Sagittarius A* size measurements
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Sagittarius A* Size Measurements Across Radio Wavelengths
Intrinsic Size of Sagittarius A* and Wavelength Dependence
Multiple high-resolution radio observations have established that the intrinsic size of Sagittarius A* (Sgr A*)—the supermassive black hole at the center of our galaxy—scales with observing wavelength following a power law. Studies using Very Long Baseline Interferometry (VLBI) and the Event Horizon Telescope (EHT) have found that the intrinsic size is proportional to λ^γ, with γ typically between 1.2 and 1.7, depending on the wavelength range and scattering model used Bower2006Cho2021Bower2006+1 MORE. For example, at 0.35 cm (3.5 mm), the intrinsic size is about 13 Schwarzschild radii (Rs), and at 7 mm, it is measured to be around 24 Rs Bower2006Bower2004Bower2006+1 MORE.
Impact of Interstellar Scattering on Size Measurements
Interstellar scattering significantly affects radio images of Sgr A*, especially at longer wavelengths. Recent work has refined the scattering law, showing that the major axis size of the scattering kernel is about 6% smaller than previously thought, which in turn increases the estimated intrinsic size of Sgr A* by up to 20% at shorter wavelengths Bower2006Oyama2024. Physically motivated scattering models confirm that an anisotropic Gaussian kernel is a good approximation for wavelengths longer than 1 cm, but refractive scattering effects become less significant at millimeter wavelengths, allowing for more accurate intrinsic size measurements Issaoun2019Oyama2024.
High-Resolution Imaging and Morphology
Recent VLBI campaigns, including those with the EHT and the addition of ALMA, have achieved angular resolutions as fine as 87 microarcseconds at 3.5 mm, revealing a nearly symmetric, compact source with a major axis size of about 120 ± 34 microarcseconds (12 ± 3.4 Rs) . At 1.3 mm, the EHT finds the emission is dominated by a ring-like structure with a diameter of about 50 microarcseconds, consistent with emission from near the event horizon Akiyama2022Johnson2018. The intrinsic structure is well described by a symmetric Gaussian at both 1.3 cm and 7 mm, with axial ratios close to unity, indicating near-isotropy Cho2021Oyama2024.
Constraints on Black Hole Models and Mass
The measured sizes and their wavelength dependence place strong constraints on models of the accretion flow and emission mechanisms around Sgr A*. The data support models where the radio emission originates very close to the event horizon, and the observed sizes are consistent with the predictions for a black hole of about 4 million solar masses Lo1998Melia1992Johnson2018. The EHT results further confirm that the observed image size is within about 10% of the predictions for a Kerr black hole, providing strong evidence for the black hole nature of Sgr A* .
Conclusion
In summary, the intrinsic size of Sagittarius A* decreases with shorter observing wavelengths, following a power-law relationship, and is best measured at millimeter wavelengths where scattering effects are minimized. The most recent and precise measurements show that the emission region is compact, nearly symmetric, and consistent with theoretical expectations for a supermassive black hole, providing critical insights into the nature of Sgr A* and its immediate environment Bower2006Cho2021Bower2004+5 MORE.
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