Phase-Space Correlations among Systems of Satellite Galaxies

Round 1

Reviewer 1 Report

Dear Editors,

I have read with interest the manuscript entitled "Phase-Space Correlations Among Systems of Satellite Galaxies".  It is a comprehensive description of the status of this field, as it has developed over the last few years and especially with the advent of Gaia.

I have no concerns with the paper as presented. It is primarily a review, with few new results but with very good commentary on the status of the field, and some very useful and attractive figures that help expand on the issues that the author highlights. I noticed only one typo, in the introduction: "lineare".I  only had one other small comment, which is that the auhor seems to suggest that recent work confidently excludes Carina and Fornax from membership of the Magellanic group of galaxies (page 18). However, I note that this issue is far from settled - a more recent paper by Santos-Santos et al. (2021) suggests that it is possible for these galaxies, and especially Carina, to be members (see also Battaglia et al. 2021).

I expect that this paper will be a very useful, and likely well cited, introduction to this field for students and professional astronomers alike.

Author Response

Reply to reviewer reports, Galaxies special issue review on Satellite Phase-Space Correlations

I would like to thank all three reviewers for their reports, and for the helpful suggestions and questions contained therein. I have revised the manuscript according to these comments, and have also added some other discussions because highly relevant literature became available since the original version was submitted. Changes or additions in the manuscript (except for simply corrections of typos) are highlighted in boldface. Below, I summarize all changes. I hope that with these minor revisions the manuscript can now be accepted for publication.

Sect. 1:

Following the comment by one of the reviewers, at the end of the introduction I now explicitly point out that the review, due to its scope and in order to keep it sufficiently brief, will not focus on the many internal properties of dwarf galaxies (such as their age, SFHs, velocity dispersions, …) that are being studied and can provide important hints at their origins. I also cite an additional paper comparing such properties for on- and off-plane satellites of M31 as a highly relevant example (Collins et al. 2015).

Sect. 2.2.2:

Three recent publications have addressed signatures of satellite planes outside of the Local Group. One is focussed on the NGC 253 system and confirms a recently proposed spatial alignment with additional satellite discoveries. Another reports a strong kinematic correlation among the satellites of NGC 2750 and argues this to be an analog of the Centaurus A satellite system. The other is an extensive study of the spatial distribution of dwarf galaxy candidates around host galaxies in the MATLAS survey, which reports identifying a quite high fraction of significant spatially flattened arrangements. Both are now discussed in the manuscript (Mutlu-Pakdil et al. 2021, Paudel et al. 2021, and Heesters et al. 2021, respectively).

Section 2.3:

I have added the references of Santos-Santos et al. (2021) and Battaglia et al. (2021) to the discussion about whether Carina and Fornax can be considered satellites of the LMC. The text previously suggested that this is a settled issue, though as correcly highlighted by one of the referees, these two recent references do still allow for this possibility. The respective studies do, however, stress that such an association appears unlikely, in particular in case of Fornax. My added discussion reflects this.

Finally, there is one reviewer comment I decided not to modify the manuscript for, but I would like to respond in more detail here. This is in regards to the suggestion to discuss commonalities in the physics of planets orbiting the Sun in a common plane and satellite galaxy planes, and whether the satellite planes could have a similar origin as planetary systems.

The planets are believed to form out of a gas and dust cloud at the center of which their host star forms. Due to cooling and angular momentum conservation, a spinning disk of gas and debris forms, and within this planets assemble, thereby explaining why they share a common orbital plane and direction. What is essential for this is that the gas is collisional. In contrast, in the standard cosmological model, dwarf galaxies are believed to be embedded in massive dark matter halos. Their baryonic content is only a tiny fraction (possibly as low as 1/1000) of their total mass. Consequently, their dynamics are governed by that of dark matter. Cold dark matter is, by definition, collisionless. Thus, a primordial dark matter cloud would not transform into a spinning disk out of which the dwarf galaxies could condense into a common orbital plane. Thus, unfortunately the difference to the physics of dark matter dominated dwarf galaxies prevents us from learning much from this otherwise very attractive analogy. In a sense, a scenario that maybe comes closest to the planet formation one is provided by the tidal dwarf galaxies. TDGs do form out of coherent, gaseous material, though flung out interacting galaxies.

