# Rotation curves and orbits in the scalar field dark matter halo spacetime

## TL;DR

This paper explores the properties of a scalar field dark matter halo spacetime derived from a dilaton black hole, analyzing geodesics, rotation curves, and light deflection to understand gravitational effects.

## Contribution

It demonstrates that the flat curve condition corresponds to the zero mass limit of a dilaton black hole with an exponential potential, and investigates the impact of the dilaton on orbits and light deflection.

## Key findings

- Dilaton increases overall gravitational effects.
- Dilaton decreases size of innermost stable circular orbits.
- Photon sphere radius varies with dilaton strength.

## Abstract

The spacetime satisfying the flat curve condition for galaxies is shown to be the zero mass limit of the dilaton blackhole with an exponential potential. We derive the geodesic equations and by studying rotational curves and light deflection, and find that the presence of the dilaton increases the gravitational effects overall but diminishes the contribution from the central mass when the distances are close to the black hole. Also, the dilaton reduces the size of innermost stable circular time-like orbits, while the radius of the photon sphere may be larger or smaller than its Schwrazschild counterpart, depending on the strength of the dilaton field. We also show that a generalisation of the flat-curve-condition spacetime considered elsewhere in the literature does not solve the Einstein equation.