Galactic bulge

Last updated
Artist's impression of the central bulge of the Milky Way Artist's impression of the central bulge of the Milky Way.jpg
Artist's impression of the central bulge of the Milky Way

In astronomy, a galactic bulge (or simply bulge) is a tightly packed group of stars within a larger star formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies (see galactic spheroid). Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. 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.

Contents

Classical bulges

An image of Messier 81, a galaxy with a classical bulge. The spiral structure ends at the onset of the bulge. Messier 81 HST.jpg
An image of Messier 81, a galaxy with a classical bulge. The spiral structure ends at the onset of the bulge.

Bulges that have properties similar to those of elliptical galaxies are often called "classical bulges" due to their similarity to the historic view of bulges. [2] These bulges are composed primarily of stars that are older, Population II stars, and hence have a reddish hue (see stellar evolution). [3] These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. [3] Due to the lack of dust and gases, bulges tend to have almost no star formation. The distribution of light is described by a Sersic profile.

Classical bulges are thought to be the result of collisions of smaller structures. Convulsing gravitational forces and torques disrupt the orbital paths of stars, resulting in the randomised bulge orbits. If either progenitor galaxy was gas-rich, the tidal forces can also cause inflows to the newly merged galaxy nucleus. Following a major merger, gas clouds are more likely to convert into stars, due to shocks (see star formation). One study has suggested that about 80% of galaxies in the field lack a classical bulge, indicating that they have never experienced a major merger. [4] The bulgeless galaxy fraction of the Universe has remained roughly constant for at least the last 8 billion years. [5] In contrast, about two thirds of galaxies in dense galaxy clusters (such as the Virgo Cluster) do possess a classical bulge, demonstrating the disruptive effect of their crowding. [4]

Disk-like bulges

Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges. ESO 498-G5.jpg
Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges.

Many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. [6] [7] [8] They are often referred to as pseudobulges or disky-bulges. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies.

Subsequent studies (using the Hubble Space Telescope) show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. [3] 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. Where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rate (per area) than is typically found in outer disks, as shown in NGC 4314 (see photo).

Central region of NGC 4314, a galaxy with a star-forming nuclear ring NGC 4314HST1998-21-b-full.jpg
Central region of NGC 4314, a galaxy with a star-forming nuclear ring

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 are less certain than those for classical bulges. Pseudobulges may be the result of extremely gas-rich mergers that 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 outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) are often observed in such galaxies; both spiral disks and galactic bars 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 the galaxy, and thus make a bulge that has properties similar to those of disk galaxies.

If secular evolution, or the slow, steady evolution of a galaxy, [9] 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. [3] [4] [5]

Boxy/peanut bulge for edge-on galaxies

Exposing the Milky Way's "X".jpg
The X-shape of the bulge of the Milky Way.
NGC 1175 - Flickr - geckzilla.png
The prominent X-shape of the bulge of NGC 1175 as seen by Hubble.

Edge-on galaxies can sometimes have a boxy/peanut bulge with an X-shape. The boxy nature of the Milky Way bulge was revealed by the COBE satellite and later confirmed with the VVV survey with the help of red clump stars. The VVV survey also found two overlapping populations of red clump stars and an X-shape of the bulge. The WISE satellite later confirmed the X-shape of the bulge. The X-shape makes up 45% of the mass of the bulge in the Milky Way. [10] The boxy/peanut bulges are in fact the bar of a galaxy seen edge-on. [11] Other edge-on galaxies can also show a boxy/peanut bar sometimes with an X-shape.

Central compact mass

ESO 495-21 may host a supermassive black hole, an unusual feature for a galaxy of its size. A tiny galaxy with a big heart.jpg
ESO 495-21 may host a supermassive black hole, an unusual feature for a galaxy of its size.

Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, which is traditionally assumed to be a supermassive black hole. Such black holes by definition cannot be observed directly (light cannot escape them), but various pieces of evidence 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 M–sigma relation relates black hole mass to the velocity dispersion of bulge stars, [13] [14] while other correlations involve the total stellar mass or luminosity of the bulge, [15] [16] [17] the central concentration of stars in the bulge, [18] the richness of the globular cluster system orbiting in the galaxy's far outskirts, [19] [20] and the winding angle of the spiral arms. [21]

Until recently it was thought that one could not have a supermassive black hole without a surrounding bulge. Galaxies hosting supermassive black holes without accompanying bulges have now been observed. [4] [22] [23] The implication is that the bulge environment is not strictly essential to the initial seeding and growth of massive black holes.

