High-redshift star formation in the ALMA era

The Atacama Large Millimetre/submillimetre Array (ALMA) is currently in the process of transforming our view of star-forming galaxies in the distant ($z\gtrsim1$) universe. Before ALMA, most of what we knew about dust-obscured star formation in distant galaxies was limited to the brightest submillimetre sources$-$the so-called submillimetre galaxies (SMGs)$-$and even the information on those sources was sparse, with resolved (i.e., sub-galactic) observations of the obscured star formation and gas reservoirs typically restricted to the most extreme and/or strongly lensed sources. Starting with the beginning of early science operations in 2011, the last nine years of ALMA observations have ushered in a new era for studies of high-redshift star formation. With its long baselines, ALMA has allowed observations of distant dust-obscured star formation with angular resolutions comparable to$-$or even far surpassing$-$the best current optical telescopes. With its bandwidth and frequency coverage, it has provided an unprecedented look at the associated molecular and atomic gas in these distant galaxies through targeted follow-up and serendipitous detections/blind line scans. Finally, with its leap in sensitivity compared to previous (sub-)millimetre arrays, it has enabled the detection of these powerful dust/gas tracers much further down the luminosity function through both statistical studies of color/mass-selected galaxy populations and dedicated deep fields. We review the main advances ALMA has helped bring about in our understanding of the dust and gas properties of high-redshift ($z\gtrsim1$) star-forming galaxies during these first nine years of its science operations, and we highlight the interesting questions that may be answered by ALMA in the years to come.

This review article is a remarkable work, well written and quite complete about high-z star forming galaxies, ALMA has revolutionize the domain, and it is timely to make the census of all new results now. The authors have explained in detail what was the state of the art before ALMA, and described all the progress due to ALMA, keeping also space to SCUBA2 and NOEMA. The text is well structured, (continuum dust studies, line molecular gas, etc..) and the Figures very well done and relevant.
I have only very minor comments. Figure 2 computes the sensitivity for a PWV of 3mm, whatever the sites. It might be good to add which percentage of the time this is obtained in each site. Indeed, the ALMA altitude of 5500m and desertic site makes these conditions very common, and even much lower PWV are possible, while they are quite rare in the other sites. This will explain better the amplitude of the ALA revolution.
p.18: comparison between local ULIRGs and SMG, the latter being bluer. "This difference suggests a lower average dust attenuation, which could be due to the fact that high-redshift SMGs may be more extended than local ULIRGs.". I guess this is not the obvious conclusion, the most obvious being that at high-z galaxies have lower metallicity, and therefore lower dust attenuation, for the same radial extension. Independent observations show that galaxies at high-z are more compact, more clumpy, even if more gaseous. So the stellar component could be bluer, may be also because the stars are in average younger.
In Section 2.3.3 when the GMC and star formation scaling laws are described, and only a few clumps are derived from lensing galaxies, you should cite the recent work on the Snale https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1115D/abstract where a lot more clumps have been detected In Section 3.2.1, when discussing Sigma_SFR going higher than the Eddington limit, you should caution that the FIR could be excited by a central AGN, and the true SFR could be over-estimated. The fraction of AGN-heated FIR is increasing with mass and L_FIR.
Note that in Section 3.2.8, the discussion on the existence or not of the clump could be enlighted by the example of the Snake (Dessauges-Zavadsky et al 2019), that was mentioned earlier.
Section 3.5: influence of the CMB. Indeed, the contrast of the dust/gas emission in the outer parts of a galaxy is lower, as mention in Zhang et al (2016). However, it is true at any redshift, even at z=0 "For example, in galaxies where the dust temperature decreases with radius, the cool dust in the outer regions might not be visible against the CMB background, and therefore the size of the dust-emitting region might be underestimated for galaxies at high redshifts." >> at any redshift in fact.

