Review Reports - On the Origin and Nature of Double-Double Radio Galaxies

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

The authors explain an observed correlation between the age of the inner jet and quiescent time in a DDRG with the picture of continued accretion but a change in the rotation of the accretion disk with respect to the spinning black hole. They present a first order theoretical framework for this idea, and make predictions. They also discuss the role and impact of AGN jet feedback.

The paper as it is written is rather difficult to critique. It is too sweeping in places and lacks robustness. There are not enough references from DDRG observations, theory or simulations that are not the authors’ own work. No comparison is made between the authors’ theory and other theories that exist for DDRGs in the literature, which might in fact explain the observations better.

I list my comments below that must be considered by the authors before this paper can be considered for publication.

1. Considering that (accretion disk - jet) simulations will be essential and in fact be the first step towards the testing of ideas presented in this paper, the paper should only be presented as an alternate suggestion that needs future testing. The tone of the paper should be reflective of that.

2. The tables and figures need proper formatting - it is currently extremely sloppy. The empty column in Table 2 is not even explained in the caption. The caption to Figure 2 is not following Figure 2. The equations need proper formatting.

3. For the reader to not have to go to Marecki et al (2021), the authors must clearly state how t_in, t_out and t_quies, were estimated. Which methods were used for obtaining these ages? Were they obtained similarly or consistently for the entire DDRG sample? Please explain in a new paragraph in Section 2.1.

4. Please look at Konar et al. 2013, MNRAS, 430, 2137 (Table 9) and add these data to your analysis. Please look at Brocksopp et al. 2011, MNRAS, 410, 484, and add those data as well. If some of these data have already been included, mention these references in the paper.

5. The paper makes several speculative claims about FRI/FRII formation (spin values and direction, plus entrainment) and AGN feedback - that it actually operates and AGN jets enhance or suppress star-formation. Surely these are all topics that are not settled and some nuances are in order. It will be good to edit the paper to present the main ideas without going into too many sweeping generalisations about spins, entrainment, AGN feedback.

Author Response

I list my comments below that must be considered by the authors before this paper can be considered for publication.

Considering that (accretion disk - jet) simulations will be essential and in fact be the first step towards the testing of ideas presented in this paper, the paper should only be presented as an alternate suggestion that needs future testing. The tone of the paper should be reflective of that.

Author’s reply: We have changed the tone appropriately.

The tables and figures need proper formatting - it is currently extremely sloppy. The empty column in Table 2 is not even explained in the caption. The caption to Figure 2 is not following Figure 2. The equations need proper formatting.

Author’s reply: Fixed.

For the reader to not have to go to Marecki et al (2021), the authors must clearly state how t_in, t_out and t_quies, were estimated. Which methods were used for obtaining these ages? Were they obtained similarly or consistently for the entire DDRG sample? Please explain in a new paragraph in Section 2.1.

Author’s reply” We have commented on this and therefore on the nature of the uncertainties.

Please look at Konar et al. 2013, MNRAS, 430, 2137 (Table 9) and add these data to your analysis. Please look at Brocksopp et al. 2011, MNRAS, 410, 484, and add those data as well. If some of these data have already been included, mention these references in the paper.

Author’s reply: Unfortunately, there are no new sources. In fact, many of the sources in Marecki et al 2021 come from Konar et al 2013. We have added these references.

The paper makes several speculative claims about FRI/FRII formation (spin values and direction, plus entrainment) and AGN feedback - that it actually operates and AGN jets enhance or suppress star-formation. Surely these are all topics that are not settled and some nuances are in order. It will be good to edit the paper to present the main ideas without going into too many sweeping generalisations about spins, entrainment, AGN feedback.

Author’s reply: These are features of the model that have allowed us to explain within our framework many observations (references added). But these physical effects are forced to occur in tandem in a well-constrained way which makes them easy to test. The general tone of our paper, however, is now different as you can see.

Reviewer 2 Report

Comments and Suggestions for Authors

A pdf file of the report is also attached.