See also

Related Research Articles

The study of galaxy formation and evolution is concerned with the processes that formed a heterogeneous universe from a homogeneous beginning, the formation of the first galaxies, the way galaxies change over time, and the processes that have generated the variety of structures observed in nearby galaxies. Galaxy formation is hypothesized to occur from structure formation theories, as a result of tiny quantum fluctuations in the aftermath of the Big Bang. The simplest model in general agreement with observed phenomena is the Lambda-CDM model—that is, that clustering and merging allows galaxies to accumulate mass, determining both their shape and structure. Hydrodynamics simulation, which simulates both baryons and dark matter, is widely used to study galaxy formation and evolution.

An active galactic nucleus (AGN) is a compact region at the center of a galaxy that emits a significant amount of energy across the electromagnetic spectrum, with characteristics indicating that the luminosity is not produced by stars. Such excess, non-stellar emissions have been observed in the radio, microwave, infrared, optical, ultra-violet, X-ray and gamma ray wavebands. A galaxy hosting an AGN is called an active galaxy. The non-stellar radiation from an AGN is theorized to result from the accretion of matter by a supermassive black hole at the center of its host galaxy.

<span class="mw-page-title-main">Triangulum Galaxy</span> Spiral galaxy in the constellation Triangulum

The Triangulum Galaxy is a spiral galaxy 2.73 million light-years (ly) from Earth in the constellation Triangulum. It is catalogued as Messier 33 or NGC (New General Catalogue) 598. With the D25 isophotal diameter of 18.74 kiloparsecs (61,100 light-years), the Triangulum Galaxy is the third-largest member of the Local Group of galaxies, behind the Andromeda Galaxy and the Milky Way.

<span class="mw-page-title-main">Elliptical galaxy</span> Spherical or ovoid mass of stars

An elliptical galaxy is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

<span class="mw-page-title-main">Spiral galaxy</span> Class of galaxy that has spiral structures extending from their cores.

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

<span class="mw-page-title-main">Messier 87</span> Elliptical galaxy in the Virgo Galaxy Cluster

Messier 87 is a supergiant elliptical galaxy in the constellation Virgo that contains several trillion stars. One of the largest and most massive galaxies in the local universe, it has a large population of globular clusters—about 15,000 compared with the 150–200 orbiting the Milky Way—and a jet of energetic plasma that originates at the core and extends at least 1,500 parsecs, traveling at a relativistic speed. It is one of the brightest radio sources in the sky and a popular target for both amateur and professional astronomers.

<span class="mw-page-title-main">Supermassive black hole</span> Largest type of black hole

A supermassive black hole is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (M☉). Black holes are a class of astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not even light. Observational evidence indicates that almost every large galaxy has a supermassive black hole at its center. For example, the Milky Way galaxy has a supermassive black hole at its center, corresponding to the radio source Sagittarius A*. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering active galactic nuclei (AGNs) and quasars.

<span class="mw-page-title-main">Lenticular galaxy</span> Class of galaxy between an elliptical galaxy and a spiral galaxy

A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.

<span class="mw-page-title-main">Galactic Center</span> Rotational center of the Milky Way galaxy

The Galactic Center is the rotational center and the barycenter of the Milky Way. Its central massive object is a supermassive black hole of about 4 million solar masses, which is called Sagittarius A*, a compact radio source which is almost exactly at the galactic rotational center. The Galactic Center is approximately 8 kiloparsecs (26,000 ly) away from Earth in the direction of the constellations Sagittarius, Ophiuchus, and Scorpius, where the Milky Way appears brightest, visually close to the Butterfly Cluster (M6) or the star Shaula, south to the Pipe Nebula.

<span class="mw-page-title-main">Sombrero Galaxy</span> Galaxy in the constellation Virgo

The Sombrero Galaxy is a peculiar galaxy of unclear classification in the constellation borders of Virgo and Corvus, being about 9.55 megaparsecs from the Milky Way galaxy. It is a member of the Virgo II Groups, a series of galaxies and galaxy clusters strung out from the southern edge of the Virgo Supercluster. It has an isophotal diameter of approximately 29.09 to 32.32 kiloparsecs, making it slightly bigger in size than the Milky Way.

<span class="mw-page-title-main">Barred spiral galaxy</span> Spiral galaxy with a central bar-shaped structure composed of stars

A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where the Solar System is located, is classified as a barred spiral galaxy.

<span class="mw-page-title-main">Messier 32</span> Dwarf elliptical galaxy in the constellation Andromeda

Messier 32 is a dwarf "early-type" galaxy about 2,650,000 light-years (810,000 pc) from the Solar System, appearing in the constellation Andromeda. M32 is a satellite galaxy of the Andromeda Galaxy (M31) and was discovered by Guillaume Le Gentil in 1749.