Very complete and impressive work! The referee
Review form: Reviewer 2 Is the manuscript scientifically sound in its present form? Yes Two, the SkA. While it is too low n frequency for molecular one work, continuum sensitivity, and potential resolution is very impressive, and continuum and radio-recombination lines might make a substantial contribution here. The author's net rest in CMB effects could provide a route in here?
The most substantial suggestion I have is to include more information about ALMA's Surface brightness sensitivity being inevitably limited at finest resolution, which I thin should be noted, as integration times for interesting observations can become "challenging". For non-experts, I suspect this issue might be under appreciated, and the minute-long integrations that can produce impressive results seem strange when compared with the hours required to resolve CO lines.
I wonder whether there's a way to highlight this near Fig. 2? Minor comments: Very end of Section 2.2.2 -isn't the multiplicity changing with depth already largely clear? It ught help to be more specific about the future approach required to getting samples suitable to understand this better at this point in the manuscript, or to highlight the relevant discussion at the conclusion. Section 2.2.5. Don't the substantial SPT samples indicate that a high-flux cutoff must be modest? With decent lending models, they do seem to have some sources substantially brighter than 10mJy? This seems to be revisited in Section 2.3.1, but it might be a good idea to link it here, and discuss this factor. Section 2.2.6. Median redshift is perhaps not all you'd like to know -there's also issue of the shape of the whole distribution, and perhaps especially of the high-redshift tail, since detecting the restframe FIR vs UV continues to get easier at higher z? The absence of a long tail I suggest gives a general hint that the first light might be a job for JWST, or points beyond. Section 3.2.7. Does SKA not have potential to compare with next-generation VLA here? Sensitive low-frequency continuum and radio recombination lines seem to have potential value. If the authors disagree, it would still be worth a gentle note about this large facility that's on its way.
Boundary of P46/47. Can substantial AGN contamination not be ruled out already by spatial information? The AGN emission would have to lurk unresolved? The lenses samples having complex, but enhanced resolution in the source plane might help to address this, even if the ratios are difficult to pin down? Multiplicity itself also suggests AGN can't dominate?
On P49. Young dust with low-density/colloidal/dendritic structure can provide more opacity for lower mass. Early dust may indeed be different, and something that is discussed a little later, but perhaps should appear earlier as a caveat, which could be a positive thing.for future investigations, in [CII] vs continuum, Nd perhaps JWST mid-IR spectra?
Overall, I thought it was a comprehensive and informative review. The biggest concern I had was a warning about surface brightness sensitivity at the highest resolution. At present I suspect the presentation doesn't reflect this challenge to ALMA's rate of discovery; without being critical, perhaps more managing expectations.