The authors suggest a possible relationship between the lifetime of the retriggered jet activity and the period of quiescence in a sample of double-double radio galaxies, defined as those with two distinct cycles of episodic jet activity. They interpret this relationship in terms of transition from counter-rotation to co-rotation of supermassive black holes and the time scales involved depending on the accretion rates.

Exploring any relationship between periods of activity or periods of quiescence with other physical parameters is interesting for further understanding the nature of radio galaxies with episodic jet activity, but this needs to be done more rigorously and systematically using a large sample of double-double radio galaxies.

The authors started with a small sample of ten sources (Table 1 of their paper) from a compilation by Marecki et al. (2021) and stated that because these show a relation of “low statistical significance”, they “searched for additional objects to improve the numbers” and added five more from the literature (Table 2 of their paper). Of the additional five, 4C29.30 is already listed in Table 1 under its IAU name (J0840+2949), J1548-3216 occurs in both the Tables with the same name, while 3C293 is also listed in Table 1 under its IAU name (J1352+3126). The sample compiled by Marecki et al. (2021) appears to be largely to compare the parameters they derived for the source they studied in this paper with other sources from the literature. The assumptions and methodology adopted by the different authors referred to in the references cited by Marecki et al. (2021) for the list of sources vary widely. Model parameters used by Machalski et al. (2011) referred to in the compilation differ from those of Brocksopp et al. (2011). For J0840+2949, Marecki et al. (2021) refer to the paper by Jamrozy et al. (2007) but there does not seem to be a period of quiescence quoted in that paper. This source has the highest value in the compilation. If it has been derived from the Jamrozy et al. (2007) paper the assumptions are unclear. The values quoted in Marecki et al. (2021) for the quiescent time scale of J1835+6204 are 1.0–6.6 Myr taken from Konar et al. (2012). These values estimated by Konar et al. were under the assumption that the North-Western hotspot, which has a prominent hotspot unlike the South-Eastern one, is on the far side. This assumption is questionable as the location of the hotspots relative to the core do not appear to be consistent with this assumption. However, the authors list the quiescent time scale in their paper as 21.0 – 6.6 Myr.

A minor point on the classification of the radio structure of 3C293. Although lower resolution observations show some diffuse emission beyond the hotspots, the MERLIN 5-GHz image shows clear hotspots on opposite sides of the inner double (see Floyd et al. 2006 and references therein). Regions of diffuse emission beyond hotspots have been seen in other radio sources as well. I would classify it as an FRII. Its radio luminosity would also be consistent with this classification.

The relationship needs to be explored using a well-defined large sample of double-double radio galaxies and estimating their periods of activity and quiescence in a systematic, uniform and transparent way stating clearly the assumptions made and equations used. At present there are over 200 double-double radio galaxies known. There are 111 giant double-double radio galaxies in the LoTSS DR2 region alone (Dabhade et al. 2025). In order to explore a range of time scales for such a study, it is also advisable to consider a wide range of source sizes. In its present form, I am not confident of the relationship and unless explored more rigorously as outlined above, I am unable to recommend the manuscript for publication.

Comments for author File: Comments.pdf

Author Response

Author’s reply: Unfortunately, the data does not yet exist. We changed some of the language emphasizing that the data is not close to conclusive and that this is an attempt at starting a conversation.

Author’s reply: Thank you for noticing this. We have redone the statistics with only 12 objects.

The sample compiled by Marecki et al. (2021) appears to be largely to compare the parameters they derived for the source they studied in this paper with other sources from the literature. The assumptions and methodology adopted by the different authors referred to in the references cited by Marecki et al. (2021) for the list of sources vary widely. Model parameters used by Machalski et al. (2011) referred to in the compilation differ from those of Brocksopp et al. (2011). For J0840+2949, Marecki et al. (2021) refer to the paper by Jamrozy et al. (2007) but there does not seem to be a period of quiescence quoted in that paper. This source has the highest value in the compilation. If it has been derived from the Jamrozy et al. (2007) paper the assumptions are unclear. The values quoted in Marecki et al. (2021) for the quiescent time scale of J1835+6204 are 1.0–6.6 Myr taken from Konar et al. (2012). These values estimated by Konar et al. were under the assumption that the North-Western hotspot, which has a prominent hotspot unlike the South-Eastern one, is on the far side. This assumption is questionable as the location of the hotspots relative to the core do not appear to be consistent with this assumption. However, the authors list the quiescent time scale in their paper as 21.0 – 6.6 Myr.

