Turning AGN Bubbles into Radio Relics with Sloshing: Modeling CR Transport with Realistic Physics
Round 1
Reviewer 1 Report
I have two questions regarding the content of the paper:
When you give the mass of a cluster. Is that total mass, or baryonic mass? In case of the latter, how large is each component?
In the merger, plasma-beta is constant, but what is its value?
Author Response
"When you give the mass of a cluster. Is that total mass, or baryonic mass? In case of the latter, how large is each component?"
We have added the details about the total mass and the relative masses of the components to Section 2.1. We have also restructured this section somewhat for clarity.
"In the merger, plasma-beta is constant, but what is its value?"
The initial plasma-beta is 100, which we have now noted in Section 2.1.
Reviewer 2 Report
The paper, “Turning AGN bubbles into radio relics with sloshing: modelling CR transport with realistic physics” presents more sophisticated computational results to back their proposed mechanism of the formation of radio relics. Namely, that the shape of some relics can be explained by a pre-existing population of CRs. To this extent, they update their previous calculation to include varying degrees of CR physics. This paper offers interesting results on the origin of radio relics and will of significant interest for those seeking to understand the origin of radio relics. I have below some major and minor suggestions that I hope will be taken into account to improve this manuscript.
Major points
- Please state, at least generally, what the initial conditions are. For instance, while you mention the mass ratio between the two clusters was 5, and that they include DM and hot plasma, upon looking at ZuHone+2021a, I saw that the “Merger1” simulation had the subcluster with dark matter only. Is there a rational for choosing the model where the subcluster has no baryonic versus with baryonic matter?
- In Figure 1 (and throughout the paper), you mention that the 3rd row is “including diffusion and Alfven losses”, but it seems this is meant to align with the “Model 2” which is “purely advective CRs (without CRs)”. The two descriptions should be made the same if they’re referring to the same model. The annotation of that 3rd row also says “no cooling or diffusion”, so this is incompatible with the legend. The same error appears in Figure 2, also.
- This paper presents an interesting mechanism to explain radio relics! Since there are observational analogues, it would greatly enhance the results of the paper to see a plot comparing their simulation results to an observed radio relic (e.g. the Sausage)
Minor:
- To avoid having to do a “heirarchical paper delving” of previous works, if there is space, it would be useful to show the density distribution of the simulation as well.
- For the plots of X_jet, I would suggest the use of a different colormap, since the “jet”/“spectrum” style maps are not guaranteed to be Color-blind safe and may result in some features appearing stronger. I’d recommend a perceptually uniform map, like the top row. While not entirely necessary, I believe it would be beneficial.
Author Response
"Please state, at least generally, what the initial conditions are. For instance, while you mention the mass ratio between the two clusters was 5, and that they include DM and hot plasma, upon looking at ZuHone+2021a, I saw that the “Merger1” simulation had the subcluster with dark matter only. Is there a rational for choosing the model where the subcluster has no baryonic versus with baryonic matter?"
We have restructured Section 2.1 to clarify this. We have also added some rationale for choosing the "Merger1" simulation instead of "Merger2".
"In Figure 1 (and throughout the paper), you mention that the 3rd row is “including diffusion and Alfven losses”, but it seems this is meant to align with the “Model 2” which is “purely advective CRs (without CRs)”. The two descriptions should be made the same if they’re referring to the same model. The annotation of that 3rd row also says “no cooling or diffusion”, so this is incompatible with the legend. The same error appears in Figure 2, also."
We thank the reviewer for pointing out this discrepancy, which arose because of text that was left from an earlier version of this manuscript. We have corrected the error in the figure captions and made sure the rest of the paper is consistent.
"This paper presents an interesting mechanism to explain radio relics! Since there are observational analogues, it would greatly enhance the results of the paper to see a plot comparing their simulation results to an observed radio relic (e.g. the Sausage)"
In this paper, we studied the interaction of a jet lobe filled with cosmic rays, their escape from the lobe and how they are transported in the sloshing ICM. We plan to study the radio emission, that a putative cosmic ray electron component in these lobes is expected to power, in a separate publication. Giant radio relics are the result of (re-)acceleration of cosmic ray electrons by the passage of a (merger) shock which we did not simulate here. Relic electrons from AGN lobes are indeed a possible source for the electrons that can be (re-)accelerated at a cluster shock, but details matter and need to be carefully addressed before we can claim a successful comparison to radio data (see e.g., the recent paper by Inchingolo et al., 2021, astro-ph:2110.11668, for such a comparison).
"To avoid having to do a “heirarchical paper delving” of previous works, if there is space, it would be useful to show the density distribution of the simulation as well."
We have added a new figure (the new Figure 1) showing slices of the gas density at the same epochs as the following two figures.
"For the plots of X_jet, I would suggest the use of a different colormap, since the “jet”/“spectrum” style maps are not guaranteed to be Color-blind safe and may result in some features appearing stronger. I’d recommend a perceptually uniform map, like the top row. While not entirely necessary, I believe it would be beneficial."
