New data from the European Gaia mission shows that galaxies long considered Milky Way satellites are actually newcomers to our cosmic neighborhood. Some may join our galaxy, while others will pass by.
Dwarf galaxies, the universe's smallest and faintest with thousands to billions of stars, were once thought to be longstanding companions of the Milky Way. However, fresh analysis from the European Space Agency (ESA) paints a different picture. The findings are detailed in The Astrophysical Journal.
Launched by ESA in 2013, Gaia is a pioneering satellite designed to map the positions, motions, distances, and physical properties of billions of stars in our galaxy. It orbits the Sun-Earth Lagrange point 2 (L2), 1.5 million kilometers from Earth, where gravitational balance ensures stability.
To date, Gaia has cataloged over 1.8 billion stars. Recently, a team led by François Hammer at the Observatoire de Paris leveraged this data to precisely track the movements of forty dwarf galaxies near the Milky Way.
Astronomers computed the three-dimensional velocities of these galaxies, deriving their orbital and angular momentum energies.
Surprisingly, these galaxies move far faster than bound objects like giant stars or clusters orbiting the Milky Way. True satellites would exhibit lower energies, indicating these dwarfs arrived in our vicinity within the last billion years.
This aligns with prior revelations about the Large Magellanic Cloud (LMC), a southern sky dwarf once deemed a Milky Way satellite but later found to be unbound due to its high speed. The same pattern now applies to dozens more.
Their futures remain uncertain: some may be captured as true satellites, others will escape. Predictions hinge on the Milky Way's exact mass, which current estimates vary by a factor of two.
