UCLA

Stellar orbits around the supermassive black hole (SMBH) at Galactic center (GC) provide unique insight into the physics and astrophysics of SMBHs and how they interact with their host galaxies. In this dissertation, I use stellar orbits at the GC to directly measure the distribution of extended mass around the Milky Way’s central SMBH. Dynamical theories predict that there should be a steep increase in mass density, or a cusp, towards the center of relaxed stellar populations around supermassive black holes. While the theory of stellar cusp formation is well motivated by physics and therefore widely adopted, it has never been observationally verified in the direct vicinity (within 0.5 parsecs) of a supermassive black hole. Observational verification is important because the assumption that cusp models are correct underlies a wide range of calculations that impact understandings of astrophysical processes such as galaxy evolution, the growth of and dynamics around SMBHs, and the origins and rates of gravitational wave emission. The GC provides a unique laboratory to test cusp theories as it hosts stellar populations old enough to be dynamically relaxed, and is the only galactic nucleus close enough to measure the stellar density distribution of the innermost region around an SMBH. This work uses multi-star General Relativistic orbit fits to probe the extended mass within the central 0.01 pc around the SMBH at the GC. Assuming a canonical Bahcall-Wolf cusp with a power-law index of 1.5, measurements of orbital precession of the 16-year-period star S0-2 suggest ∼18,000 � 12,000 � 6,000 M⊙ of extended mass within the central 0.01 pc of the Galaxy. This measurement from S0-2 alone is consistent with predictions from dynamical theories and allows for the possibility of a dark cusp, though still allows for the possibility of a missing cusp (no extended mass) at the 2σ-level. One of the key advancements of this work is the ability to move beyond fits to S0-2 alone and simultaneously constrain multiple stars with a relativistic orbit model. Added constraints from additional stars make it possible to leave the power-law index of the density profile as a free parameter. Multi-star relativistic orbit fits with the power-law index left as a free parameter suggest evidence for a non-zero extended mass within the central 0.01 pc of the GC that follows a core-like density profile rather than a cusp-like density profile. The inferred value of extended mass is ∼60,000 � 20,000 � 6,000M⊙, following a density profile with a 1σ upper limit of γ=0.46 (2σ<0.76, 3σ<1.66)—consistent with the core-like profile inferred from observations of late-type stars in the GC. Updated values for the mass of and distance to the SMBH at the GC, which are fundamental parameters for modeling the central SMBH, are also presented: MBH = (4.18 � 0.03 � 0.01) � 10^6 M⊙ and R0= (8.16 � 0.04 � 0.01) kpc.