How chameleons core dwarfs with cusps

The presence of a scalar field that couples nonminimally and universally to matter can enhance gravitational forces on cosmological scales while restoring general relativity in the Solar neighborhood. In the intermediate regime, kinematically inferred masses experience an additional radial dependence with respect to the underlying distribution of matter, which is caused by the increment of gravitational forces with increasing distance from the Milky Way center. The same effect can influence the internal kinematics of subhalos and cause cuspy matter distributions to appear core-like. Specializing to the chameleon model as a worked example, we demonstrate this effect by tracing the scalar field from the outskirts of the Milky Way halo to its interior, simultaneously fitting observed velocity dispersions of chemo-dynamically discriminated red giant populations in the Fornax and Sculptor dwarf spheroidals. Whereas in standard gravity these observations suggest that the matter distribution of the dwarfs is cored, we find that in the presence of a chameleon field the assumption of a cuspy Navarro-Frenk-White profile becomes perfectly compatible with the data. Importantly, chameleon models also predict the existence of slopes between two stellar subcomponents that in Newtonian gravity would be interpreted as a decline of matter density toward the dwarf center. Hence, an observation of such an apparently pathological scenario may serve as a smoking gun for the presence of a chameleon field or a similar modification of gravity, independent of baryonic feedback effects. In general, measuring the dynamic mass profiles of the Milky Way dwarfs provides stronger constraints than those inferred from the screening scale of the Solar System since these are located at greater distances from the halo center.

Figure 1

Velocity dispersions and half-light radii of the chemo-dynamically discriminated stellar subcomponents of Fornax (left panel) and Sculptor (middle panel) measured by Ref. [12]. Whereas the assumption of a cuspy NFW matter density profile ($\gamma =0$) does not provide good fits in Newtonian gravity, it becomes compatible with observations in chameleon gravity. Right panel: The slope $\mathrm{\Gamma }$ of the dynamic NFW mass profile of Fornax inferred from its two red giant populations when alternately varying its location in the Milky Way $r_{\mathrm{MW}}$, the position of the mean half-light radius $r_{\mathrm{h}}$ of the stars in the dwarf, as well as its virial mass and concentration while keeping other parameters fixed. In general, in both gravitational models $\mathrm{\Gamma }$ becomes larger if baryonic feedback flattens the matter distribution ($0\le \gamma <1$). In Newtonian gravity $\mathrm{\Gamma }>3$ would be interpreted as a decline of matter density toward the center of the dwarf. Hence, an observation of such an apparently pathological scenario may serve as a smoking gun for modified gravity.