tokens: ALMA data (Decarli et al. 2017 ; Champagne et al. 2018 ). In Figure 9 we show the postage stamps of our follow-up observations. Figure 9. Source adjacent to the quasar J2211, detected solely in the dust-continuum emission, i.e., with no secure redshift measure. Top: postage stamps (20′′ × 20′′) of our follow-up observations; labels are as in Figure 3 . Bottom: spectral energy distribution of J2211c. We assume that the source is located at the same redshift of the quasar. We report our photometric measurements/limits and, for comparison, various templates of local galaxies and the best SED fit from MAGPHYS -high- z . The labels and templates are as in Figure 6 . J2211c SED results to be intermediate between the low- z ULIRG Arp220 and the starbursting galaxy M82 (see Section 3.1 ). On the basis of our follow-up observations, and considering the predicted density of millimeter-sources, we are not able to exclude that this source is a fore/background (see the text for details, and Champagne et al. 2018 ). In analogy with the companions discussed in the main body of the paper, we compare the SED of J2211c with those of local galaxies, and we fit our photometric data with MAGPHYS -high- z (see Figure 9 ). From the latter, we find that the SED of J2211c is better reproduced by a galaxy model in between Arp 220 and M82 (i.e., a powerful local ULIRG and a starburst), with M * ∼ 3 × 10 10 M ⊙ and SFR ∼ 130 M ⊙ yr −1 . We further measure the obscured/unobscured SFR ratio of J2211c, following the procedure used for PJ167c (see Section 3.2 ). The star formation rate is dominated by the obscured contribution (SFR UV ∼ 2 M ⊙ yr −1 and f obscured ∼ 0.99). We report all these estimates in Table 6 . The lack of a secure redshift confirmation prevents us from drawing further conclusions on the nature of this source, or from placing it in the context of previous observations. Appendix B Quasar Photometry In the framework of our study of companion galaxies, we also perform forced photometry at the position of the quasars in the Spitzer /IRAC and HST /WFC3 images (see Section 2 for methodology). In Table 7 we report the derived quasars’ photometry. In Figure 10 , we show the quasars SEDs. The fluxes measured in our follow-up data are consistent with those expected from the observed optical/NIR spectra, when available, and/or from a lower- z quasar template (Selsing et al. 2016 ) rescaled to match the observed J band magnitude. Figure 10. Spectral energy distribution of the quasars in our sample. The observed photometric measurements (filled points) are obtained from our new follow-up data and from the literature (see Table 7 ; the filter responses are reported in the lower right panel). We also show the available optical/NIR spectra (light gray; see also Mazzucchelli et al. 2017b ), and a lower-redshift composite template shifted at the redshift of the quasar (black solid line; Selsing et al. 2016 ). The location of the Ly α line is highlighted with a light blue
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