Galactic halo

A galaxy has a big, round shape. It is called a halo. This halo goes far past the bright parts. It holds many old stars. These stars are very old. The halo also has hot gas. Some parts of the halo are invisible. This invisible part is very heavy. It helps hold the whole galaxy together. Scientists study the halo to learn more. It is a very big place.

A galaxy has a big, round part called a halo. It sits outside the main, bright parts of a galaxy. This halo has three main parts. First is the stellar halo. It is made of old stars and star clusters. These stars are very old. Most are over 12 billion years old. This part is not very bright. It only has about one percent of a galaxy's star mass.

Next is the galactic corona. This is a layer of hot gas. It reaches far from the center of the galaxy. We can find it using X-ray tools.

The third part is the dark matter halo. This part is invisible. We think it is there because of its gravity. This gravity helps hold the galaxy together. It is much heavier than the other parts.

How do halos form? They may form when small things merge into big ones. This is called a bottom-up way of growing. When things merge, gas moves to the center. But stars and dark matter stay in the halo. The Milky Way's halo may come from something called the Gaia Sausage.

A galaxy has a huge, round part called a halo. It sits outside the bright, main parts of a galaxy. This shape is easy to see in spiral galaxies. There, the round halo sits around a flat disc. In elliptical galaxies, the halo blends in more smoothly. We can study halos by looking at light from distant objects. These objects, like quasars, shine from far behind the galaxy. Scientists watch how the halo affects that passing light.

The stellar halo is one part of this big structure. It is made of field stars and globular clusters. This part is not very bright at all. It holds only about one percent of a galaxy's star mass. In the Milky Way, these stars are very old. Most are more than 12 billion years old. They are also metal-poor, which means they have few heavy elements. These stars move with a radial velocity dispersion of 200 kilometres per second.

Another part is the galactic corona. This is a layer of hot gas that spreads out far. It is made of plasma, which is a type of hot gas. We can find this gas using X-ray spectroscopy. This tool lets us see the light the gas gives off. It shows us atomic neutral hydrogen, also called the H I region. This gas reaches very far from the center of the galaxy. It is a key part of the halo's makeup.

Scientists also believe in a dark matter halo. This part is invisible and reaches far beyond the visible parts. Its mass is much greater than all other parts combined. We think it exists because of its gravity. This gravity helps control how things move inside a galaxy. Researchers use the Navarro–Frenk–White profile to model its density. This model shows how mass changes based on distance from the center. The profile uses a special radius called r-star. It also uses the Hubble constant to help with math.

Halos form through a bottom-up way of growing. This means small objects merge to create large structures. When things merge, the gas moves to the galaxy's center. However, the stars and dark matter stay in the halo. Some say gravity or primordial black holes help this happen. The Milky Way's halo may come from the Gaia Sausage. This is a specific event in our galaxy's history. It shows how halos help build the universe we see.

A galactic halo is an enormous, roughly spherical structure. It surrounds the main, visible parts of a galaxy. This component extends far beyond the bright center. In spiral galaxies, the halo shape is very easy to see. It looks like a round shell around a flat disc. In elliptical galaxies, the transition is less sharp. The halo blends into the other parts of the galaxy. Scientists study these halos by watching light from distant objects. They look at light from bright quasars located far behind the galaxy. This light passes through the halo on its way to us. By observing this, researchers can learn about the halo's properties.

The stellar halo is a major component of this structure. It consists of field stars and globular clusters. These are groups of stars held together by gravity. The stellar halo surrounds most disk galaxies. It also surrounds some specific elliptical galaxies called cD types. This part of the galaxy is not very bright. It contains only about one percent of a galaxy's total stellar mass. Because of this, its luminosity is much lower than the galaxy's disc. In the Milky Way, this halo contains RR Lyrae stars. These stars have low metallicity, meaning they have few heavy elements. The stars in our stellar halo are mostly very old. Most of them are greater than 12 billion years old.

Another important part is the galactic corona. This is a vast distribution of gas. It extends very far away from the galactic center. The corona is made of plasma, which is a type of hot gas. Scientists detect this gas by looking at its emission spectrum. This is the specific pattern of light the gas gives off. Using X-ray spectroscopy, researchers can find different features. One feature is the H I region. This region contains atomic neutral hydrogen. The corona represents a massive, hot layer surrounding the galaxy.

There is also a theorized dark matter halo. This component is invisible to our eyes and telescopes. It extends throughout the galaxy and far beyond its visible parts. The mass of the dark matter halo is huge. It is much greater than the mass of all other components. Scientists hypothesize its existence to explain galactic dynamics. Dark matter provides the gravitational potential that controls how bodies move. The nature of these halos is a major topic in cosmology. It helps us understand how galaxies form and evolve over time.

Researchers use the Navarro–Frenk–White profile to model dark matter. This is a widely accepted density profile. It describes mass density as a function of distance from the center. The model uses a characteristic radius called r-star. It also includes the critical density, which involves the Hubble constant. This profile also uses a dimensionless constant. The density cannot extend indefinitely according to this model. If it did, the calculated mass would become infinite. However, the profile still provides a finite gravitational potential. Most measurements are not very sensitive to the outer halo's mass. This is due to Newton's laws regarding spheroidal shapes.

Galactic halos form through a process called bottom-up evolution. This happens in a cold dark matter model of the universe. In this model, large structures start from small objects. These small objects merge together over time. Halos are composed of both baryonic and dark matter. When halos merge, they create larger ones. During these mergers, the gas behaves in a specific way. The gas moves toward the center to form galactic components. However, the stars and dark matter stay in the halo. Some evidence suggests gravity or primordial black holes help this. The Milky Way's halo may come from the Gaia Sausage.

Understanding the halo helps us see the whole system. The halo is not just an empty space around a galaxy. It is a complex mix of stars, gas, and invisible matter. Each part plays a role in the galaxy's life. The stellar halo tells us about the galaxy's ancient history. The corona shows us the state of galactic gas. The dark matter halo explains the gravity that holds everything together. Together, these components make up the massive scale of a galaxy.