Exploring millicharged dark matter components from the shadows

Dark matter sectors with hidden interactions have been of much interest in recent years. These frameworks include models of millicharged particles as well as dark sector bound states, whose constituents have electromagnetic gauge interactions. These exotic, charged states could constitute a part of the total dark matter density. In this work, we explore in some detail the various effects, on the photon sphere and shadow of spherically symmetric black holes, due to dark matter plasmas furnished by such sectors. Estimating physically viable parameter spaces for the particle physics models and taking semi-realistic astrophysical scenarios that are amenable to theoretical analyses, we point out various modifications and characteristics that may be present. Many of these effects are unique and very distinct from analogous situations with conventional baryonic plasmas, or neutral perfect fluid dark matter surrounding black holes. While in many physically viable regions of the parameter space the effects on the near-horizon regions and black hole shadows are small, in many parts of the low particle mass regions the effects are significant, and potentially measurable by current and future telescopes. Such deviations, for instance, include characteristic changes in the photon sphere and black hole shadow radii, unique thresholds for the dark matter plasma dispersion where the photon sphere or black hole shadow vanishes, and where the dark matter plasma becomes opaque to electromagnetic waves. Alternatively, we point out that a non-observation of such deviations and characteristics, in future, could put constraints on interesting regions of the particle physics parameter space.