Thursday, February 01, 2024

Stars travel more slowly at Milky Way’s edge suggesting less dark matter in the core

Amazing stuff!

Maybe dark matter is not so dark! Reminds of how the terra incognita disappeared as exploration increased and knowledge accumulated!

"By clocking the speed of stars throughout the Milky Way galaxy, MIT physicists have found that stars further out in the galactic disk are traveling more slowly than expected compared to stars that are closer to the galaxy’s center. The findings raise a surprising possibility: The Milky Way’s gravitational core may be lighter in mass, and contain less dark matter, than previously thought.
The new results are based on the team’s analysis of data taken by the Gaia and APOGEE instruments. Gaia is an orbiting space telescope that tracks the precise location, distance, and motion of more than 1 billion stars throughout the Milky Way galaxy, while APOGEE is a ground-based survey. The physicists analyzed Gaia’s measurements of more than 33,000 stars, including some of the farthest stars in the galaxy, and determined each star’s “circular velocity,” or how fast a star is circling in the galactic disk, given the star’s distance from the galaxy’s center. ..."

From the abstract:
"In this paper, we construct the circular velocity curve of the Milky Way out to ∼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 GaiaDR3, 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 ... than a generalized Navarro–Frenk–White (NFW) profile. The virial mass of the best-fitting dark matter halo profile is only 
 M⊙, 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-fitting 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."

Study: Stars travel more slowly at Milky Way’s edge | MIT News | Massachusetts Institute of Technology The findings suggest our galaxy’s core may contain less dark matter than previously estimated.


Figure 3 Galactocentric XY-plane map of the 33 335 stars used for calculating circular velocities, plotted in 0.5 kpc bins. The vectors represent the mean velocity of stars within each bin, colour coded by the number of stars in each bin.



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