Decision letter (RSOS-200556.R0)
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Once again, thank you for submitting your manuscript to Royal Society Open Science and I look forward to receiving your revision. If you have any questions at all, please do not hesitate to get in touch. This review article is a remarkable work, well written and quite complete about high-z star forming galaxies, ALMA has revolutionize the domain, and it is timely to make the census of all new results now. The authors have explained in detail what was the state of the art before ALMA, and described all the progress due to ALMA, keeping also space to SCUBA2 and NOEMA. The text is well structured, (continuum dust studies, line molecular gas, etc..) and the Figures very well done and relevant.
I have only very minor comments. Figure 2 computes the sensitivity for a PWV of 3mm, whatever the sites. It might be good to add which percentage of the time this is obtained in each site. Indeed, the ALMA altitude of 5500m and desertic site makes these conditions very common, and even much lower PWV are possible, while they are quite rare in the other sites. This will explain better the amplitude of the ALA revolution.
p.18: comparison between local ULIRGs and SMG, the latter being bluer. "This difference suggests a lower average dust attenuation, which could be due to the fact that high-redshift SMGs may be more extended than local ULIRGs.". I guess this is not the obvious conclusion, the most obvious being that at high-z galaxies have lower metallicity, and therefore lower dust attenuation, for the same radial extension. Independent observations show that galaxies at high-z are more compact, more clumpy, even if more gaseous. So the stellar component could be bluer, may be also because the stars are in average younger.
In Section 2.3.3 when the GMC and star formation scaling laws are described, and only a few clumps are derived from lensing galaxies, you should cite the recent work on the Snale https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1115D/abstract where a lot more clumps have been detected In Section 3.2.1, when discussing Sigma_SFR going higher than the Eddington limit, you should caution that the FIR could be excited by a central AGN, and the true SFR could be over-estimated. The fraction of AGN-heated FIR is increasing with mass and L_FIR.
Note that in Section 3.2.8, the discussion on the existence or not of the clump could be enlighted by the example of the Snake (Dessauges-Zavadsky et al 2019), that was mentioned earlier.
Section 3.5: influence of the CMB. Indeed, the contrast of the dust/gas emission in the outer parts of a galaxy is lower, as mention in Zhang et al (2016). However, it is true at any redshift, even at z=0 "For example, in galaxies where the dust temperature decreases with radius, the cool dust in the outer regions might not be visible against the CMB background, and therefore the size of the dust-emitting region might be underestimated for galaxies at high redshifts." >> at any redshift in fact.
Very complete and impressive work! The referee Reviewer: 2 Comments to the Author(s) Overall, I found the manuscript a useful and comprehensive review.
I would suggest perhaps that the conclusions section is made more "punchy", and that the authors consider a list format for some of the most crucial results there, and perhaps a more highlighted set of future recommendations for key work?
A couple of future opportunities seem to be potentially missing, One is the capacity of very-wide-band correlations, as being highlighted by SmA and NOEMA, and while could update ALMA.
Two, the SkA. While it is too low n frequency for molecular one work, continuum sensitivity, and potential resolution is very impressive, and continuum and radio-recombination lines might make a substantial contribution here. The author's net rest in CMB effects could provide a route in here?
The most substantial suggestion I have is to include more information about ALMA's Surface brightness sensitivity being inevitably limited at finest resolution, which I thin should be noted, as integration times for interesting observations can become "challenging". For non-experts, I suspect this issue might be under appreciated, and the minute-long integrations that can produce impressive results seem strange when compared with the hours required to resolve CO lines.
I wonder whether there's a way to highlight this near Fig. 2? Minor comments: Very end of Section 2.2.2 -isn't the multiplicity changing with depth already largely clear? It ught help to be more specific about the future approach required to getting samples suitable to understand this better at this point in the manuscript, or to highlight the relevant discussion at the conclusion.
Section 2.2.5. Don't the substantial SPT samples indicate that a high-flux cutoff must be modest? With decent lending models, they do seem to have some sources substantially brighter than 10mJy? This seems to be revisited in Section 2.3.1, but it might be a good idea to link it here, and discuss this factor. Section 2.2.6. Median redshift is perhaps not all you'd like to know -there's also issue of the shape of the whole distribution, and perhaps especially of the high-redshift tail, since detecting the restframe FIR vs UV continues to get easier at higher z? The absence of a long tail I suggest gives a general hint that the first light might be a job for JWST, or points beyond. Section 3.2.7. Does SKA not have potential to compare with next-generation VLA here? Sensitive low-frequency continuum and radio recombination lines seem to have potential value. If the authors disagree, it would still be worth a gentle note about this large facility that's on its way.
Boundary of P46/47. Can substantial AGN contamination not be ruled out already by spatial information? The AGN emission would have to lurk unresolved? The lenses samples having complex, but enhanced resolution in the source plane might help to address this, even if the ratios are difficult to pin down? Multiplicity itself also suggests AGN can't dominate?
On P49. Young dust with low-density/colloidal/dendritic structure can provide more opacity for lower mass. Early dust may indeed be different, and something that is discussed a little later, but perhaps should appear earlier as a caveat, which could be a positive thing.for future investigations, in [CII] vs continuum, Nd perhaps JWST mid-IR spectra?
Overall, I thought it was a comprehensive and informative review. The biggest concern I had was a warning about surface brightness sensitivity at the highest resolution. At present I suspect the presentation doesn't reflect this challenge to ALMA's rate of discovery; without being critical, perhaps more managing expectations.

