GALACTIC HALOS are an apparent spherical part of the galaxy that extend beyond the galaxy's main body. These halos are more easily seen and observable in spiral galaxies rather than elliptical galaxies. It appears as a glowy, outer surroundings of a galaxy which is why it's called a Halo.
There are three main components of a galactic halo:
1. The Stellar part
2. The Corona part
3. The Dark Matter part.
The stellar part consists of outer field stars and globular clusters. These stars are older, sometimes greater than 10 billion years old. Because we are on the outer parts of our galaxy (still far away from the corona relatively) we can observe our halo very well. The Milky Way's stellar halo has stars that are older than 12 billion years old. The star formation in our halo however, ceased eons ago.
Galactic Corona or the corona part of the halo, is the gas of the galaxy that extends way beyond the centre of the galaxy. These can be observed in different spectrums of light.
The dark matter halo, is of course the most complicated because, well, it's dark matter. This is a theory backed up by a well established density profile (NFW profile) that dark matter extends throughout the galaxy and well beyond the parts we can see and thus must contribute to the halo. It is theorised that the mass of the dark matter halo is far greater than the entire galaxy altogether.
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(Picture Credits: Science Daily, astronomy.swin.edu.au, ESA, NASA)