<span class="mw-page-title-main">NGC 3115</span> Galaxy in the constellation Sextans

NGC 3115 is a field lenticular (S0) galaxy in the constellation Sextans. The galaxy was discovered by William Herschel on February 22, 1787. At about 32 million light-years away from Earth, it is several times bigger than the Milky Way. It is a lenticular (S0) galaxy because it contains a disk and a central bulge of stars, but without a detectable spiral pattern. NGC 3115 is seen almost exactly edge-on, but was nevertheless mis-classified as elliptical. There is some speculation that NGC 3115, in its youth, was a quasar.

<span class="mw-page-title-main">NGC 4395</span> Galaxy in the constellation Canes Venatici

NGC 4395 is a nearby low surface brightness spiral galaxy located about 14 million light-years from Earth in the constellation Canes Venatici. The nucleus of NGC 4395 is active and the galaxy is classified as a Seyfert Type I known for its very low-mass supermassive black hole. It is one of the lowest

<span class="mw-page-title-main">M–sigma relation</span>

The M–sigmarelation is an empirical correlation between the stellar velocity dispersion σ of a galaxy bulge and the mass M of the supermassive black hole at its center.

In astronomy, the velocity dispersion (σ) is the statistical dispersion of velocities about the mean velocity for a group of astronomical objects, such as an open cluster, globular cluster, galaxy, galaxy cluster, or supercluster. By measuring the radial velocities of the group's members through astronomical spectroscopy, the velocity dispersion of that group can be estimated and used to derive the group's mass from the virial theorem. Radial velocity is found by measuring the Doppler width of spectral lines of a collection of objects; the more radial velocities one measures, the more accurately one knows their dispersion. A central velocity dispersion refers to the σ of the interior regions of an extended object, such as a galaxy or cluster.

<span class="mw-page-title-main">NGC 7469</span> Galaxy located in the constellation Pegasus

NGC 7469 is an intermediate spiral galaxy in the constellation of Pegasus. NGC 7469 is located about 200 million light-years away from Earth, which means, given its apparent dimensions, that NGC 7469 is approximately 90,000 light-years across. It was discovered by William Herschel on November 12, 1784.

<span class="mw-page-title-main">IC 1459</span> Elliptical galaxy in the constellation of Grus

IC 1459 is an elliptical galaxy located in the constellation Grus. It is located at a distance of circa 85 million light-years from Earth, which, given its apparent dimensions, means that IC 1459 is about 130,000 light-years across. It was discovered by Edward Emerson Barnard in 1892.

<span class="mw-page-title-main">NGC 973</span> Galaxy in the constellation Triangulum

NGC 973 is a giant spiral galaxy located in the constellation Triangulum. It is located at a distance of circa 200 million light-years from Earth, which, given its apparent dimensions, means that NGC 973 is about 230,000 light years across. It was discovered by Lewis Swift on October 30, 1885.