Author’s reply: We have added comments that address these issues and fixed the range referred to.

Author’s reply: We eliminated the comment on FRI jet as a result.

Author’s reply: As stated above, the data does not seem to exist. We changed the tone of the paper by emphasizing that we are encouraging observers to investigate this in a more detailed way.

Reviewer 3 Report

Comments and Suggestions for Authors

The manuscript "The Origin and Nature of Double-Double Radio Galaxies" by D. Garofalo et al. (galaxies-3993465) proposes an explanation for the observed properties of the so-called double-double radio galaxies (DDRGs) as a class of objects. It promotes a model where the accretion on the central supermassive black hole (SMBH) is continuous, while the jet production is determined by its spin parameter. Initial jet activity assumes an SMBH that is counter-rotating with respect to the accretion disk. Then the black hole spin approaches 0, causing a pause in the jet production. Finally, an already co-rotating black hole which gains high enough spin leads to a new start of jet activity. According to the authors, their model is consistent with the claimed correlation between the length of the new activity cycle and the time of quiescence, as well as the fact that DDRGs belong to the Fanaroff-Riley II (FRII) class of radio galaxies.

In principle, I find this idea worth publishing because it may trigger discussions in the community, eventually leading to a better understanding of the DDRG phenomenon. However, at this stage, I believe the arguments supporting this model are not particularly strong. Therefore, this study must be labelled and presented as simplistic and quite speculative. This could start with a less imperative title, e.g. "On the Origin and Nature of Double-Double Radio Galaxies".

Below I collect some points supporting my opinion, starting with concerns about the data, and then questions about the model. Finally, I also list a bunch of formal issues which must be dealt with, before this manuscript can be seriously reviewed and considered for acceptance for publication, following a major revision.

(1) Concerns about the data

A. The authors say that "the sample size of 10 is small", and then they add just 5 more sources from the literature. I agree, and I would say even the extended sample is very small - probably insufficient to draw general conclusions for the DDRGs as a class. Note that in the order of 100 DDRGs are currently known in the literature. Even though not all of them have age estimates, a deeper literature search might reveal some more. How much effort did the authors spend on compiling a more complete list? I can accept the fact that ~15 is the largest possible sample to date, but would like to read about how the authors made the search.

B. These age estimates are rather uncertain, and come from different sources, derived with possibly varying methodologies. In real life, they must be associated with errors but I see here no attempt to estimate or characterise them. Moreover, some ages fall into quite wide ranges. These are apparently plotted as "error bars" in the figures, but I doubt any errors or uncertainties are actually taken into account when calculating the correlation coefficients. Am I right? If yes, it is not surprising while the correlation coefficients are so suspiciously large. My point is that this kind of statistics does not make sense without considering "realistic" uncertainties of the measurements. A side note: it is said in line 63 that average values are plotted. Taking a look at the top point in Fig. 1 - the one with T_quies = 100 -, it does not seem to be the average of anything for the source 4C 29.30 from Table 2. In any case, the authors should make an attempt to handle and discuss the errors, and to explain what the error bars indicate in the plots.

C. The statement that DDRGs are FRIIs (line 84) is based on a rather old reference (Schoenmakers et al. 2020). It may well be still valid in the light of more recent discoveries made in the past quarter of a century, but more recent references would be useful here. What I miss the most is the discussion or at least a mention of alternative scenarios. Especially the role of selection effects here which must be huge I believe. Isn't it more difficult to identify DDRGs with diffuse FRI-like inner lobes? Aren't the FRI-like relic outer lobes intrinsically less powerful, and fading more quickly below the detection threshold, making them less likely to be detected? The bottom line is that the statistics of DDRGs in terms of FR classification may be heavily affected by selection biases. See, for instance, Mahatma et al. (2019) who, by definition, excluded candidates with FRI morphology from their DDRG sample (see their Sect. 2.2.2). If DDRGs are intentionally chosen as sources displaying edge-brightened FRII morphology, it is not surprising that we end up with a sample containing FRIIs only...

