One of the greatest mysteries in astrophysics today is the nature of dark matter, the elusive substance that is believed to make up the majority of the matter in the universe. While dark matter cannot be directly observed, its presence can be inferred through its gravitational effects on visible matter. One of the most important pieces of evidence for the existence of dark matter is the circular velocity curve of galaxies, which measures how fast objects move at a given distance from the center of the galaxy.
In a recent study published in 2023, Xiaowei Ou and colleagues present new measurements of the circular velocity curve for the Milky Way galaxy out to a distance of 30 kiloparsecs. The team used data from the APOGEE DR17 spectra combined with photometry measurements from Gaia, 2MASS, and WISE to derive precise parallaxes for over 120,000 stars. The resulting sample size was approximately 50% larger than in a previous study, allowing for more accurate measurements of the circular velocity curve.
The team found that the circular velocity curve for the Milky Way declines at a faster rate at larger radii compared to inner radii. This suggests that the dark matter density profile of the Milky Way is not well described by a generalized or contracted Navarro-Frank-White (NFW) profile, as was previously believed. Instead, a cored Einasto profile with a slope parameter of 1.13 was found to be a better fit to the data.
The team’s measurements also allowed them to calculate the virial mass and local dark matter density of the Milky Way’s dark matter halo. The virial mass was found to be lower than that estimated from a generalized NFW profile, but the local dark matter density at the solar position was consistent with previous estimates from the literature.
Additionally, the team calculated the J-factor for annihilating dark matter towards the galactic center, which is an important quantity for detecting dark matter particles. The J-factor was found to be approximately 8% of the value obtained from a standard NFW profile used in previous studies.
Overall, the team’s study highlights the importance of the circular velocity curve for understanding the nature of dark matter halos in galaxies. By extending the circular velocity curve measurements to larger radii with a larger sample size, the team was able to refine our understanding of the Milky Way’s dark matter density profile. Their findings suggest that the Milky Way’s dark matter halo may have a cored Einasto profile, which could have implications for the formation and evolution of the Milky Way.
While the study provides important insights into the nature of dark matter in the Milky Way, much work remains to be done to fully understand this mysterious substance. Future studies combining the Milky Way’s star formation and accretion history with the circular velocity curve measurements may provide further insights into the formation and evolution of the Milky Way and its dark matter halo. In the meantime, the circular velocity curve remains a powerful tool for probing the properties of dark matter halos in galaxies, providing clues to one of the most fundamental mysteries in astrophysics.
Source: Ou, Xiaowei et al. “The dark matter profile of the Milky Way inferred from its circular velocity curve.” (2023). https://doi.org/10.48550/arXiv.2303.12838