Re: Newtonian simulations of dark matter only sphere

From: Sten Delos <>
Date: Wed, 9 Sep 2020 16:02:51 -0400

While I'm not quite clear on the initial conditions here, I would propose
that the cuspless outcome may be real. Cusps appear in simulations of
overdense regions surrounded by a dark matter background. I've found that
if I surround the region with vacuum instead, I obtain a much less
pronounced cusp.

In other words, if you're just simulating a uniformly dense sphere, try
surrounding it with a less uniformly dense background. You can still have
vacuum boundary conditions at some radius, of course---that radius just has
to be large enough to not be collapsing at the same time as the halo is
first forming.

- Sten Delos

On Wed, Sep 9, 2020, 2:38 PM Cameron McBride <>

> My guess here is that you need to adapt your gravitational softening in
> Gadget-2. I've been out of actively following the literature for the last
> ~5ish years, but there was a host of papers that talk about cusp - core
> connection to gravitational softening that can change the cusp vs core.
> It's a little bit of a numerical quagmire, as you can tune softening to act
> differently (and there are tradeoffs even if you force a strict N^2
> calculation, which Gadget-2 typically doesn't). I also suggest you don't
> use grid, but tree only configurations.
> I think Zavala et al 2019 or Kuhlen et al 2012 might discuss aspects of
> this. There are some older articles, like Boylan-Kolchin et al 2005 that
> might also discuss some of it.
> Others might have more insight, but hopefully that might help you start
> digging deeper.
> Cameron
> On Wed, Sep 9, 2020 at 8:32 AM ABINASH KUMAR SHAW <
>> wrote:
>> Hello,
>> I am trying to study time evolution of the density profile of a single
>> dark matter only halo in Newtonian space (without boundary) using Gadget-2.
>> My initial condition file contains a spherical distribution of only the
>> cold dark matter particle which is written in gadget format. I am following
>> the example of the gassphere (provided with the code) to evolve the CDM
>> halo. I expect the density profile to show the cusp at the halo centre
>> whereas I am getting a core. I understand that a core forms when there are
>> baryons in addition to the CDM. There are no baryons here, however I expect
>> that somehow baryonic interaction is still taking place. So, do I need to
>> modify the code of my simulations; or am I doing something wrong?
>> Thank you in advance.
>> Sincerely,
>> Abinash Kumar Shaw
>> *Senior Research Fellow*
>> *Department of Physics*
>> *Indian Institute of Technology Kharagpur*
>> *Kharagpur, India -- 721302.*
Received on 2020-09-09 22:03:17

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