(2) Concerns about the model

A. The theoretical framework of this study is of Garofalo et al. (2010). If I understand correctly, it postulates that powerful FRII jets are produced when the SMBH has high-enough retrograde spin with respect to that of the accretion disk. On the other hand, SMBHs with high prograde spins tend to be radio-quiet AGN. In this model, it is just unclear to me how the jet activity is restarted in FRII-type DDRGs, after the initially retrograde-accreting SMBH (a<-0.1) gains sufficiently high spin (a>0.1) in the prograde regime. In other words, why do jets "turn on" once again after the SMBH spin direction is reversed? Doesn't the Garofalo et al. (2010) model predict otherwise?

B. There are a couple of triple-double radio galaxies known (at least 4 of them, see 2023MNRAS.525L..87C). These have evidence for 3 cycles of activity. How the model presented here explains this phenomenon in terms of the spin-direction changes?

C. There is much discussion about the role of accretion rate determining the length of jet activity periods. On would naively think that not only the rate of accretion but the total amount of the matter available to be accreted by the SMBH from its environment would play a significant role here. Is it the same for all AGN? How does this fit in the model?

(3) Concerns about how the manuscript is presented

A. Sometimes the terms "jets" and "lobes" are not used consequently. For example, the introduction says "Double-double radio galaxies (DDRG) display inner and outer jets thought to result from intermittent accretion", although, strictly speaking, the outer lobes are no longer fed by jets because the activity cycle once produced them is already over.

B. Table 1 list the literature compilation from Marecki et al. (2021). It would be fair to refer to the original publications here as well. Last row, last column of Table 1: instead of "21.0 - 6.6", Marecki et al. list in their Table 7 "1.0 - 6.6". Please double-check for typos the data entered in the tables.

C. While error bars (for certain data points) are plotted in Figs. 1-3, they are not presented in the tables (with one exception). This makes the correlation analysis impossible to reproduce.

D. The 3rd column of Table 2 is empty. Why is it needed then?

E. The typesetting of the formulae (mostly on page 5) is so unpretending that is makes the equations barely readable.

F. lines 148-150: "...have been fleshed out in the literature by this author over the past decade so we refer the reader to that literature for more detail" - I guess "this author" means here the first author, since there are 2 other co-authors of the paper; I suggest to specifically cite those literature references here. In line 235, "this author" appears once again.

G. The numbering of equations suddenly starts over in line 167.

H. Sect. 2.2 is overly lengthy, I suggest to divide it into smaller logical blocks.

I. The mandatory section on author contributions is missing from the end.

J. Two of the listed references (#3 and #20) are actually not cited in the text, this should be fixed, as well as the missing bibliographic details of reference #3 (Symmetry, 17, 171)

Author Response

Author’s reply: Indeed, we have modified the title but also added language in the data analysis section emphasizing the lack of statistical significance and pointing out that we are in fact trying to start a conversation.

(1) Concerns about the data

The authors say that "the sample size of 10 is small", and then they add just 5 more sources from the literature. I agree, and I would say even the extended sample is very small - probably insufficient to draw general conclusions for the DDRGs as a class. Note that in the order of 100 DDRGs are currently known in the literature. Even though not all of them have age estimates, a deeper literature search might reveal some more. How much effort did the authors spend on compiling a more complete list? I can accept the fact that ~15 is the largest possible sample to date, but would like to read about how the authors made the search.

Author’s reply: All three authors have conducted slightly different searches over many months and have converged on the existence of a small sample size. In fact, 3 of the sources were discovered to be the same source but listed under a different catalog. We have made a comment to this effect to encourage observers to investigate this further.

