Address increase of galaxy-dark matter offset with time

[wadawson]

Referee comment:

On more than one occasion, the author claims that for SIDM mergers the offset between the galaxies and the dark matter increases with time after core passage (i.e., t_coll) until the subcluster reaches the turn around radius. I suspect that this is not generally the case, and that there is a similar "slingshot" effect with the SIDM as observed with ram pressure stripped gas in merging galaxies and subclusters (i.e., at some time after t_coll and before reaching d_max the gravitational influence of the subcluster DM halo on the subcluster galaxies leads to a decrease in the separation). Has this been carefully checked, or simply assumed?

I need to find all "occasions" in the paper and think about editing.

• [x] 2nd paragraph of introduction
• [x] 1st paragraph of section 4.2
• [x] 3rd paragraph of section 5

In short, this has simply been assumed. I attempted to check this by running a simple multibody simulation that containted two galaxy particles and two dark matter particles (one for each subcluster). The galaxy particles only interacted via the gravitational force, and the dark matter particles interacted via gravity and a self-interaction force. After running this simplistic simulation through it was clear that the results were unphysical, largely due to treating the galaxies as a single coherent body and lack of dissapative effects. My collaboration is working on fullblown self-intearcting dark matter merger simulations but these results are still a ways off.

[wadawson]

Response to the referee:

On all occasions in the paper I clarified that the offset will initially grow with time, then at later TSC the offset will decrease due to the gravitational attraction between the galaxies and dark matter halo.

As an aside, I had previously attempted to explore the temporal dependence of the offset by running a simple multibody simulation that containted two galaxy particles and two dark matter particles (one for each subcluster). The galaxy particles only interacted via the gravitational force, and the dark matter particles interacted via gravity and a self-interaction force. After running this simplistic simulation through it was clear that the results were unphysical, largely due to treating the galaxies as a single coherent body and lack of dissapative effects. My collaboration is working on fullblown self-intearcting dark matter merger simulations but these results are still a ways off.