et al. 2010; Bouwens et al. 2011; Bowler et al. 2012). Each target was observed for 10 min in Band 6 (centred at $233\, {\rm GHz}$ or $1.29\, {\rm mm}$; corresponding to ∼170 ${\mu}$m in the rest frame). The integration time was chosen to provide limits on the ratio of FIR to UV luminosity sufficient to distinguish between the previous results at $z$ > 5 from Watson et al. (2015) and Capak et al. (2015). The data were taken in a compact configuration C36-2, with a maximum baseline of 330 m, on 2016 January 25. In order to maximize continuum sensitivity, the correlator was set up in time division mode with 2 GHz bandwidth across each of four spectral windows (centred at 224, 226, 240, 242 GHz) in two polarizations. The data were reduced with the ALMA pipeline of casa version 4.5.1. Calibration was performed by ALMA personnel using J1058+0133, J0948+0022, and Ganymede for bandpass, phase, and flux calibration, respectively. We then processed the data through the imaging pipeline, collapsing the full 8 GHz bandwidth and both polarizations into a single-continuum image of each field, initially using natural weighting of baselines to obtain the optimal root-mean-square (rms) sensitivity for point sources. Imaging was achieved using the tclean task, and the data were cleaned by defining a mask for any source in the field with a peak signal-to-noise ratio (SNR) > 3 that also corresponds to a Hubble Space Telescope(HST)-detected galaxy (whether the target or serendipitous), with a clean threshold of 60 ${\mu}$Jy (≃ 2 × rms). The natural weighted images have a beam size of 1.1 × 1.4 arcsec full width at half-maximum (FWHM) and reach an average rms sensitivity of $27\, {\mu} {\rm Jy}\,{\rm beam}^{-1}$, with small deviations of $2 \, {\mu} {\rm Jy}\,{\rm beam}^{-1}$ between the different pointings. To increase the sensitivity to extended flux, we also tried imaging the targets with a 1 arcsec taper, which results in a beam size of 1.4 × 1.7 arcsec and rms sensitivity of $31\, {\mu} {\rm Jy}\,{\rm beam}^{-1}$. As the natural weighted images have a smaller beam and better rms sensitivity, we chose to present this weighting throughout the paper, however comment on the tapered results where relevant. We measured fluxes using two complementary methods. First, fluxes were measured from the peak flux within an aperture of radius 1 arcsec from the HST/WFC3 centroid. The peak flux assumes that the source is unresolved in the ALMA data. For the stacks and the detected individual source, we also measured the total flux using a Gaussian fit. In the case of a non-detection at the 3σ level for the individual LBGs, we conservatively provide 2σ upper limits calculated as $f_{\rm peak} + 2\, \times \, {\rm rms}$.
Citation: Bowler et al. (2012
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Citation: Bowler et al. 2012
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