These age estimates are rather uncertain, and come from different sources, derived with possibly varying methodologies. In real life, they must be associated with errors but I see here no attempt to estimate or characterise them. Moreover, some ages fall into quite wide ranges. These are apparently plotted as "error bars" in the figures, but I doubt any errors or uncertainties are actually taken into account when calculating the correlation coefficients. Am I right? If yes, it is not surprising while the correlation coefficients are so suspiciously large. My point is that this kind of statistics does not make sense without considering "realistic" uncertainties of the measurements. A side note: it is said in line 63 that average values are plotted. Taking a look at the top point in Fig. 1 - the one with T_quies = 100 -, it does not seem to be the average of anything for the source 4C 29.30 from Table 2. In any case, the authors should make an attempt to handle and discuss the errors, and to explain what the error bars indicate in the plots.

Author’s reply: We have added a comment on the nature of the error bars plotted and that they reflect the use of different methodologies and cannot therefore be used as rigorous uncertainties.

The statement that DDRGs are FRIIs (line 84) is based on a rather old reference (Schoenmakers et al. 2020). It may well be still valid in the light of more recent discoveries made in the past quarter of a century, but more recent references would be useful here.

Author’s reply: Fair enough. We have added an additional more recent reference on this.

What I miss the most is the discussion or at least a mention of alternative scenarios. Especially the role of selection effects here which must be huge I believe. Isn't it more difficult to identify DDRGs with diffuse FRI-like inner lobes? Aren't the FRI-like relic outer lobes intrinsically less powerful, and fading more quickly below the detection threshold, making them less likely to be detected? The bottom line is that the statistics of DDRGs in terms of FR classification may be heavily affected by selection biases. See, for instance, Mahatma et al. (2019) who, by definition, excluded candidates with FRI morphology from their DDRG sample (see their Sect. 2.2.2). If DDRGs are intentionally chosen as sources displaying edge-brightened FRII morphology, it is not surprising that we end up with a sample containing FRIIs only...

Author’s reply: Good point. We discussed.

(2) Concerns about the model

The theoretical framework of this study is of Garofalo et al. (2010). If I understand correctly, it postulates that powerful FRII jets are produced when the SMBH has high-enough retrograde spin with respect to that of the accretion disk. On the other hand, SMBHs with high prograde spins tend to be radio-quiet AGN. In this model, it is just unclear to me how the jet activity is restarted in FRII-type DDRGs, after the initially retrograde-accreting SMBH (a<-0.1) gains sufficiently high spin (a>0.1) in the prograde regime. In other words, why do jets "turn on" once again after the SMBH spin direction is reversed? Doesn't the Garofalo et al. (2010) model predict otherwise?

Author’s reply: Jets are “on” in the model for prograde spin and thin-disk accretion for 0.1 < a < 0.7. Above that spin value they become suppressed. If the accretion is advection-dominated, no jet suppression occurs. We add a brief discussion of this but related to your next comment below.

There are a couple of triple-double radio galaxies known (at least 4 of them, see 2023MNRAS.525L..87C). These have evidence for 3 cycles of activity. How the model presented here explains this phenomenon in terms of the spin-direction changes?

Author’s reply: We briefly comment on the possibility of two quiescent phases in the model but suggest it is unlikely to be a good description of triple-double radio galaxies due to the timescale involved in the second quiescent phase.

There is much discussion about the role of accretion rate determining the length of jet activity periods. On would naively think that not only the rate of accretion but the total amount of the matter available to be accreted by the SMBH from its environment would play a significant role here. Is it the same for all AGN? How does this fit in the model?

Author’s reply: If the amount of fuel is not sufficient to get the system nicely past the corotation spin threshold, it cannot become a DDRG and once the fuel ends, the system is no longer an AGN of any kind.

(3) Concerns about how the manuscript is presented

Sometimes the terms "jets" and "lobes" are not used consequently. For example, the introduction says "Double-double radio galaxies (DDRG) display inner and outer jets thought to result from intermittent accretion", although, strictly speaking, the outer lobes are no longer fed by jets because the activity cycle once produced them is already over.

Author’s reply: Added the term “or lobes” to this in various places.

Table 1 list the literature compilation from Marecki et al. (2021). It would be fair to refer to the original publications here as well. Last row, last column of Table 1: instead of "21.0 - 6.6", Marecki et al. list in their Table 7 "1.0 - 6.6". Please double-check for typos the data entered in the tables.

