Exploring millicharged dark matter components from the shadows

TL;DR

This paper investigates how millicharged dark matter components could influence black hole shadows and photon spheres, revealing potential observable effects or constraints on dark matter models through astrophysical measurements.

Contribution

It provides a detailed analysis of the effects of millicharged dark matter plasmas on black hole shadows, highlighting unique signatures and potential for observational constraints.

Findings

Significant effects on black hole shadows in low particle mass regions.

Distinct modifications to photon sphere and shadow radii due to dark matter plasmas.

Potential to constrain dark matter models through future astronomical observations.

Abstract

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.