An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

Abstract: We present new, more precise measurements of the mass and distance of our Galaxy's central supermassive black hole, Sgr A*. These results stem from a new analysis that more than doubles the time baseline for astrometry of faint stars orbiting Sgr A*, combining two decades of speckle imaging and adaptive optics data. Specifically, we improve our analysis of the speckle images by using information about a star's orbit from the deep adaptive optics data (2005 - 2013) to inform the search for the star in the speckle years (1995 - 2005). When this new analysis technique is combined with the first complete re-reduction of Keck Galactic Center speckle images using speckle holography, we are able to track the short-period star S0-38 (K-band magnitude = 17, orbital period = 19 years) through the speckle years. We use the kinematic measurements from speckle holography and adaptive optics to estimate the orbits of S0-38 and S0-2 and thereby improve our constraints of the mass ($M_{bh}$) and distance ($R_o$) of Sgr A*: $M_{bh} = 4.02\pm0.16\pm0.04\times10^6~M_{\odot}$ and $7.86\pm0.14\pm0.04$ kpc. The uncertainties in $M_{bh}$ and $R_o$ as determined by the combined orbital fit of S0-2 and S0-38 are improved by a factor of 2 and 2.5, respectively, compared to an orbital fit of S0-2 alone and a factor of $\sim$2.5 compared to previous results from stellar orbits. This analysis also limits the extended dark mass within 0.01 pc to less than $0.13\times10^{6}~M_{\odot}$ at 99.7% confidence, a factor of 3 lower compared to prior work.

• The paper refines Sgr A*'s mass and distance using a novel multistar orbit analysis of stars S0-2 and S0-38.
• It leverages over 20 years of combined speckle imaging and adaptive optics data to achieve significantly reduced uncertainties.
• The improved measurements provide tighter constraints on dark mass distribution near the galactic center, impacting Milky Way models.

An Improved Distance and Mass Estimate for Sgr A* from a Multistar Orbit Analysis

The paper under discussion presents refined measurements of the distance to, and mass of, the supermassive black hole located at the center of our galaxy, known as Sgr A*. The authors achieve this advancement by employing an extended dataset, which doubles the temporal coverage of stellar observations around Sgr A*, and applying enhanced astrometric techniques. Specifically, this research includes over 20 years of both speckle imaging and adaptive optics data, significantly extending the temporal baseline over previous studies.

A pivotal aspect of the paper is the introduction of a novel analysis technique that facilitates the integration of historical speckle imaging data with recent adaptive optics observations. The methodology allows for a comprehensive characterization of the two short-period stars, S0-38 and S0-2, which orbit the galactic center. The authors deploy speckle holography, a sophisticated image reconstruction method, to enhance the quality and depth of the speckle images, thereby enabling the tracking of challenging observational targets like S0-38.

Key findings and contributions from the analysis include:

1. Refined Parameters for Sgr A: The study reports new values for the mass of Sgr A at $4.02 \pm 0.16 \times 10^6~M_{\odot}$ and the distance to the galactic center as $7.86 \pm 0.14$ kpc. These estimates are significant improvements over previous measurements, reducing the uncertainty in both mass and distance by factors of approximately 2 and 2.5, respectively.
2. Multiple Orbital Measurements: By integrating data from two stars, the paper provides a robust multistar approach to constraining the properties of Sgr A*. The analysis of S0-2 and S0-38's orbits offers a comprehensive reflection of the central gravitational potential.
3. Constraints on Extended Mass Distributions: The research also constrains the presence of an extended distribution of dark mass around Sgr A*. It depicts an upper limit on the extended mass within 0.01 pc of less than $0.13 \times 10^6~M_{\odot}$ with 99.7% confidence, representing a significant, three-fold improvement over prior evaluations.
4. Implications for Galactic Models: The findings present revised parameters crucial for models of the Milky Way's rotation curve, dark matter distribution, and overall kinematic properties.

This research not only advances the precision of fundamental galactic center measurements but also sets the stage for future analyses using additional short-period stars. As the precision of these measurements has a direct impact on various astrophysical parameters and models, it contributes toward a more detailed understanding of both our galaxy's structure and the dynamics of supermassive black holes in other galaxies. Future work may capitalize on these methods to examine the relativistic effects expected during S0-2's next closest approach, which will further test our understanding of general relativity in extreme gravitational regimes.