Author’s reply: Done. Thanks for noticing the mistake.

While error bars (for certain data points) are plotted in Figs. 1-3, they are not presented in the tables (with one exception). This makes the correlation analysis impossible to reproduce.

Author’s reply: The correlation analysis is not based on the uncertainties but a simple Pearson correlation.

The 3rd column of Table 2 is empty. Why is it needed then?

Author’s reply: All data included in one table now.

The typesetting of the formulae (mostly on page 5) is so unpretending that is makes the equations barely readable.

Author’s reply: Font size increased for all equations and renumbered the last two.

lines 148-150: "...have been fleshed out in the literature by this author over the past decade so we refer the reader to that literature for more detail" - I guess "this author" means here the first author, since there are 2 other co-authors of the paper; I suggest to specifically cite those literature references here. In line 235, "this author" appears once again.

Author’s reply: Fixed.

The numbering of equations suddenly starts over in line 167.

Authors’s reply: Fixed

2.2 is overly lengthy, I suggest to divide it into smaller logical blocks.

Author’s reply: We broke it up into two parts, one for the derivation of the timescale and the other for the derivation of the physical origin of the correlation.

The mandatory section on author contributions is missing from the end.

Author’s reply: Fixed

Two of the listed references (#3 and #20) are actually not cited in the text, this should be fixed, as well as the missing bibliographic details of reference #3 (Symmetry, 17, 171)

Author’s reply: Fixed. Thanks.

Reviewer 4 Report

Comments and Suggestions for Authors

This manuscript presents a theory on the origin of the double-double radio galaxies. The authors argue that there is a correlation between the lifetime of the inner jet and the quiescent time seen in these objects. They explain this behaviour with continuous accretion and claim the the jet switched off due to the low spin of the central black hole as it goes through a spin reversal.

General remarks

While the basic concept of the manuscript is intriguing, its presentation can be improved.

First, it would be beneficial if the Fanaroff-Riley classification of radio galaxies is briefly introduced in the introduction with relevant references (at least including the original Fanaroff-Riley paper, https://ui.adsabs.harvard.edu/abs/1974MNRAS.167P..31F/abstract). It would also help the reader if an image of a double-double radio galaxy is shown. These details would ensure that a larger audience can be reached.

When presenting the relevant timescales (inner, outer jets, quiescent phase), it would be great if it is briefly shown how these were derived. This would serve two purposes, (i) it would show that the reported correlation cannot arise from an observational/analysis related artefact, and (ii) the time-scales are derived consistently for the sources in Table 1 and Table 2, which are coming from different samples, and publications. I wonder if different viewing angles may complicate the picture, influencing when a jet (or jet part) is visible or not. Additionally, the wavelength (and resolution, sensitivity) of the given observations are similar or not. These may have been explained in the referenced publications, but it would be important to show that they are consistent across the various publications from which the here-studied sample is created.

The manuscript would benefit a bit more careful typesetting. Most importantly, the third column of Table 2 is completely empty. Furthermore the timescales are both written with capital and normal t. In Sect. 2.2 the presentation of the theory itself would be easier to follow if it were typesetted following the standard LaTeX mathematical formalism (especially with respect to the exponents, and various indexes).

A few references are missing during the mathematical derivation. E.g., the efficiency for the zero spin black hole.

In Sect. 3, I advise the authors to clearly reference to their previous, most relevant paper(s)/work(s) on the subject, instead of just giving a general statement such as "this author and colleagues have applied the paradigm of counter-rotation and corotation to describe a wide variety of effects in AGN..." (Or just to one paper with "e.g.")

It would be also beneficial if they briefly present other explanations of the phenomena with relevant references, and discuss in the light of their findings.

Author Response

While the basic concept of the manuscript is intriguing, its presentation can be improved.

Author’s reply: Included.

It would also help the reader if an image of a double-double radio galaxy is shown. These details would ensure that a larger audience can be reached.

Author’s reply: Done.

