Bulge

In astronomy, a bulge is a tightly packed group of stars. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happen to have a disk of stars around them. Yet, high resolution images, using Hubble Space Telescope reveal that many bulges have properties that are more like spiral galaxies. It is now thought that there are at least two types of bulges, bulges that are like ellipticals and bulges that are like spiral galaxies.

Classical Bulges

An image of Messier 81, a galaxy with a classical bulge. Notice that the spiral structure ends at the onset of the bulge.

Bulges that have properties similar to elliptical galaxies^[1] are often called classical bulges due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older Population II, and hence redder (see stellar evolution). They are also in orbits that are essentially random compared to the plane of the galaxy, whence the round shape arises. Furthermore, they have very little dust and gas compared to the disk portion of the galaxy, explaining why there are so few young stars (that is, there is little material left from which to form stars). The distribution of light is well described by de Vaucouleurs' law. At right, we show an example of a galaxy that harbors a bulge with properties similar to an elliptical galaxy, Messier object 81. Notice that the bulge is devoid of spiral structure, and the blue stars (indicating younger stars) are mainly in the outer disk surrounding the bulge.

It is this set of properties, that leads many astronomers to conclude that classical bulges are a product of the galactic merging process. It is thought that classical bulges are the result of the coalescences of smaller structures. This is a violent process, and thus disrupts the path of the stars, result in the randomness of bulge orbits. Also during the merger, gas clouds are more likely to be converted into stars, due to the shocks from the mergers. Thus the majority of the gas is converted into stars. A bulge may be the end result of many mergers. This process is more likely in the distant past, when the mergers were more common. Thus most classical bulges are old today, and have not evolved significantly in the past 10 billion years. Then the remaining gas and stars, that did not participate in the merger, could settle around the bulge, thus making the outer disk.

Disk-like Bulges

An image of Messier 63, a galaxy with a non-classical bulge. Notice that the spiral structure goes all the way to the center of the galaxy.

A Hubble Space Telescope image of the central region of NGC 4314, a galaxy with a star-forming nuclear ring.

Many bulges have properties more similar to spiral galaxies than elliptical galaxies.^[2][3][4] They are often referred to as pseudobulges or disky-bulges. It was first discovered that the stars in some bulges orbit around the galaxy like disk stars. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the outer disk. This is very different than elliptical galaxies.

Subsequent studies (using the Hubble Space Telescope) show that bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a spiral galaxy, but is much smaller. Giant spiral galaxies are typically 2-100 times the size of those spirals that exist in bulges. When they exist these central spirals dominate the light of the bulge in which they reside. Many bulges also have young stars and ongoing star formation, this is not a feature commonly found in elliptical galaxies. Typically the rate at which new stars are formed in pseudobulges is similar to the rates at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rates than (per area) is typically found in outer disks, as shown in NGC 4314 (right).

These properties (such as spiral structure and young stars) suggest that some bulges did not form through the same process that made elliptical galaxies and classical bulges. Yet the theories for the formation of pseudobulges is less certain than those of classical bulges. Pseudobulges may be the result of extremely gas-rich mergers than happened more recently than those mergers that formed classical bulges (within the last 5 billion years). However, it is difficult for disks to survive the merging process, casting doubt on this scenario.

Many astronomers suggest that bulges that appear similar to disks form internally out of the disk, and are not the product of the merging process. When left alone disks galaxies can rearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) are often observed in disk galaxies; both spiral disks and galactic bars are can result from secular evolution of galaxy disks. Secular evolution is also expected to send gas and stars to the center of a galaxy. If this happens that would increase the density at the center of a galaxy, and thus make a bulge that has properties similar to disk galaxies.

If secular evolution is responsible for the formation of a significant number of bulges, then that many galaxies have not experienced a merger since the formation of their disk. This would then mean that current theories of galaxy formation and evolution greatly over-predict the number of mergers in the past few billion years.

Most bulges are thought to host a supermassive black hole at their center. Such black holes by definition can not be observed (light cannot escape them), but various pieces of evidence strongly suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly with bulge properties; the tightest such correlation, the M-sigma relation, is between black hole mass and the velocity dispersion of stars in the bulge.^[5] Until recently it was thought that one could not have a supermassive black hole without a bulge around it, but galaxies hosting supermassive black holes without accompanying bulges have now been observed.^[6]

See also

• Galaxy formation and evolution
• Galactic coordinate system
• Disc galaxy
  • Spiral arm
• Galactic halo
• Galactic spheroid
• Galactic corona
• M-sigma relation

References

The Galactic Bulge: A Review