Of course, if one were willing to give up on the standard model of cosmology and adopt e.g. a modified gravity approach, one might envision to construct a more similar scenario to the planet formation. I suppose that in a modified gravity framework, large gas clouds might cool, form a disk, and fragment on scales of 100s of kpc. However, assessing this scenario’s internal consistency and feasibility in light of observational constraints is difficult, and so determining whether and is possible or even realistic would require a dedicated research program. I am not aware of any published studies in this regard, and since performing such work myself is clearly beyond the scope of this review, I have decided not to include such a discussion in the manuscript.

Reviewer 2 Report

The manuscript reviews the phase-space correlations among satellite galaxy systems.  It is well-written, clear and timely.

Author Response

Reply to reviewer reports, Galaxies special issue review on Satellite Phase-Space Correlations

I would like to thank all three reviewers for their reports, and for the helpful suggestions and questions contained therein. I have revised the manuscript according to these comments, and have also added some other discussions because highly relevant literature became available since the original version was submitted. Changes or additions in the manuscript (except for simply corrections of typos) are highlighted in boldface. Below, I summarize all changes. I hope that with these minor revisions the manuscript can now be accepted for publication.

Sect. 1:

Following the comment by one of the reviewers, at the end of the introduction I now explicitly point out that the review, due to its scope and in order to keep it sufficiently brief, will not focus on the many internal properties of dwarf galaxies (such as their age, SFHs, velocity dispersions, …) that are being studied and can provide important hints at their origins. I also cite an additional paper comparing such properties for on- and off-plane satellites of M31 as a highly relevant example (Collins et al. 2015).

Sect. 2.2.2:

Three recent publications have addressed signatures of satellite planes outside of the Local Group. One is focussed on the NGC 253 system and confirms a recently proposed spatial alignment with additional satellite discoveries. Another reports a strong kinematic correlation among the satellites of NGC 2750 and argues this to be an analog of the Centaurus A satellite system. The other is an extensive study of the spatial distribution of dwarf galaxy candidates around host galaxies in the MATLAS survey, which reports identifying a quite high fraction of significant spatially flattened arrangements. Both are now discussed in the manuscript (Mutlu-Pakdil et al. 2021, Paudel et al. 2021, and Heesters et al. 2021, respectively).

Section 2.3:

I have added the references of Santos-Santos et al. (2021) and Battaglia et al. (2021) to the discussion about whether Carina and Fornax can be considered satellites of the LMC. The text previously suggested that this is a settled issue, though as correcly highlighted by one of the referees, these two recent references do still allow for this possibility. The respective studies do, however, stress that such an association appears unlikely, in particular in case of Fornax. My added discussion reflects this.

Finally, there is one reviewer comment I decided not to modify the manuscript for, but I would like to respond in more detail here. This is in regards to the suggestion to discuss commonalities in the physics of planets orbiting the Sun in a common plane and satellite galaxy planes, and whether the satellite planes could have a similar origin as planetary systems.

The planets are believed to form out of a gas and dust cloud at the center of which their host star forms. Due to cooling and angular momentum conservation, a spinning disk of gas and debris forms, and within this planets assemble, thereby explaining why they share a common orbital plane and direction. What is essential for this is that the gas is collisional. In contrast, in the standard cosmological model, dwarf galaxies are believed to be embedded in massive dark matter halos. Their baryonic content is only a tiny fraction (possibly as low as 1/1000) of their total mass. Consequently, their dynamics are governed by that of dark matter. Cold dark matter is, by definition, collisionless. Thus, a primordial dark matter cloud would not transform into a spinning disk out of which the dwarf galaxies could condense into a common orbital plane. Thus, unfortunately the difference to the physics of dark matter dominated dwarf galaxies prevents us from learning much from this otherwise very attractive analogy. In a sense, a scenario that maybe comes closest to the planet formation one is provided by the tidal dwarf galaxies. TDGs do form out of coherent, gaseous material, though flung out interacting galaxies.

Of course, if one were willing to give up on the standard model of cosmology and adopt e.g. a modified gravity approach, one might envision to construct a more similar scenario to the planet formation. I suppose that in a modified gravity framework, large gas clouds might cool, form a disk, and fragment on scales of 100s of kpc. However, assessing this scenario’s internal consistency and feasibility in light of observational constraints is difficult, and so determining whether and is possible or even realistic would require a dedicated research program. I am not aware of any published studies in this regard, and since performing such work myself is clearly beyond the scope of this review, I have decided not to include such a discussion in the manuscript.

