Can tides explain the low dark matter density in Fornax?

• Main Authors: Genina, Anna, Read, Justin I, Fattahi, Azadeh, Frenk, Carlos S
• Format: Journal Article
• Language: English
• Published: 01-12-2021
• Subjects: Physics - Astrophysics of Galaxies
• DOI: 10.48550/arxiv.2011.09482

The low dark matter density in the Fornax dwarf galaxy is often interpreted
as being due to the presence of a constant density `core', but it could also be
explained by the effects of Galactic tides. The latter interpretation has been
disfavoured because it is apparently inconsistent with the orbital parameters
and star formation history of Fornax. We revisit these arguments with the help
of the APOSTLE cosmological hydrodynamics simulations. We show that simulated
dwarfs with similar properties to Fornax are able to form stars after infall,
so that star formation is not necessarily a good tracer of infall time. We also
examine the constraints on the pericentre of Fornax and point out that small
pericentres (<50 kpc) are not currently ruled out by the data, allowing for
Fornax to be tidally influenced on its current orbit. Furthermore, we find that
some dwarfs with large orbital pericentres can be stripped prior to infall due
to interactions with more massive galaxies. Tidal effects lead to a reduction
in the dark matter density, while the profile remains cuspy.
Navarro-Frenk-White profiles are consistent with the kinematic data within
3$\sigma$ in the innermost regions, while profiles with shallow cusps or cores
provide a better fit. We predict that if the reduction of the dark matter
density in Fornax occurs, at least in part, because of the action of Galactic
tides, then tidal tails should be visible with a surface brightness limit of
$\sim35-36$ mag arcsec$^{-2}$ over a survey area of $\gtrsim$100 deg$^2$.