Artist's impression of the central bulge of the Milky Way[1]

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.

Classical bulges

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.

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

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

The X-shape of the bulge of the Milky Way.
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.[12]

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

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: 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. 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.
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