Author’s reply: There is no consistency in the way the timescales are obtained which is a serious problem. We have discussed this as well as the nature of the uncertainties. We have emphasized that the data is not statistically robust to support the theory but we view it rather as a way of starting a conversation and encouraging observers to analyze this for larger data sets.

Author’s reply: We have eliminated Table 2 and used a different font that make the equations easier to read but ultimately they will appear in the journal's format if accepted.

A few references are missing during the mathematical derivation. E.g., the efficiency for the zero spin black hole.

Author’s reply: Fixed.

It would be also beneficial if they briefly present other explanations of the phenomena with relevant references, and discuss in the light of their findings.

Author’s reply: We discussed in the conclusions.

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The authors have indeed changed the tone of the manuscript and mentioned the caveats. I would recommend publication after a few minor corrections. The authors should improve the quality of their figures. Currently, the axes labels are tiny and almost unreadable. Plus, the captions to all the figures can be more detailed. Figure 1 caption incorrectly says "$\micro$GMRT" - it should be "uGMRT". There are typos in the added text in red colored font. A more careful look at the manuscript is warranted.

Author Response

Author’s reply: The figures have been improved and Figure 1 was changed.

Reviewer 2 Report

Comments and Suggestions for Authors

My report on the revised version is attached as a pdf file.

Comments for author File: Comments.pdf

Author Response

Report on the revised version of the paper “On the origin and nature of double-double radio galaxies” by Garofalo et al. The authors have mentioned that the correlation is not robust, corrected the errors in the carelessly compiled information in the Tables, and stated that their objective is to start a conversation on a possible correlation. This is more acceptable than the earlier version, although I do not agree that data are not available to make a rigorous study. The authors have chosen not to make a detailed study using available information in the literature to estimate ages for inner and outer doubles themselves for a reasonably large sample of double-double radio galaxies, but take the numbers essentially from one small compilation. This may be okay for starting a conversation, if consistent with the journal policy. However, I would suggest considering the following while revising the paper. I have listed these as they occurred to me while going through the paper. These are not in order of importance.

Both DDRG (e.g. lines 9 and 35) and DDRGs (e.g. line 52) have been used to describe double-double radio galaxies. Be consistent. I would advise using DDRG for a single source and DDRGs for more than one as is usually done in the literature.

Author’s reply: Corrected.

Figure 1. The source shown here can hardly be a good example of a DDRG. There are clear hotspots visible on the outer lobes, and peaks of emission in the bridges, which are often diffuse, do NOT usually denote another cycle of activity. I would not call it a DDRG, and the authors themselves call it a “Candidate Double-Double Radio Galaxy”. There are many much better examples in the literature if the authors choose to illustrate it with an example. It may be better to choose one from their list of sources so that readers can relate easily (Please take permission to reproduce if necessary).

Author’s reply: Figure changed.

Figure 1. It is NOT μGMRT but uGMRT which refers to the upgraded GMRT. The source name is J2349-0003, where the 2 has been left out.

Author’s reply: No longer relevant.

Figure 1 caption, line 70 and other places: et al => et al. This is okay in some places and not in others.

Author’s reply: Corrected.

Lines 56-57: “If the accretion disk changes direction, an FRI jet will form and otherwise not.” As I understand it, this is specific to their model, and not a widely accepted scenario as this section seems to suggest. Please be explicit on this and give a reference to your work. Besides differences in lobe structure, there are also differences in jet structure and magnetic field orientations between FRI and FRII sources (cf. Bridle & Perley 1984; Saikia 2022 for reviews) which we also need to take on board while trying to understand the FRI-FRII dichotomy.

Author’s reply: Correct. Fixed and added discussion.

In Table 1 of the list of sources, please list all the sources with its IAU name followed by its common/alternative name, and list basic relevant properties like redshift, luminosity and projected linear size stating clearly the cosmological parameters used. Also indicate by numbers in the last column which reference is relevant for which source.

Author’s reply: Done.

