The mysterious galactic halo: the enigma of the edges of the Milky Way

The Milky Way, our home galaxy, is a vast and complex system of stars, planets, gas and dust. However, one of its most enigmatic and least understood components is the galactic halo.

What is the Galactic Halo?

The galactic halo is a spherical region surrounding the disk and central bulge of the Milky Way. Although less dense compared to these brighter, star-filled regions, the halo is of fundamental importance in the galactic structure. This halo extends far beyond the visible edge of the galaxy, reaching distances of hundreds of thousands of light years.

Composition of the Galactic Halo

The galactic halo is composed of several key elements:

1. Stars: Stars in the halo are generally old and metal-poor, indicating that they formed early in the galaxy’s history. These stars are more sparse and fewer in number than those in the galactic disk.

2. Dark Matter: One of the most intriguing components of the halo is dark matter, a form of matter that does not emit or interact with electromagnetic radiation in an observable way, but whose existence is inferred by its gravitational effects. It is estimated that most of the mass of the galactic halo is composed of dark matter, which plays a crucial role in maintaining the structure of the galaxy.

3. Gas and Dust: The halo also contains ionized gas and, to a lesser extent, cosmic dust. The hot gas in the halo extends to great distances and can have temperatures of millions of degrees, as detected through X-ray emission.

(Photo: ESO/S. Brunier)

Importance of the Galactic Halo

1. Galactic Evolution: The galactic halo is essential to understanding the evolution of the Milky Way. The motions and compositions of the stars in the halo provide clues about the formation and early history of the galaxy. Stars in the halo often display elliptical and disordered trajectories, suggesting a violent past of galactic mergers and accretions.

2. Dark Matter: The distribution of dark matter in the galactic halo influences the motion of stars and gas in the galaxy. Studies of the rotation rates of stars in the galactic disk have shown that dark matter is essential to explain the stability and dynamics of the Milky Way.

3. Interaction with Other Galaxies: The galactic halo interacts with satellite galaxies and star streams orbiting the Milky Way. These interactions can result in the transfer of matter and the disruption of galactic structures. Star streams, which are remnants of disintegrated minor galaxies, provide evidence of these past interactions.

Recent Discoveries

1. Detailed Maps: Recent space missions, such as the European Space Agency’s Gaia, have produced detailed maps of the stars in the galactic halo. These data allow astronomers to trace stellar motions and compositions with unprecedented precision, offering new insights into the structure and dynamics of the halo.

2. X-ray studies: X-ray observations have revealed the presence of hot, extended gas in the galactic halo. This gas may play a role in feeding new stars and regulating the intergalactic medium.