References

  1. ↑ "The Peanut at the Heart of our Galaxy". ESO Press Release. Retrieved 14 September 2013.
  2. ↑ Sandage, Allan, The Hubble Atlas of Galaxies, Washington: Carnegie Institution, 1961
  3. 1 2 3 4 The Galactic Bulge: A Review
  4. 1 2 3 4 Kormendy, J.; Drory, N.; Bender, R.; Cornell, M. E. (2010). "Bulgeless Giant Galaxies Challenge Our Picture of Galaxy Formation by Hierarchical Clustering". The Astrophysical Journal . 723 (1): 54–80. arXiv: 1009.3015 . Bibcode:2010ApJ...723...54K. doi:10.1088/0004-637X/723/1/54. hdl:2152/35173. S2CID   119303368.
  5. 1 2 Sachdeva, S.; Saha, K. (2016). "Survival of Pure Disk Galaxies over the Last 8 Billion Years". The Astrophysical Journal Letters. 820 (1): L4. arXiv: 1602.08942 . Bibcode:2016ApJ...820L...4S. doi: 10.3847/2041-8205/820/1/L4 . S2CID   14644377.
  6. ↑ The formation of galactic bulges edited by C.M. Carollo, H.C. Ferguson, R.F.G. Wyse. Cambridge, U.K.; New York : Cambridge University Press, 1999. (Cambridge contemporary astrophysics)
  7. ↑ Kormendy, J.; Kennicutt, Jr. R. C. (2004). "Secular Evolution and the Formation of Pseudobulges in Disk Galaxies". Annual Review of Astronomy and Astrophysics . 42 (1): 603–683. arXiv: astro-ph/0407343 . Bibcode:2004ARA&A..42..603K. doi:10.1146/annurev.astro.42.053102.134024. S2CID   515479.
  8. ↑ Athanassoula, E. (2005). "On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations". Monthly Notices of the Royal Astronomical Society . 358 (4): 1477–1488. arXiv: astro-ph/0502316 . Bibcode:2005MNRAS.358.1477A. doi:10.1111/j.1365-2966.2005.08872.x.
  9. ↑ SAO Encyclopedia of Astronomy
  10. ↑ Ness, Melissa; Lang, Dustin (2016-07-01). "The X-shaped Bulge of the Milky Way Revealed by WISE". The Astronomical Journal. 152 (1): 14. arXiv: 1603.00026 . Bibcode:2016AJ....152...14N. doi: 10.3847/0004-6256/152/1/14 . ISSN   0004-6256.
  11. ↑ Athanassoula, E. (2005-04-01). "On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations". Monthly Notices of the Royal Astronomical Society. 358 (4): 1477–1488. arXiv: astro-ph/0502316 . Bibcode:2005MNRAS.358.1477A. doi: 10.1111/j.1365-2966.2005.08872.x . ISSN   0035-8711.
  12. ↑ "Hubble Observes Tiny Galaxy with Big Heart". www.spacetelescope.org. Retrieved 17 June 2019.
  13. ↑ Ferrarese, L.; Merritt, D. (2000). "A Fundamental Relation between Supermassive Black Holes and Their Host Galaxies". The Astrophysical Journal Letters. 539 (1): L9–L12. arXiv: astro-ph/0006053 . Bibcode:2000ApJ...539L...9F. doi:10.1086/312838. S2CID   6508110.
  14. ↑ Xiao, T.; Barth, A. J.; Greene, J. E.; Ho, L. C.; Bentz, M. C.; Ludwig, R. R.; Jiang, Y. (2011). "Exploring the Low-mass End of the M $_BH$-$\sigma$$_*$ Relation with Active Galaxies". The Astrophysical Journal . 739 (1): 28. arXiv: 1106.6232 . Bibcode:2011ApJ...739...28X. doi:10.1088/0004-637X/739/1/28. S2CID   118444825.
  15. ↑ Magorrian, J.; Tremaine, S.; Richstone, D.; Bender, R.; Bower, G.; Dressler, A.; Faber, S. M.; Gebhardt, K.; Green, R.; Grillmair, C.; Kormendy, J.; Lauer, T. (1998). "The Demography of Massive Dark Objects in Galaxy Centers". The Astronomical Journal . 115 (6): 2285–2305. arXiv: astro-ph/9708072 . Bibcode:1998AJ....115.2285M. doi:10.1086/300353. S2CID   17256372.
  16. ↑ Häring, N.; Rix, H.-W. (2004). "On the Black Hole Mass-Bulge Mass Relation". The Astrophysical Journal Letters. 604 (2): L89–L92. arXiv: astro-ph/0402376 . Bibcode:2004ApJ...604L..89H. doi:10.1086/383567. S2CID   119431361.
  17. ↑ Giulia A.D. Savorgnan, et al. (2016), Supermassive Black Holes and Their Host Spheroids. II. The Red and Blue Sequence in the MBH-M*,sph Diagram
  18. ↑ Graham et al. (2001), A Correlation between Galaxy Light Concentration and Supermassive Black Hole Mass
  19. ↑ Spitler, L. R.; Forbes, D. A. (2009). "A new method for estimating dark matter halo masses using globular cluster systems". Monthly Notices of the Royal Astronomical Society . 392 (1): L1–L5. arXiv: 0809.5057 . Bibcode:2009MNRAS.392L...1S. doi:10.1111/j.1745-3933.2008.00567.x. S2CID   16818778.
  20. ↑ Sadoun, R.; Colin, J. (2012). "MBH–σ relation between supermassive black holes and the velocity dispersion of globular cluster systems". Monthly Notices of the Royal Astronomical Society . 426 (1): L51–L55. arXiv: 1204.0144 . Bibcode:2012MNRAS.426L..51S. doi:10.1111/j.1745-3933.2012.01321.x. S2CID   117185846.
  21. ↑ Seigar, M., et al. (2008), Discovery of a Relationship between Spiral Arm Morphology and Supermassive Black Hole Mass in Disk Galaxies
  22. ↑ SPACE.com - Even Thin Galaxies Pack Hefty Black Holes
  23. ↑ Simmons, B. D.; Smethurst, R. J.; Lintott, C. (2017). "Supermassive black holes in disk-dominated galaxies outgrow their bulges and co-evolve with their host galaxies". Monthly Notices of the Royal Astronomical Society . 470 (2): 1559–1569. arXiv: 1705.10793 . Bibcode:2017MNRAS.470.1559S. doi:10.1093/mnras/stx1340.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.