Jittering Jets by Negative Angular Momentum Feedback in Cooling Flows

Round 1

Reviewer 1 Report

The manuscript "Jittering jets by negative angular momentum feedback in cooling flows" by Noam Soker provides a theoretical interpretation for the observation of the galaxy cluster RBS 797 using a jittering jets in a cooling flow scenario. The paper is interesting and worth publishing. However, I have some concerns about the presentation of the results. Indeed, I find worth mentioning already in the introduction that some of the calculations are really crude, and must be followed up by simulations to confirm the results. Also, I find some passages quite hard to follow. I anyway believe that these will be mostly minor revision, so I recommend acceptance after these issues have been addressed. I list a few bullet points (mostly minor) that should be checked. 

line 110: processes --> process. Same in line 111. 

line 138: the term "outburst" in the literature does not necessarily refer to a jet-launching episode. I recommend clarifying its definition here, where it is used for the first time. 

line 165: please clarify here if these two episodes are different accretion episodes, as it is not easily understood. 

section 2.1:  I find this whole section quite cryptic and hard to follow. I strongly recommend rephrasing to make it more clear, and making more use of the sketch in Fig. 1. 

line 283: "by a large angle", please report a value for reference. 

line 331: remove extra ")"

line 367: the calculations that follow are very crude indeed, and I recommend to at least be a bit more precise. For instance, what does "a few" mean? Is it 2, or is it 10? How much do the results change if a different definition of "a few" is adopted? 

line 427: this note should be moved above, at the beginning of section 2 or even in the introduction, as it is an important distinction that should be immediately clear to the reader.  

line 546: missing ")"

Author Response

I thank the referee for the useful comments. I made changes according to all comments. After making changes I read the entire paper fixed some writing.

I find the paper to be clearer now. 

(1) I added in the last paragraph of section 2: " My crude calculations should be confirmed by future three-dimensional hydrodynamical simulations.". 

(2) I fixed the typos. 

(3) In the first place (as well as some other places) where I use outbursts in the Abstract and in the text I explain that I refer to jet-launching episodes: " . .outbursts (jet-launching episodes) . . ".

(4) Where I mention two jet-launching episodes in that place I added: "In the present study I consider each jet-launching episode to results from a different mass accretion episode. "

(5) I improved section 2.1. 

(6) I mention that angle of $\approx 40-50$ degrees. 

(7) The calculation in (old) line 367 (equation 6). I now explain this part, like added that few means $\approx 5$. The scaled equation shows the dependence of the results on the time. 

(8)  I moved the entire paragraph that mentions wide jets to the introduction. 

Reviewer 2 Report

The manuscript seeks to explain the two sets of perpendicular bubbles in the cool core cluster RBS797.  A scenario is discussed in terms of gas clouds which are uplifted by a jet which then later fall into the central black hole with an angular momentum which causes the orientation of the jet to change.

This is an interesting idea. It is timely and will be important to the field of cooling flows and deserves to be published.  There are a few places where the scenario may not quite hang together yet and these should be addressed in more detail.

The model depends on the existence of clouds of cold molecular gas. These have been detected in other cool core clusters, but not yet in RBS797, as far as I can tell. Is this a problem for the model for RBS797. 

At this point, jet orientation changes seem to be rare. Is this a problem, given that cold gas clouds are detected in other clusters. Shouldn’t the clusters with cold gas clouds show bubbles with different orientations?

The model relies on Babul et al. 2013 for the jet orientation change. But Babul et al. suggest that the orientation change is associated with a thin accretion disk. Even if the disk is hidden from view by an obscuring torus, the photons from the bright thin disk should produce high excitation emission lines which are not seen in cooling flows.

The author assumes the jets are “wide-slow-massive” which may not characterize actual jets in brightest cluster galaxies. What are the consequences of the jet properties being different from this assumption?

The author extrapolates the results of Hillel & Soker 2014 determined at 20 kpc to size scales a hundred times smaller.  How good is this extrapolation?

Minor comments:

Figure 1 could use some additional explanation/clarification.

Lines 110-112, fix English

Line 119. “already study” should be “previously studied”

Line 138. “mention” should be “suggest”

Line 145. “be” should be “are”

The idea that radio sources can provide feedback in clusters should include a reference to the study of PKS 0745-191 by Baum & O’Dea 1991

Comments for author File: Comments.pdf

Author Response

I thank the referee for useful comments.  I made changes according to all comments. 

(1) The model is based on the cold feedback mechanism, where the central AGN accretes relatively cold gas. Therefore, there must be some clumps/clouds, but in many cases they might contain little mass, below detection limit. So the non-detection is not a problem. But if there are no clumps/louds at all, and there were no clouds during the accretion phase, then here is a problem. I added this in the first paragraph of section 2.1"

"I base the mechanism I consider on the presence of clouds that are cooler than the ICM. Therefore, there must be some clouds in the ICM. In some clusters they might be below detection limit. In those clusters I predict that very deep observations will reveal such clouds close to the center. "

(2) The comment on cases with no change in jets' orientation is good.

It depends on the competition between the ICM angular momentum and that of the falling clouds. I now explain this in section 2.1. I added two paragraphs in section 2.1 (second and last paragraphs in section 2.1): 

"I consider in this study the case where the angular momentum of the ICM is negligible (zero). Therefore, the clouds that are formed by thermal instabilities in the ICM are born with zero angular momentum"

"Again, this conclusion holds for cases where the specific angular momentum (per unit mass) of the ICM is much smaller than what the jets deposit to the clouds. Oppositely, in cases where the ICM does have a large specific angular momentum all clouds are born with the same angular momentum direction as that of the ICM. The angular momentum axis of the accretion disk will be then the axis of the ICM angular momentum, and so is the axis of the jets. In those cases different jet-launching episodes will share more or less the same axis." 

(3) I do not rely on Babul et al. (2013) for the jet orientation change. Actually, Babul et al. assumed that the jets are along the angular momentum axis of the black hole. This is not the case. I now added in the first paragraph of section 2.2, and added the reference to Gottlieb et al. (2022): 

"In any case, in a new paper Gottlieb et al. (2022) demonstrate with 3D general-relativity magnetohydrodynamic simulations that the jets are along the angular momentum axis of the accretion disk more than they are along the black hole angular momentum axis." 

(4) I do not think that using narrow jets instead of wide-slow-massive jets will change much for the present scenario. It will change the properties of inflated bubbles though. I prefer not to get into this in the present study.  I continue the line of research of my group in using wide jets. More than that, in a recent paper Fujita (2022) use wide jets to explain the Fermi bubbles of our Galaxy. I now mention this paper (I moved the mentioning of wide jets to the first section, next to last paragraph).  

(5) I am not sure how good is the extrapolation of the results of Hillel & Soker (2014) from 20 kpc to <1kpc. I assume that the results hold. Again, as I now added also in section 1, the entire scenario I describe here requires 3D hydrodynamical simulations to confirm it.

(6) I fixed the typos.  

(7) I added the reference in the second paragraph of section 1.