Author's Response to Decision Letter for (RSOS-200556.R0)
See Appendix A.

Decision letter (RSOS-200556.R1)
We hope you are keeping well at this difficult and unusual time. We continue to value your support of the journal in these challenging circumstances. If Royal Society Open Science can assist you at all, please don't hesitate to let us know at the email address below.
Dear Elisabete, It is a pleasure to accept your manuscript entitled "High-redshift star formation in the ALMA era" in its current form for publication in Royal Society Open Science.
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Reply to the Referees
We would like to thank both referees for taking the time to read our manuscript and provide valuable feedback which has improved the review. We would also like to apologize for the delay in our response.

Reviewer 1:
This review article is a remarkable work, well written and quite complete about high-z star forming galaxies, ALMA has revolutionize the domain, and it is timely to make the census of all new results now. The authors have explained in detail what was the state of the art before ALMA, and described all the progress due to ALMA, keeping also space to SCUBA2 and NOEMA. The text is well structured, (continuum dust studies, line molecular gas, etc..) and the Figures very well done and relevant.
We thank referee #1 for the extremely positive reaction and specific comments, which we address below.
I have only very minor comments. Figure 2 computes the sensitivity for a PWV of 3mm, whatever the sites. It might be good to add which percentage of the time this is obtained in each site. Indeed, the ALMA altitude of 5500m and desertic site makes these conditions very common, and even much lower PWV are possible, while they are quite rare in the other sites. This will explain better the amplitude of the ALA revolution.
We thank the referee for this good suggestion, and we have added the following to the caption of Figure  2: "A PWV of ≤3mm occurs ∼10%, ∼35%, ∼65%, and ∼80% of the time for CARMA, the PdBI/NOEMA, the SMA, and ALMA, respectively." p.18: comparison between local ULIRGs and SMG, the latter being bluer. "This difference suggests a lower average dust attenuation, which could be due to the fact that highredshift SMGs may be more extended than local ULIRGs.". I guess this is not the obvious conclusion, the most obvious being that at high-z galaxies have lower metallicity, and therefore lower dust attenuation, for the same radial extension. Independent observations show that galaxies at high-z are more compact, more clumpy, even if more gaseous. So the stellar component could be bluer, may be also because the stars are in average younger.
We agree with referee #1 that this is true in general. However, if we compare SMGs with ULIRGs, which are a very specific class of IR-bright galaxies, this actually isn't necessarily the case. SMGs are more extended than ULIRGs and there's evidence that they're already quite metal-rich (also from lines of evidence like high dust masses). We have therefore edited this section of the text to read: "This difference suggests a lower average dust attenuation despite similar dust masses (e.g., da Cunha et al. 2010), which could be due to the fact that high-redshift SMGs may be more extended than local ULIRGs and/or the dust and stellar distributions are not co-located (e.g., Hodge et al. 2016)." In Section 2.3.3 when the GMC and star formation scaling laws are described, and only a few clumps are derived from lensing galaxies, you should cite the recent work on the Snale https://ui.adsabs.harvard.edu/abs/2019NatAs...3.1115D/abstract where a lot more clumps have been detected

We thank the referees for pointing us toward this impressive work, which was still embargoed when we wrote this section. We have added a discussion on it in section 2.3.3: "Meanwhile, Dessauges-Zavadsky et al. (2019) used 30-pc ALMA mapping of the CO(4-3) emission in the z = 1.036 'Cosmic Snake'
to identify 17 molecular clouds in this Milky Way progenitor. They measured the masses, surface densities and supersonic turbulence implied by these clouds, reporting values 10-100 times higher than present-day analogues, and bringing into question the universality of GMCs. It is important to note that the Cosmic Snake has one of the largest magnification factors known for a giant arc (80±10; Ebeling et al. 2009), making this sort of study rare even in the era of ALMA." In Section 3.2.1, when discussing Sigma_SFR going higher than the Eddington limit, you should caution that the FIR could be excited by a central AGN, and the true SFR could be over-estimated. The fraction of AGN-heated FIR is increasing with mass and L_FIR.

