Dynamical Constraints on Dark Matter Spike at the Galactic Centre from Stellar Orbits
In this paper, Thomas Lacroix investigates the dynamics of dark matter (DM) profiles at the Galactic Centre, specifically around the supermassive black hole (SMBH) Sgr A*. Using astrometric and spectroscopic data up to 2016, the author examines the orbits of the S2 star to constrain the size of a potential DM spike—a highly dense region of DM expected near the SMBH if it has grown adiabatically. This analysis provides the most direct constraints on such a spike for non-annihilating DM, a pertinent consideration in the context of indirect DM searches.
Lacroix acknowledges the longstanding debate regarding dark matter distribution in galaxies, known as the cusp/core problem. Numerical simulations tend to predict cuspidal profiles, while observations often favor cored profiles. This dichotomy extends to subparsec scales near SMBHs where the enhancement of DM density is hypothesized. A potential spike, characterized by a power-law density increase, could significantly influence indirect DM detection due to heightened annihilation signatures. However, the formation of such a spike is contingent on various dynamical processes, including mergers and BH growth patterns, rendering its existence and morphology uncertain.
The methodology employed involves orbit-fitting procedures to examine deviations from the standard point-mass gravitational potential of Sgr A*. The paper translates the physics of celestial mechanics into constraints on the DM profile by analyzing the characteristics of the star S2's orbit. Specifically, this entails using both existing dynamical data of the Milky Way and extensive time-series data on S2 from the VLT and Keck observatories to fit the mass model parameters and ascertain the gravitational influence of a potential spike.
Numerical modeling confirms that constraints on the DM spike radius vary widely based on the characteristics of the outer halo. For non-annihilating DM, spike radii greater than several tens of parsecs for halos with slopes (gamma) around 1, and a few hundred parsecs for more cored distributions (gamma less than 0.6), are excluded with high confidence. These results are critical as they rule out scenarios involving substantial, concentrated DM spikes at the Galactic Centre for the specified halo models.
Beyond presenting these constraints, the paper highlights the importance of future observations, such as those expected with the Extremely Large Telescope, which could further refine or lower the constraints on DM density profiles. Understanding the spike's properties not only has implications for our comprehension of galactic dynamics but could also impact the search for non-luminous DM particles through gravitational effects.
In summary, this research underlines the utility of stellar kinematics in probing DM density at galactic cores, offering evidence against significant DM spikes under the assumption of non-annihilating or weakly annihilating DM—notably contributing to the body of work attempting to reconcile galactic dynamics with the DM profiles proposed by simulations.