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    Dear Arepo community,

    I’m trying to simulate a MW-like galaxy, including gas. However, when Arepo adds the background, the disk gas changes its distribution (see the figure in this link: https://drive.google.com/open?id=1orpenpM5Ib7_f9yv-w2ACwk1hej_sNOA). I’m wondering if this is due to some issues with the grid generation.

    I would greatly appreciate any help/hint.


    Here is another figure illustrating better our problem: https://drive.google.com/open?id=1jBvpxkEja_y4goIrx6ZKqi05m6qqBp_u


    Hi Josue,
    Let me first understand what you are trying to do:
    You have initial conditions of an isolated disk galaxy, which contains gas in the disk and some collisionless component. So something you would be able to run with gadget right away. And what you need now some background medium, but somehow this impacts your gas initial conditions.
    So the overall setup is similar to the ‘galaxy_merger_star_formation_3d’ example provided, right?

    Could you let me know the Config.sh and param.txt settings?




    Dear Rainer,

    Your description of our problem is correct. We use the setup “galaxy_merger_star_formation_3d” as a template. In the “param.txt” file we only edit the BoxSize, ReferenceGasPartMass, and MeanVolume parameters according to our ICs, so probably we’re missing something important here. On the other hand, for the files related to “ADDBACKGROUNDGRID” we didn’t change anything. You can find the “Setup.sh”, “param.txt”, and the ICs files in this link:


    For the moment, the ICs file only contains gas particles as we want to characterize the Arepo’s HD. The next step is adding collisionless particles (DM halo, stellar disk, and bulge) trying to reproduce a MW-type galaxy.

    Best regards and thanks a lot for your kind help.


    I just had a look at your setup. Generally, this looks as expected to me, just a couple of comments:
    Using ADDBACKGROUNDGRID will re-bin also your SPH particles in the disk, so it will degrade the density field to a certain degree. The only way around this is to set up a grid yourself (i.e. just manually set the mesh generating points, but then in a volume-filling fashion).
    If you don’t want to do this, another ‘problem’ is the difference in resolution between the disk and the background, which will degrade the edges. This can be improved by increasing the parameter ‘GridSize’ in the ADDBACKGROUNDGRID step. This parameter refers to the number of grid points per dimension that are added in the background, and is chosen quite aggressively with 16 in the example run. For production runs, I would recommend to choose it such that GridSize^3 is of the order of the number of grid cells in the disk (but not more, otherwise the computation would be dominated by the background).
    I hope this helps. If you need further help, let me know.

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