We agree this is a good point. We have edited the relevant paragraph in Section 3.2.1 to read: "Taking the extent of the FIR emission as a proxy for the extent of the dusty star formation, one of the immediate implications of the measured FIR sizes is for the global SFR surface densities (ΣSFR) of highredshift sources. (We note that the assumption is generally made that the FIR emission is heated primarily by star formation, with negligible AGN contribution; this could be wrong especially for the most massive sources)."
Note that in Section 3.2.8, the discussion on the existence or not of the clump could be enlighted by the example of the Snake (Dessauges-Zavadsky et al 2019), that was mentioned earlier.

Indeed, we have now re-introduced the cosmic snake earlier in that section "For example, Dessauges-Zavadsky et al. (2019) reported the discovery of 17 GMCs in the CO(4-3) emission of the z = 1.036 Milky
Way progenitor the 'Cosmic Snake'. This is an exceptionally strongly lensed galaxy, providing a sourceplane resolution as high as 30 pc, and allowing the GMCs to be studied at a resolution comparable to CO observations of nearby galaxies." Then, during the discussion on clumps, we have added: " Dessauges-Zavadsky et al. (2019) report comparable mass distributions for their (CO-identified) GMCs and stellar clumps as those seen in (respectively) the gravitationally bound gas clouds and stellar clumps produced by simulations of fragmenting gas-rich disks (Tamburello et al. 2015); however, the GMCs and stellar clumps are still not co-located." Section 3.5: influence of the CMB. Indeed, the contrast of the dust/gas emission in the outer parts of a galaxy is lower, as mention in Zhang et al (2016). However, it is true at any redshift, even at z=0 "For example, in galaxies where the dust temperature decreases with radius, the cool dust in the outer regions might not be visible against the CMB background, and therefore the size of the dust-emitting region might be underestimated for galaxies at high redshifts." >> at any redshift in fact.
Indeed. To this end, and in the context of this review, we have added "(this is true for galaxies at any redshift, but particularly relevant for ALMA observations at high redshifts)."

Very complete and impressive work!
We once again thank the referee for their thorough reading and the good points they raised.

