Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The review is comprehensive and clearly written, a very useful addition to the literature. However the total absence of figures makes its reading quite hard for a non-specialist. 

I would then suggest at least the addition of two figures to illustrate the underlying model:

• A model of the accretion disc embedded within the AGN showing the relative position of the water maser source clouds, so that one can appreciate if the lie in the inner or the outer disc, their position w.r.t. Broad- and Narrw Line region clouds, jet, etc;
• A schematic view of the three-peaks line profile of 22 GHz line for rotating water masers.

 

Author Response

General comments :  The review is comprehensive and clearly written, a very useful addition to the literature. However the total absence of figures makes its reading quite hard for a non-specialist. I would then suggest at least the addition of two figures to illustrate the underlying model: 

Comment 1 : A model of the accretion disc embedded within the AGN showing the relative position of the water maser source clouds, so that one can appreciate if the lie in the inner or the outer disc, their position w.r.t. Broad- and Narrw Line region clouds, jet, etc.

Response 1 : I have added a figure (see Figure 1) to provide a schematic view of the maser disk  within the standard AGN paradigm, highlighting its location relative to the torus, broad-line region (BLR), narrow-line region (NLR), and the central black hole. See Page 2 of the revised manuscript.

Comment 2 : A schematic view of the three-peaks line profile of 22 GHz line for rotating water masers. 

Response 2 : A representative spectrum of the triple-peaked maser line profile is included in the revised manuscript as Figure 3. See Page 6.

Reviewer 2 Report

Comments and Suggestions for Authors

This manuscript summarizes the method for measuring the mass of supermassive black holes using water masers and the results obtained. Although the overall content is generally good, it would be better to include some figures that quantitatively show the results obtained.

Major comments

1. Section 3.3. The content of this section is abstract. For example, it is necessary to quantitatively demonstrate, using figures, how much the detection rate of water masers increases for galaxies with high OIII luminosity or red infrared colors.
2. Section 4. It is difficult to understand the necessity of this section. If this section is to be included, please include appropriate figures or tables.
3. Section 5.1.2. It is difficult to understand the concept of the warped disk model through text alone. To help readers understand the warped disk model, diagrams explaining the model and tables summarizing the parameters are necessary.
4. Section 5.3. Since the content of this section is highly relevant to Section 5.1, it would be better to move this section to Section 5.2.
5. Section 5.3. L492. Although it is stated that the SMBH mass obtained from the rotation curve method generally agrees with that obtained from the full disk model method, it would be better to demonstrate this quantitatively. 
6. Although it is stated that the SMBH mass obtained from the rotating curve method generally agrees with that obtained from the 3D disk model method, it would be better to demonstrate this quantitatively. Please include a figure that directly compares the SMBH mass obtained via rotation curve method with that obtained via the full disk model method.curve method with the SMBH mass obtained via the full disk model method.

Minor comments

1. L025. active galactic nuclei -> active galactic nuclei (AGNs)
2. L028. In the case of reverberation mapping, it should be noted that only the SMBH mass of Type 1 AGNs can be measured.
3. L029. active galactic nuclei (AGNs) -> AGNs
4. L034. It should be explicitly stated that SMBH mass measurements using the water maser method are applicable to Type 2 AGNs.
5. Table 1. Column 4. 10^7 solar masses -> solar mass.
6. L570. maser-based black holes -> maser-based black hole masses

Author Response

General comments : This manuscript summarizes the method for measuring the mass of supermassive black holes using water masers and the results obtained. Although the overall content is generally good, it would be better to include some figures that quantitatively show the results obtained.

Response for general comments : I have added five figures to make the points in this paper clearer. Figure 1 provides schematic illustration of a maser disk within the standard AGN paradigm, highlighting its location relative to the torus, broad-line region (BLR), narrow-line region (NLR), and the central black hole. Figure 2 shows a representative maser map and Keplerian rotation curve. In Figure 3, I show the characteristic “triple-peaked” maser spectrum of a maser disk. Figure 4 illustrates how maser detection rates increase with mid-IR colors. Finally, Figure 5 provides a schematic illustration of the 3-dimensional model for maser disk fitting.

