Galactic bulge

A galactic bulge is a tightly packed group of stars located near the center of most spiral galaxies.

For many years, astronomers believed these bulges were actually separate, elliptical galaxies. They thought a disk of stars simply sat around an existing elliptical galaxy. However, high-resolution images from the Hubble Space Telescope changed this historical view. We now know that many bulges are integrated parts of the spiral galaxy itself. Scientists currently categorize these structures into at least two distinct types: classical bulges and pseudobulges.

Classical bulges are defined by their similarity to elliptical galaxies. These structures are composed mostly of older stars known as Population II stars. Because these stars are older, they often have a noticeable reddish hue. Unlike the stars in a flat disk, these stars move in essentially random orbits. This lack of a single orbital plane gives the bulge a distinct, spherical shape. The distribution of light in these bulges follows a Sersic profile. This means the light intensity increases dramatically as you move toward a lower radius.

The formation of a classical bulge is a violent process. They are thought to result from the collisions of smaller galactic structures. During these mergers, convulsing gravitational forces and torques disrupt the existing orbital paths of stars. This disruption results in the randomized orbits we observe today. If the merging galaxies are gas-rich, tidal forces can cause gas to flow into the new nucleus. Following a major merger, gas clouds are likely to convert into stars due to shocks. This process explains why classical bulges often lack the dust and gas needed for new star formation.

In contrast, pseudobulges, or disky-bulges, behave very differently. These bulges have stars that orbit in an ordered fashion. Instead of moving randomly, they follow the same plane as the stars in the outer disk. Many pseudobulges also contain a complex structure of dust. They often look like much smaller versions of a spiral galaxy. A giant spiral galaxy is typically 2 to 100 times larger than the spirals found in these bulges.

Some pseudobulges even contain nuclear rings. These rings can form new stars at a much higher rate per area than the outer disks.

Astronomers are still studying how pseudobulges form. One theory suggests they come from extremely gas-rich mergers that occurred within the last 5 billion years. However, it is difficult for a disk to survive a major merger, which makes this theory uncertain. Many astronomers instead suggest a process called secular evolution. This is a slow, steady evolution where a galaxy rearranges its own stars and gas. This process can create galactic bars or spiral disks. Secular evolution is also expected to send gas and stars toward the center. This increases the density at the galactic core and builds a bulge with disk-like properties.

Evidence for these different formation paths is found in galaxy populations. About 80% of galaxies in the field lack a classical bulge. This suggests they have never experienced a major merger. This number of bulge-less galaxies has remained roughly constant for at least 8 billion years. However, about two-thirds of galaxies in dense clusters, like the Virgo Cluster, do possess a classical bulge. This demonstrates the disruptive effect of crowded environments.

Some galaxies, like the Milky Way, even show a boxy or peanut-shaped bulge when seen from the edge. This X-shape is actually the galaxy's bar seen edge-on and makes up 45% of the Milky Way bulge's mass.

Most bulges and pseudobulges are thought to host a central relativistic compact mass. This is traditionally assumed to be a supermassive black hole. While we cannot observe black holes directly because light cannot escape them, we see their effects. The mass of a black hole correlates tightly with various bulge properties. The M–sigma relation relates the black hole mass to the velocity dispersion of the bulge stars. Other correlations involve the total stellar mass, the luminosity of the bulge, or the richness of the surrounding globular clusters. Interestingly, astronomers have now observed galaxies with supermassive black holes that do not have an accompanying bulge. This suggests that a bulge environment is not strictly essential for a black hole to grow.