Reviewer 2:
Overall, I found the manuscript a useful and comprehensive review.
We thank referee #2 for the positive report and suggestions, which we address below.
I would suggest perhaps that the conclusions section is made more "punchy", and that the authors consider a list format for some of the most crucial results there, and perhaps a more highlighted set of future recommendations for key work?
A couple of future opportunities seem to be potentially missing, One is the capacity of very-wide-band correlations, as being highlighted by SmA and NOEMA, and while could update ALMA.
Two, the SkA. While it is too low n frequency for molecular one work, continuum sensitivity, and potential resolution is very impressive, and continuum and radio-recombination lines might make a substantial contribution here. The author's net rest in CMB effects could provide a route in here?
We appreciate the referees suggestions, and we have worked to address them as best as possible. To begin with, although we prefer to keep the paragraph format, we have edited the conclusions to try to better emphasize our main points. (We will not list all of the changes here, but direct the referee to the pdf). Regarding the capacity of very-wide-band correlations, we thank the referee for bringing up this excellent point. Please note that we already discuss the potential update to ALMA's instantaneous BW and the opportunities it would lead to in the final paragraph of conclusions. We have now updated this discussion to explicitly mention the NOEMA and SMA correlators. Finally, regarding the SKA, indeed, there was previously only a limited discussion of the SKA. We have added a new paragraph in the conclusions to focus on non-traditional line studies, including dense gas tracers and radiorecombination lines, and the potential for future SKA/ngVLA/ALMA B1 there (where the SKA is actually not necessarily too low frequency if we consider SKA Band-5). We have avoided a discussion of the continuum impact of SKA as it is beyond the scope of this review.
The most substantial suggestion I have is to include more information about ALMA's Surface brightness sensitivity being inevitably limited at finest resolution, which I thin should be noted, as integration times for interesting observations can become "challenging". For non-experts, I suspect this issue might be under appreciated, and the minute-long integrations that can produce impressive results seem strange when compared with the hours required to resolve CO lines.
I wonder whether there's a way to highlight this near Fig. 2? We understand the referee's point and have made a number of edits throughout the manuscript to address it. In particular: -We have edited the caption of Figure 2 to add "We caution that for all interferometers (including ALMA), there is an inherent tradeoff between spatial resolution and surface brightness sensitivity, which is not reflected in this figure." -We have added a caveat in the Introduction (Section 1.2) where we discuss ALMA's improved sensitivity: "We note that, like all interferometers, ALMA is still limited by the unavoidable tradeoff between spatial resolution and surface brightness sensitivity. ALMA offers the ACA to help improve the imaging of extended structures, but this limitation should nevertheless be kept in mind, particularly for observations with the most extended configurations." -When discussing SDP.81 in Section 2.3.3 on 'kpc and pc-scale studies', we have added this caveat to the first paragraph: "We note that all of the targets for the Long Baseline Campaign were chosen specifically to demonstrate the suitability of the long baseline capability (ALMA Partnership et al. 2015a), and that even despite the relatively compact size of SDP.81's Einstein ring (θE ∼ 1.5ʹʹ), a large amount of total observing time was required (∼9-12 hours per band) in order to achieve good uvcoverage (ALMA Partnership et al. 2015b). As a result, high-resolution ALMA imaging of this quality is still relatively uncommon." -In the following paragraph, we have edited the the first sentence (adding part between **) to read: "From the source plane reconstructions, ALMA imaging of strongly lensed sources *with sufficiently good image quality* allows detailed investigations of the dusty star formation and ISM…" -We have added the caveat to the opening paragraph of Section 3.2 on resolved studies: "*We note that due to surface brightness sensitivity limitations,* much of the most detailed/highest-resolution work with ALMA has necessarily still focused on submillimetre-bright sources…" -Regarding the referee's point about resolved CO lines, we note that the time required to resolve CO lines isn't due purely to surface brightness sensitivity, but rather due to the intrinsic strength (or weakness) of the lines and the narrower bandwidths of observability. We have clarified this point in Section 3.2.3: "...resolved CO studies are particularly time-intensive due to the brightness of the lines and the more limited bandwidths over which they are observable (compared to the dust continuum), and this remains true even with ALMA." -We have added a clarifying sentence in the Conclusions in the paragraph on resolved observations: "While the highest resolution imaging possible with ALMA will necessarily remain limited to select sources due to the inherent tradeoff between spatial resolution and surface brightness sensitivity…"

Minor comments:
Very end of Section 2.2.2 -isn't the multiplicity changing with depth already largely clear? It ught help to be more specific about the future approach required to getting samples suitable to understand this better at this point in the manuscript, or to highlight the relevant discussion at the conclusion.

