# Bulge (astronomy)

In astronomy, a bulge is a tightly packed group of stars within a larger 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.

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

## 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).

A Hubble Space Telescope image of the 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]

## Central compact mass

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 can not 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[10][11], while other correlations involve the total stellar mass or luminosity of the bulge[12][13] [14], the central concentration of stars in the bulge[15], the richness of the globular cluster system orbiting in the galaxy's far outskirts[16][17], and the winding angle of the spiral arms.[18]

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][19][20] The implication is that the bulge environment is not strictly essential to the initial seeding and growth of massive black holes.

## 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. ^ a b c d The Galactic Bulge: A Review
4. ^ a b c d 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.
5. ^ a b 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.
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.
8. ^
9. ^ SAO Encyclopedia of Astronomy
10. ^ 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.
11. ^ 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.
12. ^ 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.
13. ^ 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.
14. ^ 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
15. ^ Graham et al. (2001), A Correlation between Galaxy Light Concentration and Supermassive Black Hole Mass
16. ^ 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.
17. ^ 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.
18. ^
19. ^ SPACE.com - Even Thin Galaxies Pack Hefty Black Holes
20. ^ 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.

Baade's Window is an area of the sky with relatively low amounts of interstellar "dust" along the line of sight from the Earth. This area is considered an observational "window" as the normally obscured Galactic Center of the Milky Way is visible in this direction. It is named for astronomer Walter Baade who first recognized its significance. This area corresponds to one of the brightest visible patches of the Milky Way.

Dark matter halo

A dark matter halo is a theoretical component of a galaxy that envelops the galactic disc and extends well beyond the edge of the visible galaxy. The halo's mass dominates the total mass. Thought to consist of dark matter, halos have not been observed directly. Their existence is inferred through their effects on the motions of stars and gas in galaxies. Dark matter halos play a key role in current models of galaxy formation and evolution. The dark matter halo is not fully explained by the presence of massive compact halo objects (MACHOs).

Galactic core

Galactic core or galaxy core can refer to:

AstronomyThe Galactic Center of the Milky Way.

The nucleus of a regular galaxy.

Bulge (astronomy), the core of galaxies in general

Central massive object, the mass concentration at the center of a galaxy

Supermassive black hole, the core of most galaxiesSmartphonesSamsung Galaxy Core

Samsung Galaxy Core LTEOtherA computer video game developed by Spiderweb Software.

Galaxy

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million (108) stars to giants with one hundred trillion (1014) stars, each orbiting its galaxy's center of mass.

Galaxies are categorized according to their visual morphology as elliptical, spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers. The Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, and observed as it existed just 400 million years after the Big Bang.

Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion (2×1011) to a suggested 2 trillion (2×1012) or more, containing more stars than all the grains of sand on planet Earth. Most of the galaxies are 1,000 to 100,000 parsecs in diameter (approximately 3000 to 300,000 light years) and separated by distances on the order of millions of parsecs (or megaparsecs). For comparison, the Milky Way has a diameter of at least 30,000 parsecs (100,000 LY) and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs (2.5 million LY).

The space between galaxies is filled with a tenuous gas (the intergalactic medium) having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups, clusters, and superclusters. The Milky Way is part of the Local Group, which is dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. The largest structure of galaxies yet recognised is a cluster of superclusters that has been named Laniakea, which contains the Virgo supercluster.

Galaxy formation and evolution

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.

Galaxy merger

Galaxy mergers can occur when two (or more) galaxies collide. They are the most violent type of galaxy interaction. The gravitational interactions between galaxies and the friction between the gas and dust have major effects on the galaxies involved. The exact effects of such mergers depend on a wide variety of parameters such as collision angles, speeds, and relative size/composition, and are currently an extremely active area of research. Galaxy mergers are important because the merger rate is fundamental measurement of galaxy evolution. The merger rate also provides astronomers with clues about how galaxies bulked up over time.

OGLE-TR-10b

OGLE-TR-10b is an extrasolar planet orbiting the star OGLE-TR-10.

The planet was first detected by the Optical Gravitational Lensing Experiment (OGLE) survey in 2002. The star, OGLE-TR-10, was seen dimming by a tiny amount every 3 days. The transit lightcurve resembles that of HD 209458 b, the first transiting extrasolar planet. However, the mass of the object had to be measured by the radial velocity method, because other objects like red dwarfs and brown dwarfs can mimic the planetary transit. In late 2004 it was confirmed as the 5th planetary discovery by OGLE.The planet is a typical "hot Jupiter", a planet with a mass half that of Jupiter and orbital distance only 1/24th that of Earth from the Sun. One revolution around the star takes a little over three days to complete. The planet is slightly larger than Jupiter, probably due to the heat from the star.

OGLE-TR-10 was identified as a promising candidate by the OGLE team during their 2001 campaign in three fields towards the Galactic Center.

The possible planetary nature of its companion was based on spectroscopic follow-up.

A reported a tentative radial velocity semi-amplitude (from Keck-I/HIRES) of 100±43 m/s, and a mass for the putative planet of 0.7 ± 0.3 MJup was confirmed in 2004 with the UVES/FLAMES radial velocities. However, the possibility of a blend could not be ruled out.A blend scenario as an alternative explanation from an analysis combining all available radial velocity measurements with the OGLE light curve. OGLE-TR-10b has a mass of 0.57 ± 0.12 MJup and a radius of 1.24 ± 0.09 RJup. These parameters bear close resemblance to those of the first known transiting extrasolar planet, HD 209458 b.Note that the planets with the longer periods in the hot Jupiter class all have small masses (~0.7 MJup), while all the short-period planets (i.e., very hot Jupiters) have masses roughly twice as large. This trend may be related to the survival of planets in proximity to their parent stars.

WR 30a

WR 30a is a massive spectroscopic binary in the constellation Carina. The primary is an extremely rare star on the WO oxygen sequence and the secondary a massive class O star.

Morphology
Structure
Active nuclei
Energetic galaxies
Low activity
Interaction
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