Major comments 

Comment 1 : Section 3.3. The content of this section is abstract. For example, it is necessary to quantitatively demonstrate, using figures, how much the detection rate of water masers increases for galaxies with high OIII luminosity or red infrared colors.

Response 1 : I have substantially revised Section 3.1.1 to illustrate how much the detection rate increases with [OIII] luminosity “quantitatively”. Note that I do not include a figure here because I am among the coauthors for this work. Due to the copyright issue as well as the relevant numbers are not given in the original paper (Zhu et al. 2011), I cannot use their figure directly for the review paper.

Comment 2 : Section 4. It is difficult to understand the necessity of this section. If this section is to be included, please include appropriate figures or tables. 

Response 2 : This section is important for this review paper because H2O megamaser is a narrow field and not many people in the world know how to conduct proper VLBI observations to achieve maser-based black hole mass measurement. So, this section is essential for those beginners who want to enter this field and they can learn some keys points for maser observation in this section. The author of the current review paper thinks that adding figures and tables would not make this section clearer. For those who really want to learn maser observations, it would be necessary to work with an expert in person. Nevertheless, the guidances provided in this section can serve as an entering point.          

Comment 3 : Section 5.1.2. It is difficult to understand the concept of the warped disk model through text alone. To help readers understand the warped disk model, diagrams explaining the model and tables summarizing the parameters are necessary.

Response 3 : I have added Figure 5 (see Page 13) to better illustrate the 3D model. The new Table 1 (see Page 14) provides an example for the typical outputs of the 3-dimensional modeling. In addition, I add a few new paragraphs to explain the 3D modeling in more details.

Comment 4 : Section 5.3. Since the content of this section is highly relevant to Section 5.1, it would be better to move this section to Section 5.2.

Response 4 : I have exchanged the order of Section 5.2 and Section 5.3 such that the discussion of maser-based BH mass measurement follows immediately after Section 5.1. The discussion on the error budget now moves to Section 5.3.

Comment 5 : Section 5.3. L492. Although it is stated that the SMBH mass obtained from the rotation curve method generally agrees with that obtained from the full disk model method, it would be better to demonstrate this quantitatively.

Response 5 : I have added Table 2 (see Page 15) in the new Section 5.2 to compare SMBH masses from both methods quantitatively. Note that the number of galaxies that have maser-based BH masses measured with both methods are small (i.e. only five). So, this table already shows all cases which have BH masses measured from both techniques.

1. Although it is stated that the SMBH mass obtained from the rotating curve method generally agrees with that obtained from the 3D disk model method, it would be better to demonstrate this quantitatively. Please include a figure that directly compares the SMBH mass obtained via rotation curve method with that obtained via the full disk model method.curve method with the SMBH mass obtained via the full disk model method.
   Given that the number of galaxies that have maser-based BH masses measured with both methods are small (i.e. only five), using a figure to make the comparison is not necessary. So, in the revised manuscript, I only point the reader to Table 2 to see the comparison.

Minor comments : 

1. L025. active galactic nuclei -> active galactic nuclei (AGNs)
   Response 1 : Correction done.
2. L028. In the case of reverberation mapping, it should be noted that only the SMBH mass
   of Type 1 AGNs can be measured.
   Response 2 : I have added a footnote right after the sentence “ reverberation mapping and virial estimation in Type 1 active galactic nuclei and luminous quasars”. This footnote reads “The footnote “Reverberation mapping and virial estimation can only be applied to Type 1 AGNs.” 
3. L029. active galactic nuclei (AGNs) -> AGNs
   Response 3 : Correction Done.
4. L034. It should be explicitly stated that SMBH mass measurements using the water maser
   method are applicable to Type 2 AGNs.
   Response 4 : Correction done. See Line 65-67 in the revised manuscript.
5. Table 1. Column 4. 10^7 solar masses -> solar mass.
   Response 5 : The caption for column 4 in this table already states that “BH mass in units of solar mass”. 
6. L570. maser-based black holes -> maser-based black hole masses
   Response 6 : Correction made. See Line 650.