2.3).*"
Section 2.2.5. Don't the substantial SPT samples indicate that a high-flux cutoff must be modest? With decent lending models, they do seem to have some sources substantially brighter than 10mJy? This seems to be revisited in Section 2.3.1, but it might be a good idea to link it here, and discuss this factor.
We have added a footnote in this section that this hypothesis is regarding the intrinsic number counts, and not the bright-end sources seen in ultra-wide field surveys, which are dominated by lensed sources. The latter can of course be used to come at the problem from the other direction, which we have noted and pointed to Section 2.3.1, where we indeed discuss whether the magnifications factors agree with the unlensed source counts: "Note that this hypothesis is regarding the intrinsically bright sources, and not the bright end sources in ultra-wide field surveys which are found to be dominated by lensed sources. The latter, however, can also help inform the debate if the lensing magnification factors are well-constrained (see Section 2.3.1)." In addition, we have updated the quoted SFR cutoff range to 1000-2000 Msol/yr to correctly reflect the Barger+14 result, and we have corrected the average SPT magnification quoted in Section 2.3.1 to 6.3.
Section 2.2.6. Median redshift is perhaps not all you'd like to know -there's also issue of the shape of the whole distribution, and perhaps especially of the high-redshift tail, since detecting the restframe FIR vs UV continues to get easier at higher z? The absence of a long tail I suggest gives a general hint that the first light might be a job for JWST, or points beyond.
We agree with the referee that the shape of the distribution would be the ultimate goal, but this is actually quite challenging given the spectroscopic incompleteness in current samples. This is discussed in the 3rd paragraph of Section 2.2.6, and we now clarify this further by emphasizing that spectroscopic completeness rates are <50% even for the most well-studied extragalactic fields: "More importantly, even with the correct SMG counterpart(s) identified through interferometry, obtaining spectroscopic redshifts in the optical/IR is still very challenging due to the faintness/dust-obscured nature of the galaxies, *resulting in completeness rates for unlensed SMG samples of <50% for even the most wellstudied extragalactic fields (e.g., We also add a note in Table 2 regarding the reliance of z_median estimates on photometric redshifts: "Median redshift estimates for the samples are usually heavily reliant on photometric redshifts for individual sources -see Section 2.2.6." We agree that the high-z tail is the most interesting aspect, which is already discussed in the 4th paragraph. It is true that the absence of a long tail means JWST will be important, but SMGs aren't the only way to probe the EOR (as discussed later in the review, including in reference to the REBELS ALMA Large program), so we refrain from commenting further on this point here. Lastly, we have added a reference to the updated SPT paper (Reuter 2020) which has since appeared on the ArXiv. Section 3.2.7. Does SKA not have potential to compare with next-generation VLA here? Sensitive lowfrequency continuum and radio recombination lines seem to have potential value. If the authors disagree, it would still be worth a gentle note about this large facility that's on its way.
We agree and have added a reference and relevant citation: "This is therefore an area that is ripe for future work, not just with the current ALMA, but also with the future Band 1/2 receivers, the VLA, the proposed ngVLA, *and the SKA (Wagg et al. 2015)*." Boundary of P46/47. Can substantial AGN contamination not be ruled out already by spatial information? The AGN emission would have to lurk unresolved? The lenses samples having complex, but enhanced resolution in the source plane might help to address this, even if the ratios are difficult to pin down? Multiplicity itself also suggests AGN can't dominate?
Indeed the implication is that the weak AGN in the model mentioned would be spatially unresolved. Given that most of the galaxies discussed in that section are unresolved or marginally resolved, the current spatial information is not sufficient to resolve AGN. We have clarified in the text that these would be unresolved AGN.
On P49. Young dust with low-density/colloidal/dendritic structure can provide more opacity for lower mass. Early dust may indeed be different, and something that is discussed a little later, but perhaps should appear earlier as a caveat, which could be a positive thing.for future investigations, in [CII] vs continuum, Nd perhaps JWST mid-IR spectra?
We agree that the properties of the dust can change the opacity and that this may be different in the early universe, as already discussed in this section. We have edited it to emphasize this more clearly: "Typically, the dust emissivity properties at high-redshifts are assumed to be similar to those measured in the local Universe, simply because we lack empirical measurements. However, both theoretical models and laboratory studies indicate that different types of dust grains could have widely different emissivity properties ( Overall, I thought it was a comprehensive and informative review. The biggest concern I had was a warning about surface brightness sensitivity at the highest resolution. At present I suspect the presentation doesn't reflect this challenge to ALMA's rate of discovery; without being critical, perhaps more managing expectations.
We thank the referee for the detailed read and good comments they supplied. We hope our reply above on surface brightness sensitivity adequately addresses their concern.
Additional Changes: In addition to the changes motivated by the referee reports (discussed above), we have made the following improvements to the manuscript: