Feedback from Active Galactic Nuclei in Galaxy Groups

Round 1

Reviewer 1 Report

I found the manuscript to be very interesting, informative, well-written and containing new physical results. I have no serious remarks and recommend the manuscript for publication in Universe. 

Author Response

We thank you for reading our manuscript and for your positive comments.

Reviewer 2 Report

I have reviewed the manuscript, "Feedback from active galactic nuclei in galaxy groups," by Eckert et al.  I find this to be a very comprehensive review of observations of galaxy groups at a range of wavelengths, with some discussion of theoretical aspects as well.  I know there are others who are engaged in more theoretically-oriented reviews for this issue, so I don't know what is supposed to be covered where; I will leave this to the editors and judge this work on it own.  Overall,  I find it to be excellent and wide-ranging, covering all the aspects that I feel are relevant to the modern study of galaxy groups.  I have only very minor suggestions for things to consider, detailed below, but I leave it to the discretion of the authors whether to mention them.  I congratulate the authors for an excellent review article, which I will surely use for my students.

Comments:

• p9: Why are M~1-2 shocks in particular a sign of AGN feedback?  How would the jets know to be at velocities just above the sound speed? (I can think of some reasons, but I would like to hear the authors' opinion).- p18: GRGs in MIGHTEE seem more common than thought (Dalhaize+20)
• p23: Does CCA overpredict the amount of gas cooling onto the ISM/BH?
• p25 (Fig 12): What are the magenta downwards arrows?
• p26: Reorienting jets (Cielo+18) might be a way to sphericalize the energy input (beyond turbulence).
• p35: van Daalen+20 nicely connects PS suppression to the group baryon fractions in various simulations; might be worth mentioning.
• Overall, I thought that it was a bit lacking in discussions regarding groups selected optically (eg in SDSS), and how these might relate to other selections.  Perhaps other reviews will cover this?

Author Response

Dear colleague,

We thank you for your positive comments on our manuscript. In the revised version we addressed the comments listed below. The changes with respect to the original manuscript are highlighted in boldface.

• "p9: Why are M~1-2 shocks in particular a sign of AGN feedback?  How would the jets know to be at velocities just above the sound speed? (I can think of some reasons, but I would like to hear the authors' opinion)."
  We think that the phrasing of the paragraph was a bit confusing on this aspect and we have slightly modified the text accordingly. We do not necessarily think that weak shocks should be a particular sign of AGN feedback; we were rather pointing out that the detected shocks are generally weak. The Mach number of the outflowing material should not be constant as a function of distance to the SMBH, as in the central regions the jets should be strongly supersonic and they should progressively slow down. Besides, the detected shocks tend to be perpendicular to the jet axis, as highlighted in the case of NGC 5813. We do think, however, that independently of their Mach number concentric shock fronts are strong evidence for deposition of AGN-injected energy into the IGrM. We hope the new phrasing makes our view clearer.
  
  
• "- p18: GRGs in MIGHTEE seem more common than thought (Dalhaize+20)"
  We have added a sentence on this issue.
  
  
• "p23: Does CCA overpredict the amount of gas cooling onto the ISM/BH?"
  Hot mode (Bondi) accretion is highly inefficient and generally insufficient to trigger AGN activity, therefore on the contrary the increased accretion rate in CCA with respect to Bondi strongly supports the model. We have added a sentence in the manuscript to highlight this point.
  
  
• "p25 (Fig 12): What are the magenta downwards arrows?"
  The magenta arrows highlight the difference between the total AGN energy and the gravitational binding energy within the cores of galaxy groups. We have added this information explicitly in the caption of the figure.
  
  
• "p26: Reorienting jets (Cielo+18) might be a way to sphericalize the energy input (beyond turbulence)."
  We thank you for pointing out this interesting reference, we added a couple of sentences in p27 to mention this alternative possibility.
  
  
• "p35: van Daalen+20 nicely connects PS suppression to the group baryon fractions in various simulations; might be worth mentioning."
  The reference has been added.
  
  
• "Overall, I thought that it was a bit lacking in discussions regarding groups selected optically (eg in SDSS), and how these might relate to other selections.  Perhaps other reviews will cover this?"
  Indeed this particular review focuses specifically on AGN feedback in galaxy groups, independently of the selection method. While selection effects certainly play a role in a number of areas (as mentioned in particular in the case of the baryon fraction), clearly this is not the main focus of our review. A broader discussion of selection effects is provided in the companion review by Lovisari et al. on group scaling relations.