Reassessing Cosmic Standards: Dwarf Galaxies and the Puzzle of Dark Matter

A recent examination of Gaia satellite data has brought to light a significant paradigm shift in our understanding of dwarf galaxies orbiting the Milky Way. Previous assumptions regarding their long-lasting existence and rich dark matter content have been challenged by their apparent transience and reduced influence of dark matter, marking a pivotal moment in the realm of astrophysical research.

Traditionally perceived as enduring satellites of our galaxy, a groundbreaking study now hints at the ephemeral nature of most dwarf galaxies, which might meet their demise shortly after entering the Galactic halo. The revelations, made possible by the latest catalog from the European Space Agency’s Gaia satellite, have cast a shadow of doubt over the equilibrium of these celestial bodies and have far-reaching implications for our cosmological model, particularly concerning the prevalence of dark matter within our immediate cosmic vicinity.

For eons, it was widely accepted that the dwarf galaxies encircling the Milky Way had been in orbit for nearly 10 billion years, necessitating copious amounts of dark matter to shield them from the formidable tidal forces induced by our galaxy’s gravitational pull. Dark matter was thought to account for the considerable variations in the velocities of stars within these dwarf galaxies.

However, the most recent data from Gaia has unveiled an entirely different perspective on the characteristics of dwarf galaxies. A team of astronomers from esteemed institutions, including the Paris Observatory – PSL, the Centre national de la recherche scientifique (CNRS), and the Leibniz Institute for Astrophysics Potsdam (AIP), employed a method to trace the Milky Way’s history by examining the relationship between an object’s orbital energy and its entry into the halo. Objects that arrived during an earlier era, when the Milky Way was less massive, possess lower orbital energies compared to more recent arrivals.

Strikingly, the orbital energies of most dwarf galaxies significantly surpass that of the Sagittarius dwarf galaxy, which entered the halo approximately 5 to 6 billion years ago. This implies that most dwarf galaxies arrived much more recently, within the past three billion years.

This recent influx suggests that the neighboring dwarf galaxies originated outside the halo, where nearly all of them harbored substantial reserves of neutral gas. When these gas-rich galaxies collided with the hot gas within the Galactic halo, the ensuing violence of shocks and turbulence wrought transformative changes upon the dwarf galaxies. Previously dominated by the rotation of gas and stars, these galaxies, after losing their gas, reached an equilibrium in which the gravity was balanced by the random motions of their remaining stars.

The gas loss process for dwarf galaxies is so tumultuous that it disrupts their equilibrium, leading to a misalignment between the speed of their stars and their gravitational acceleration. The combination of gas loss and gravitational shocks incurred during their plunge into the Galaxy elucidates the wide dispersion of stellar velocities within the remnants of these dwarf galaxies.

This investigation has raised intriguing questions about the role of dark matter. First and foremost, the absence of equilibrium makes it impossible to estimate the dynamical mass of the Milky Way’s dwarf galaxies and their dark matter content. Second, the traditional notion that dark matter stabilized the initial rotating disks of stars within these dwarf galaxies becomes problematic for objects that have lost their equilibrium. In fact, if dwarf galaxies already contained substantial dark matter, it would have prevented the transformation of the dwarfs into galaxies with the observed random stellar motions.

This newfound perspective on dwarf galaxies and their transformations in the Galactic halo offers a coherent explanation for many of their observed characteristics, particularly the presence of stars at significant distances from their centers. Surprisingly, their properties appear to align with a dearth of dark matter, in stark contrast to the prior consensus that dwarf galaxies were among the most dark matter-dominated entities.

This revelation gives rise to a plethora of pressing inquiries. Where are the anticipated dark matter-dominated dwarf galaxies that the conventional cosmological model predicts around the Milky Way? How can we determine the dark matter content of a dwarf galaxy when equilibrium cannot be assumed? What additional observations can distinguish between the proposed out-of-equilibrium dwarf galaxies and the classical model featuring dark matter-dominated dwarfs?

Reference: “The Accretion History of the Milky Way – II. Internal Kinematics of Globular Clusters and of Dwarf Galaxies” by Francois Hammer, Jianling Wang, Gary A Mamon, Marcel S Pawlowski, Yanbin Yang, Yongjun Jiao, Hefan Li, Piercarlo Bonifacio, Elisabetta Caffau, and Haifeng Wang, 20 November 2023, Monthly Notices of the Royal Astronomical Society. DOI: 10.1093/mnras/stad2922

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More about Dark Matter Enigma

• Gaia Satellite: Information about the Gaia satellite mission.
• Milky Way Galaxy: NASA’s overview of the Milky Way.
• Dark Matter: NASA’s explanation of dark matter.
• Cosmological Model: An overview of cosmological models on Wikipedia.
• The Royal Astronomical Society: The official website of the Royal Astronomical Society where the study was published.