Reviewer 3 Report

Reviewer has made available a PDF for comments. The review is nicely done and accessible to the audience of physicists and astrophysicists.

Comments for author File: Comments.pdf

Author Response

Reply to reviewer reports, Galaxies special issue review on Satellite Phase-Space Correlations

I would like to thank all three reviewers for their reports, and for the helpful suggestions and questions contained therein. I have revised the manuscript according to these comments, and have also added some other discussions because highly relevant literature became available since the original version was submitted. Changes or additions in the manuscript (except for simply corrections of typos) are highlighted in boldface. Below, I summarize all changes. I hope that with these minor revisions the manuscript can now be accepted for publication.

Sect. 1:

Following the comment by one of the reviewers, at the end of the introduction I now explicitly point out that the review, due to its scope and in order to keep it sufficiently brief, will not focus on the many internal properties of dwarf galaxies (such as their age, SFHs, velocity dispersions, …) that are being studied and can provide important hints at their origins. I also cite an additional paper comparing such properties for on- and off-plane satellites of M31 as a highly relevant example (Collins et al. 2015).

Sect. 2.2.2:

Three recent publications have addressed signatures of satellite planes outside of the Local Group. One is focussed on the NGC 253 system and confirms a recently proposed spatial alignment with additional satellite discoveries. Another reports a strong kinematic correlation among the satellites of NGC 2750 and argues this to be an analog of the Centaurus A satellite system. The other is an extensive study of the spatial distribution of dwarf galaxy candidates around host galaxies in the MATLAS survey, which reports identifying a quite high fraction of significant spatially flattened arrangements. Both are now discussed in the manuscript (Mutlu-Pakdil et al. 2021, Paudel et al. 2021, and Heesters et al. 2021, respectively).

Section 2.3:

I have added the references of Santos-Santos et al. (2021) and Battaglia et al. (2021) to the discussion about whether Carina and Fornax can be considered satellites of the LMC. The text previously suggested that this is a settled issue, though as correcly highlighted by one of the referees, these two recent references do still allow for this possibility. The respective studies do, however, stress that such an association appears unlikely, in particular in case of Fornax. My added discussion reflects this.

Finally, there is one reviewer comment I decided not to modify the manuscript for, but I would like to respond in more detail here. This is in regards to the suggestion to discuss commonalities in the physics of planets orbiting the Sun in a common plane and satellite galaxy planes, and whether the satellite planes could have a similar origin as planetary systems.

The planets are believed to form out of a gas and dust cloud at the center of which their host star forms. Due to cooling and angular momentum conservation, a spinning disk of gas and debris forms, and within this planets assemble, thereby explaining why they share a common orbital plane and direction. What is essential for this is that the gas is collisional. In contrast, in the standard cosmological model, dwarf galaxies are believed to be embedded in massive dark matter halos. Their baryonic content is only a tiny fraction (possibly as low as 1/1000) of their total mass. Consequently, their dynamics are governed by that of dark matter. Cold dark matter is, by definition, collisionless. Thus, a primordial dark matter cloud would not transform into a spinning disk out of which the dwarf galaxies could condense into a common orbital plane. Thus, unfortunately the difference to the physics of dark matter dominated dwarf galaxies prevents us from learning much from this otherwise very attractive analogy. In a sense, a scenario that maybe comes closest to the planet formation one is provided by the tidal dwarf galaxies. TDGs do form out of coherent, gaseous material, though flung out interacting galaxies.

Of course, if one were willing to give up on the standard model of cosmology and adopt e.g. a modified gravity approach, one might envision to construct a more similar scenario to the planet formation. I suppose that in a modified gravity framework, large gas clouds might cool, form a disk, and fragment on scales of 100s of kpc. However, assessing this scenario’s internal consistency and feasibility in light of observational constraints is difficult, and so determining whether and is possible or even realistic would require a dedicated research program. I am not aware of any published studies in this regard, and since performing such work myself is clearly beyond the scope of this review, I have decided not to include such a discussion in the manuscript.