Probing the dark sector using the Fornax satellite

A long-range force acting only in the dark sector can displace the stellar component of satellite galaxies relative to their dark matter (DM) halos, thereby the weak equivalence principle (WEP) between DM and baryons. We investigate observational signatures of such WEP breaking using <a:math xmlns:a="http://www.w3.org/1998/Math/MathML" display="inline"> <a:mi>N</a:mi> </a:math> -body simulations of a Fornax-like Milky Way (MW) satellite, implementing a fifth force between DM particles of amplitude <c:math xmlns:c="http://www.w3.org/1998/Math/MathML" display="inline"> <c:mi>β</c:mi> </c:math> relative to Newtonian gravity (with a screening length much larger than the MW halo size). We find that <e:math xmlns:e="http://www.w3.org/1998/Math/MathML" display="inline"> <e:mrow> <e:mi>β</e:mi> <e:mo>≳</e:mo> <e:mn>0.6</e:mn> </e:mrow> </e:math> strips and unbinds the bulk of the stellar component, leaving at most a negligible bound remnant that fails to match the observed stellar content, surface-brightness, and line-of-sight velocity-dispersion profiles of Fornax. By contrast, <g:math xmlns:g="http://www.w3.org/1998/Math/MathML" display="inline"> <g:mi>β</g:mi> <g:mo>≲</g:mo> <g:mn>0.2</g:mn> </g:math> yields only mild stellar stripping and remains broadly consistent with current photometric and kinematic constraints within our modeling assumptions.