The dark matter profile of the Milky Way inferred from its circular velocity curve (2303.12838v2)

Abstract: In this paper, we construct the circular velocity curve of the Milky Way out to $\sim 30$ kpc, providing an updated model of the dark matter density profile. We derive precise parallaxes for 120,309 stars with a data-driven model, using APOGEE DR17 spectra combined with Gaia DR3, 2MASS, and WISE photometry. At outer galactic radii up to 30 kpc, we find a significantly faster decline in the circular velocity curve compared to the inner parts. This decline is better fit with a cored Einasto profile with a slope parameter $0.91^{+0.04}_{-0.05}$ than a generalized Navarro-Frenk-White (NFW) profile. The virial mass of the best-fit dark matter halo profile is only $1.81^{+0.06}_{-0.05}\times10^{11}$ $M_{\odot}$, significantly lower than what a generalized NFW profile delivers. We present a study of the potential systematics, affecting mainly large radii. Such a low mass for the Galaxy is driven by the functional forms tested, given that it probes beyond our measurements. It is found to be in tension with mass measurements from globular clusters, dwarf satellites, and streams. Our best-fit profile also lowers the expected dark matter annihilation signal flux from the galactic centre by more than an order of magnitude, compared to an NFW profile-fit. In future work, we will explore profiles with more flexible functional forms to more fully leverage the circular velocity curve and observationally constrain the properties of the Milky Way's dark matter halo.

• The paper presents an updated Milky Way dark matter model using high-precision circular velocity data from APOGEE, Gaia, 2MASS, and WISE over 30 kpc.
• The paper finds a steeper outer velocity decline best modeled by a cored Einasto profile, implying a lower dark matter halo mass compared to the traditional gNFW profile.
• The paper assesses systematic uncertainties and predicts a significantly reduced dark matter annihilation signal flux, impacting both direct and indirect detection strategies.

The Dark Matter Profile of the Milky Way Inferred from its Circular Velocity Curve

The paper "The dark matter profile of the Milky Way inferred from its circular velocity curve" by Xiaowei Ou et al. presents an extensive investigation into the structure of the Milky Way's dark matter halo by analyzing its circular velocity curve. Leveraging high-precision parallax data from APOGEE DR17, Gaia DR3, 2MASS, and WISE, the authors have constructed an updated model of the Milky Way's velocity curve extending out to approximately 30 kpc.

Key Findings

1. Circular Velocity Curve Analysis: The analysis reveals a significantly steeper decline in the circular velocity at outer galactic radii compared to inner galactic parts. This decline is more accurately modeled by a cored Einasto profile with a slope parameter of approximately +0.04 rather than the traditionally used generalized Navarro-Frenk-White (gNFW) profile.
2. Dark Matter Halo Profile: The derived dark matter profile indicates a virial mass of approximately +0.06, which is notably lower than predictions made with a generalized NFW profile. This suggests a less massive halo, in contrast with other mass measurements from globular clusters, dwarf satellites, and streams.
3. Systematic Uncertainties: The work presents an analysis of potential systematic effects, which primarily affect observations at larger radii.
4. Dark Matter Annihilation Signal Estimation: The paper also identifies a lower expectation for the dark matter annihilation signal flux from the galactic center, downward by more than an order of magnitude compared to expectations with an NFW profile-fit.

Implications

1. Challenges for Galactic Models: The preference for a cored profile raises questions about the formation history of the Milky Way's dark matter halo. If confirmed, such a profile may imply episodic starbursts or reduced central dark matter accretion phases during the Galaxy's evolutionary history.
2. Constraints on Dark Matter Detection:
   • Direct Detection: The local dark matter density estimated is important for calibrating direct detection experiments aimed at finding dark matter particles.
   • Indirect Detection: The estimated reduction in dark matter annihilation signals could have significant implications for gamma-ray observational strategies and theoretical models predicting particle interactions.
3. Future Prospects: The authors plan further investigations using profiles with more flexible functional forms, facilitating improved constraints based on the circular velocity curve.

This paper serves as a crucial contribution to our understanding of galactic profiles, offering insights that challenge traditional dark matter model assumptions and prompting further examination into the dynamics and history of the Milky Way. Whether these findings will prompt refinements in cosmic structure models or prove indicative of broader principles in galactic evolution remains an open question for future research.