New findings from MIT suggest that stars located on the outer edges of the Milky Way are moving at a slower pace than expected, indicating a possible lower presence of dark matter in the galaxy’s core, which challenges existing astronomical theories. Credit: SciTechDaily.comData collected by MIT physicists reveals slower rotation of outer stars in the Milky Way, implying a less massive core with reduced dark matter, contrary to previous assumptions.By determining the speed at which stars move throughout the Milky Way galaxy, MIT researchers have determined that stars located further out in the galactic disk are rotating at a slower pace than anticipated in comparison to stars closer to the core. These findings raise the possibility that the Milky Way’s central gravitational core could have a lower mass and contain less dark matter than previously believed.The latest results are derived from a thorough analysis of data obtained from the Gaia and APOGEE instruments. Gaia, a space telescope, monitors the precise positions, distances, and movements of over 1 billion stars within the Milky Way galaxy, while APOGEE is a ground-based survey. The physicists examined Gaia’s measurements of more than 33,000 stars, including some of the most distant stars in the galaxy, and determined the “circular velocity” of each star, referring to its speed as it orbits within the galactic disk, taking into consideration the star’s distance from the galaxy’s center.Gaining Insight into Galactic RotationThe researchers plotted the speed of each star against its distance in order to generate a rotation curve, a standard graph in astronomy that represents the speed at which matter rotates at a specific distance from the center of a galaxy. The shape of this curve provides scientists with information about the distribution of visible and dark matter throughout a galaxy.”We were extremely surprised to observe that this curve remained consistently flat up to a certain distance before beginning to decline,” states Lina Necib, an assistant professor of physics at MIT. “This suggests that the outer stars are rotating at a slightly slower pace than anticipated, which is a very unexpected outcome.”
Based on a study conducted by MIT physicists, it is suggested that the Milky Way’s central gravitational core may have a lower mass and contain less dark matter than previously assumed. Credit: ESA/Gaia/DPAC, Edited by MIT NewsChallenging Established Dark Matter TheoriesThe team translated the new rotation curve into a dark matter distribution capable of explaining the deceleration of outer stars and found that the resulting map indicated a less dense central region of the galaxy than expected. In other words, the core of the Milky Way may have a lower density and less dark matter compared to what scientists have previously theorized.”This result is at odds with other measurements,” Necib mentions. “There is something intriguing happening, and it’s incredibly exciting to unravel the cause, in order to establish a coherent understanding of the Milky Way.”The team has published their findings in the Monthly Notices of the Royal Society Journal this month. The study’s MIT co-authors, including Necib, are first author Xiaowei Ou, Anna-Christina Eilers, and Anna Frebel.”In the Void”Similar to most galaxies, the Milky Way rotates akin to water in a whirlpool, and its rotation is driven, in part, by all the matter within its disk. In the 1970s, astronomer Vera Rubin was the first to observe that galaxies rotate in ways that cannot be solely attributed to visible matter. She and her colleagues measured the circular velocity of stars and discovered that the resulting rotation curves remained consistently flat. This implied that some other type of invisible matter must be influencing distant stars to provide them with an additional push.Rubin’s work in rotation curves was among the initial conclusive evidence for the existence of dark matter, an invisible, unknown entity that is estimated to outweigh all the stars and visible matter in the universe.Since then, astronomers have observed similar flat curves in distant galaxies, further reinforcing the presence of dark matter. Only recently have astronomers endeavored to chart the rotation curve within our own galaxy with stars.”Measuring a rotation curve is evidently more challenging when situated within a galaxy,” Ou comments.New Insights From Gaia DataIn 2019, Anna-Christina Eilers, an assistant professor of physics at MIT, worked towards mapping the Milky Way’s rotation curve using an earlier set of data disclosed by the Gaia satellite. This dataset incorporated stars situated as far as 25 kiloparsecs, or approximately 81,000 light-years, from the galaxy’s core.Based on these data, Eilers observed that the Milky Way’s rotation curve appeared to be flat, albeit with a slight decline, similar to other distant galaxies, indicating a high density of dark matter within the galaxy’s core. However, this perspective has now shifted, following the release of a fresh batch of data by the telescope, which encompasses stars positioned as far as 30 kiloparsecs, almost 100,000 light years from the galaxy’s core.”At these distances, we’re essentially at the edge of the galaxy, where star density diminishes,” Frebel remarks. “It’s uncharted territory, where we’re essentially in empty space.”Unusual TensionFrebel, Necib, Ou, and Eilers promptly began evaluating the new data obtained from Gaia, with the intention of expanding on Eilers’ initial rotation curve. To enhance their analysis, the team supplemented Gaia’s data with measurements from APOGEE, which scrutinizes highly detailed attributes of over 700,000 stars within the Milky Way, such as their luminosity, temperature, and elemental composition.”We input all this information into an algorithm to establish connections that can offer us more reliable estimates of a star’s distance,” Ou explains. “That’s how we can extend our reach to greater distances.”The team accurately determined the distances of more than 33,000 stars and employed these findings to produce a three-dimensional map of stars spread across the Milky Way, extending to approximately 30 kiloparsecs. This map was then integrated into a circular velocity model to predict the speed at which a given star must travel, considering the distribution of all other stars within the galaxy. Subsequently, they plotted the velocity and distance of each star in a chart, resulting in an updated rotation curve of the Milky Way.”This is where things became unusual,” Necib remarks.Rather than observing a subtle decline as seen in previous rotation curves, the team noted a more pronounced dip towards the outer limits. This unexpected deceleration suggests that while stars may travel at a consistent pace up to a certain distance, they abruptly slow down at the farthest reaches. Stars located at the periphery appear to be moving slower than anticipated.Examining Galactic EnigmasUpon translating this rotation curve into the amount of dark matter dispersed throughout the galaxy, the team ascertained that the core of the Milky Way may contain less dark matter than previously conjectured.”This outcome is in disagreement with other measurements,” Necib affirms. “Comprehending this result will have significant repercussions. It could lead to the identification of more concealed masses just beyond the galactic disk or a reassessment of our galaxy’s state of equilibrium. We aim to pursue answers to these questions in future work, utilizing high-resolution simulations of galaxies similar to the Milky Way.”Reference: “The dark matter profile of the Milky Way inferred from its circular velocity curve” by Xiaowei Ou, Anna-Christina Eilers, Lina Necib, and Anna Frebel, 08 January 2024, Monthly Notices of the Royal Astronomical Society.
DOI: 10.1093/mnras/stae034This research received partial funding from the National Science Foundation.
