Galactic haloes are spherical regions of hot gas enveloping galaxies.

Those vast and virtually invisible components of galaxies extend well beyond their visible boundaries. While the spiral arms and bright cores of galaxies capture most of our attention, haloes are enormous spheroidal regions that envelop these structures like cosmic cocoons.

They are composed of dark matter, hot gas, and a sparse sprinkling of old stars and globular clusters, making them crucial to understanding the formation and evolution of galaxies.

Most of a galaxy’s mass is actually contained within its dark matter halo – an elusive substance that does not emit light but exerts its gravitational influence. These haloes provide the scaffolding that shapes galaxies and guides their motion. Embedded within this dark framework is hot gas that can reach millions of degrees, often glowing faintly in X-rays.

This matter reservoir may act as a galactic recycling system, supplying material for future star formation while also trapping remnants from supernova explosions.

Haloes also preserve a fossil record of galactic history.

Ancient stars and globular clusters, some of which formed during the galaxy’s infancy, float in this tenuous region, tracing paths that hint at past mergers and interactions. When galaxies collide or cannibalize their smaller neighbours, their haloes absorb these fragments, leaving ghostly trails of disrupted matter.

Typically, galactic haloes extend far beyond the visible parts of a galaxy, spanning up to 300,000 light-years or more in diameter. About 92 kiloparsecs.

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