Figure 2 seems to indicate the range of values for the four sources which have a range quoted. They mention that the vertical and horizontal bars denote error bars (line 90), which is not correct. Looking at Figure 2, one would then assume that there are no errors for the remaining points, which is not correct. Usually these ranges are from different assumptions which go into making these estimates and each point will have an error which was not quoted by Marecki et al. as they merely wanted a comparison with ballpark values. Even if the authors wish to confine themselves to the small sample with the readily available estimates a sincere effort should be made to go back to the original papers and estimate the errors or uncertainties involved in each point. This is important for evaluation of any correlation coefficient and its level of significance. Also what the bars denote should be explicitly mentioned in the Figure captions.

Author’s reply: Part of this was discussed already. We have corrected the statement that they are “error bars” with “range”.

Figures 3 and 4. Here the bars appear on more sources, sometimes along only one axis. How these have been obtained and what they denote are unclear.

Author’s reply: All is now consistent.

Lines 198 and also 222. Please be more precise rather than merely saying “electron cross section”. Also correct ‘it’s’ to its where necessary (e.g. line 221).

Author’s reply: Fixed.

10.Line 226. Many of the DDRGs from the compilation by Marecki et al. appear to be giant sources, which listing of the sizes in Table 1 will clarify. 3C219 is relatively smaller, although not the smallest. The source with the smallest quiescent time scale 3C293 is only about a couple of hundred kpc in size. Time scales could depend on source size although it is difficult to make any statement based on small samples. After all signs of recurrent activity, but not DDRGs so far, have also been reported for compact steep spectrum radio sources with much smaller time scales (cf. O’Dea & Saikia 2021). These aspects need to be kept in mind while interpreting the results.

Author’s reply: Added a discussion with references.

Line 299. “Garofalo, Joshi et al 2021” was published in 2020. Please check all references and equations and ensure that there are no errors.

Author’s reply: Indeed. Fixed.

12.Line 319. I did not quite understand this statement as DDRGs are defined in a way that pertains to FRII sources. With the differences in collimation and magnetic field properties of FRI jets compared with FRII sources, one may have to look for such signatures to establish whether there is a genuine FRI structure.

Author’s reply: Discussed.

Line 372. As the orientation of the accretion disk is random and not constrained by the previous cycle of activity, a brief mention of how this is consistent with the vast majority of DDRGs being reasonably well aligned would be helpful. DDRGs are also found in rich clusters of galaxies (cf. Dabhade et al. 2025).

Author’s reply: This was already discussed. It is the reason why DDRGs are few in number.

14.If the reviewer’s comments have contributed significantly to the paper, especially when significant material from the report is reproduced in the paper, I believe it is appropriate to acknowledge this in the acknowledgement even if the reviewer is an anonymous one.

Author’s reply: Indeed, the referees were extremely useful. We had intended to add an acknowledgment later in the process but have now added it due to this reminder.

Reviewer 3 Report

Comments and Suggestions for Authors

I thank the authors for implementing the suggested changes, using more careful wording in their claims, and improving the readability of the manuscript considerably (even though its format, especially the way how references are cited, does not fully conform with the journal style - but this is beyond the responsibility of a referee). Concerning its contents, I find this revised version suitable for publication. The very few minor issues I spotted are listed below.

page 2, Fig. 1 caption:
DDRG J349-0003 -> DDRG J2349-0003

line 107:
appears to be to compare -> appears to compare

line 387:
jet suppression in inhibited -> jet suppression is inhibited

Author Response

page 2, Fig. 1 caption:
DDRG J349-0003 -> DDRG J2349-0003

Author’s reply: New figure.

line 107:
appears to be to compare -> appears to compare

Author’s reply: Corrected.

line 387:
jet suppression in inhibited -> jet suppression is inhibited

Author’s reply: Corrected. Thank you for noticing all these.

Reviewer 4 Report

Comments and Suggestions for Authors

The authors addressed my comments and corrected the manuscript.

In only have one minor correction. I suggest to add a reference for this statement in the Introduction,
"Over the last half decade, in fact, we have come to recognize that a compelling picture for understanding FRI jet morphology ... "

Author Response

The authors addressed my comments and corrected the manuscript.

Author’s reply: Done. Thanks.