Reviewer 3 Report

Comments and Suggestions for Authors

The paper establishes H₂O maser VLBI as a benchmark technique for direct SMBH mass measurements, serving both as a calibrator for indirect scaling relations and as a probe into low-mass AGN populations. It addresses the main limitation, scarcity of suitable maser hosts  and outlines pathways to extend detections to higher redshifts. The review bridges technical VLBI methodology with astrophysical implications for black hole-galaxy coevolution. The review covers many different techniques presented in many papers. It addresses the Cosmology Megamaser prospects that have interesting potential for the Quasars in Cosmology branch of Cosmology (see, Marziani et al. https://www.frontiersin.org/journals/astronomy-and-space-sciences/articles/10.3389/fspas.2019.00080/full).  It would be interesting to address a bit more the improvement of M-sigma relation and Radius-Luminosity relation results, using the results from more robust methods such as megamaser mass estimates, since H₂O megamasers have emerged as the most geometric and bias-resistant method for determining supermassive black hole (SMBH) masses in active galactic nuclei (AGN).   The manuscript presents a thorough, well-organized, and technically sound review of one of the most robust methods for direct SMBH mass determination. The authors provide a clear synthesis of observational advances from the Megamaser Cosmology Project, discuss the theoretical and methodological context with precision, and highlight the cosmological implications of maser-based measurements for scaling relations. I find the paper scientifically strong, with no major methodological flaws, and highly relevant to the community. Overall, the manuscript is scientifically sound, clearly written, and of high relevance to both AGN specialists and the wider astrophysical community. I recommend it for publication.

Author Response

General comments :

The paper establishes H2O maser VLBI as abenchmark techniquefor direct SMBH mass measurements, serving both as a calibrator for indirect scaling relations and as a probe into low-mass AGN populations. It addresses the main limitation, scarcity of suitable maser hosts and outlines pathways to extend detections to higher redshifts. The review bridges technical VLBI methodology with astrophysical implications for black hole-galaxy coevolution. The review covers many different techniques presented in many papers. It addresses the Cosmology Megamaser prospects that have interesting potential for the Quasars in Cosmology branch of Cosmology (see, Marziani et al. 

https://www.frontiersin.org/journals/astronomy-and-space- sciences/articles/10.3389/fspas.2019.00080/full). It would be interesting to address a bit more the improvement of M-sigma relation and Radius-Luminosity relation results, using the results from more robust methods such as megamaser mass estimates, since H2O megamasers have emerged as the most geometric and bias-resistant method for determiningsupermassive black hole (SMBH) masses in active galactic nuclei (AGN). The manuscript presents a thorough, well- organized, and technically sound review of one of the most robust methods for direct SMBH mass determination. The authors provide a clear synthesis of observational advances from the Megamaser Cosmology Project, discuss the theoretical and methodological context with precision, and highlight the cosmological implications of maser-based measurements for scaling relations. I find the paper scientifically strong, with no major methodological flaws, and highly relevant to the community. Overall, the manuscript is scientifically sound, clearly written, and of high relevance to both AGN specialists and the wider astrophysical community. I recommend it for publication. 

Response for general comments :

We thank the reviewer's recommendation for the publication for this review paper. Regarding the point about "the improvement of the M-sigma relation",  this review paper already address this point in Section 6. As we discuss in this section, the maser-based black hole masses reveal possible deviations from the canonical M_BH–σ∗ relation. Instead of improving the relation, we pointed out that the mean maser MBH is offset downward by about –0.6 dex relative to the relation defined by early-type galaxies. In other words, adding maser-based BH mass worsens the correlation between M_BH and σ∗, suggesting that the MBH–σ∗ correlation may not be entirely universal across galaxy types. Since this point has been addressed, we do not include additional words to discuss the improvement of the relation.

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

Thank you for revising the manuscript. Thanks to the revisions, the manuscript now clearly outlines the methodology for measuring SMBH masses using water maser observations, along with the current results and future prospects.

Author Response

    I thank the reviewer for providing positive feedback for the revised manuscript !