The colormap used here is "turbo", which was designed as a replacement to "jet" to be close to perceptually uniform and has been tested to be color-blind safe:
https://ai.googleblog.com/2019/08/turbo-improved-rainbow-colormap-for.html
Reviewer 3 Report
This paper outlines an extension of work recently reported by ZuHone etal
(ApJ, 914,73) that introduced a model to account for cluster radio relic
structures as a consequence of sloshing motions resulting from a cluster merger. The earlier paper successfully simulated ICM structures with features consistent with those in radio relics. Those authors did not, however, in those simulations include explicit transport treatment of the Cosmic Ray Electrons that are responsible for the observed radio emissions; the present paper provides a first effort in that direction and addresses some of the expected consequences thereof. They find that explicit inclusion of CR transport leads to significant structure modifications that diminish the distinctive features called on to support the proposed relic formation model. The present work is an important step forward in this discussion. Once some issues with the current paper draft are addressed, it will be a useful contribution to this literature. Issues to be addressed first would include:
1) The Cosmic Rays are treated as a second, dynamically coupled fluid with a distinct adiabatic index, diffusive properties and cooling physics. While a
good step forward, two immediate limitations that need to be addressed would be that the Cosmic Rays being modeled seem only to be hadronic (except for passing mention of secondary CRes). Are we to understand here that CRe are only minor contributors to the CR properties? The radio emissions reflect only the physics of CRes, whose transport is not entirely the same as transport of Cosmic Ray protons in sometimes significant ways. Related to this, the transport physics modeled seems not to include any kinetic effect only MHD, whereas it is well established that kinetic effects are important in these weakly collisional media (e.g., Chen, etal, Nature Comm, 10, 740, 2019). Those should be addressed at some level. This is especially significant in addressing distinctions between CRp and CRe.
2) Diffusion transverse to the local magnetic field is ignored. While it is
likely to be substantially smaller than diffusion along the field, transverse
diffusion, especially in association with small scale, subgrid, magnetic field
"diffusion" can still play a significant role in effective CR transport (e.g.,
Desiati and Zwibel 2014, ApJ, 791, 51). This is likely to lead to additional
modifications of observable radio features and should be mentioned.
3) The caption in Figure 1 seems inconsistent with the figure itself. In particular, the caption specifies explicitly that the third row of the figure includes CR diffusion and cooling, whereas the text within the figure itself on the third row says explicitly that there is "no cooling or diffusion". These should be made consistent.
Author Response
"1) The Cosmic Rays are treated as a second, dynamically coupled fluid with a distinct adiabatic index, diffusive properties and cooling physics. While a
good step forward, two immediate limitations that need to be addressed would be that the Cosmic Rays being modeled seem only to be hadronic (except for passing mention of secondary CRes). Are we to understand here that CRe are only minor contributors to the CR properties? The radio emissions reflect only the physics of CRes, whose transport is not entirely the same as transport of Cosmic Ray protons in sometimes significant ways. Related to this, the transport physics modeled seems not to include any kinetic effect only MHD, whereas it is well established that kinetic effects are important in these weakly collisional media (e.g., Chen, etal, Nature Comm, 10, 740, 2019). Those should be addressed at some level. This is especially significant in addressing distinctions between CRp and CRe."
We fully agree with the reviewer that the physics of electrons and their transport in weakly collisional media is interesting. However, in this first publication on the dynamical effects of cosmic rays from AGN lobes on the structure of filaments shaped by sloshing, the electrons can be neglected because of their negligible pressure contribution in comparison to cosmic ray ions (e.g., Pfrommer & Enßlin 2014, A&A, 413, 17, Brunetti & Jones 2014, JMPD, 2330007). We plan to study the radio emission, that a putative cosmic ray electron component in these lobes is expected to power, in a separate publication. We include kinetic effect on MHD waves only approximately and postpone a study of two-moment cosmic ray transport to future work (see e.g., Thomas & Pfrommer 2019, MNRAS, 485, 2977; 2021, MNRAS, in print, arXiv:2105.08090; Thomas et al. 2020, ApJL, 890, L18, 2021; MNRAS, 503, 2242). To emulate the cosmic ray streaming transport, we supplement cosmic ray diffusion (which is intrinsically energy conserving) with Alfvén wave losses, which is a first-order approximation of cosmic ray transport as a result of the balance between the growth rate of the kinetic-scale cosmic-ray streaming instability (Kulsrud & Pearce 1969, ApJ, 156, 445; Shalaby et al. 2021, ApJ, 908, 206) and the prevalent wave damping rates, including non-linear Landau damping and “turbulent” damping (Guo & Oh 2008, MNRAS, 384, 251; Zweibel 2017, PhPl, 24, 55402). This has been shown by Wiener et al. (2017, MNRAS, 467, 906) in the context of galactic winds driven by cosmic ray pressure gradients.
"2) Diffusion transverse to the local magnetic field is ignored. While it is
likely to be substantially smaller than diffusion along the field, transverse
diffusion, especially in association with small scale, subgrid, magnetic field
"diffusion" can still play a significant role in effective CR transport (e.g.,
Desiati and Zwibel 2014, ApJ, 791, 51). This is likely to lead to additional
modifications of observable radio features and should be mentioned."
We have mentioned this possibility and commented on it in Section 2.2.
"3) The caption in Figure 1 seems inconsistent with the figure itself. In particular, the caption specifies explicitly that the third row of the figure includes CR diffusion and cooling, whereas the text within the figure itself on the third row says explicitly that there is "no cooling or diffusion". These should be made consistent."
We thank the reviewer for pointing out this discrepancy, which arose because of text that was left from an earlier version of this manuscript. We have corrected this error in the captions of both Figure 1 and Figure 2.
