Elliptical galaxy

The giant elliptical galaxy ESO 325-G004

An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless image. Unlike flat spiral galaxies with organization and structure, elliptical galaxies are more three-dimensional, without much structure, and their stars are in somewhat random orbits around the center. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae,[1] along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies[2][3][4] with their intermediate scale disks, a subset of the "early-type" galaxy population.

Elliptical galaxies range in size from tens of millions to over one hundred trillion stars. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false,[5] although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure.[6] [7] Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies.[5] Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, and they tend to be surrounded by large numbers of globular clusters. Elliptical galaxies are believed to make up approximately 10%–15% of galaxies in the Virgo Supercluster, and they are not the dominant type of galaxy in the universe overall.[8] They are preferentially found close to the centers of galaxy clusters.[9]

General characteristics

Elliptical galaxy IC 2006.[10]

Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases. The smallest known elliptical galaxy is about one-tenth the size of the Milky Way. The motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation. Furthermore, there is very little interstellar matter (neither gas nor dust), which results in low rates of star formation, few open star clusters, and few young stars; rather elliptical galaxies are dominated by old stellar populations, giving them red colors. Large elliptical galaxies typically have an extensive system of globular clusters.

The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes, although this remains controversial. The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersic's law, and a range of scaling relations between the elliptical galaxies' structural parameters unify the population.[11]

Every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies, 20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center.[12] The mass of the black hole is tightly correlated with the mass of the galaxy,[13] evidenced through correlations such as the M–sigma relation which relates the velocity dispersion of the surrounding stars to the mass of the black hole at the center.

Elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies.

Star formation

The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, leaving them to shine with only their aging stars. Elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, hot stars in their spiral arms. Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs[14] and low level star-formation.[15] Researchers with the Herschel Space Observatory have speculated that the central black holes in elliptical galaxies keep the gas from cooling enough for star formation.[16]

Sizes and shapes

The central galaxy in this image is a gigantic elliptical galaxy designated 4C 73.08.[17]
The brilliant central object is a supergiant elliptical galaxy, the dominant member of a galaxy cluster with the name MACSJ1423.8+2404. Note the gravitational lensing.

Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3000 lightyears to more than 700,000 lightyears, and masses from 105 to nearly 1013 solar masses.[18] This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals.

The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major (a) to the minor (b) axes of the galaxy's isophotes:

Thus for a spherical galaxy with a equal to b, the number is 0, and the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966[2] that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line-of-sight. This has been confirmed through spectral observations revealing the rotation of their stellar disks. [19] [20] Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated.

It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with slightly "boxy"-shaped isophotes, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion); and the "disky" normal and dwarf ellipticals, which contain disks. [21][22] This is, however, an abuse of the nomenclature, as there two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, and onto the S0 galaxies with their large-scale stellar disks that dominate the light at large radii.

Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies.

At the large end of the elliptical spectrum, there is further division, beyond Hubble's classification. Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies. These are similar to their smaller brethren, but more diffuse, with large haloes that may as much belong to the galaxy cluster within which they reside than the centrally-located giant galaxy.

Evolution

NGC 3597 is the product of a collision between two galaxies, and is slowly evolving to become a giant elliptical galaxy.

It is widely accepted that the evolution of elliptical galaxies is primarily composed of the merging of smaller galaxies. Many galaxies in the universe are gravitationally bound to other galaxies, which means that they will never escape the pull of the other galaxy. If the galaxies are of similar size, the resultant galaxy will appear similar to neither of the two galaxies merging, but will instead be an elliptical galaxy.

Such major galactic mergers are thought to have been common at early times, but may occur less frequently today. Minor galactic mergers involve two galaxies of very different masses, and are not limited to giant ellipticals. The Milky Way galaxy is, depending upon an unknown tangential component, on a collision course in 4–5 billion years with the Andromeda Galaxy.[23] It has been theorized that an elliptical galaxy will result from a merger of the two spirals.

It is believed that black holes may play an important role in limiting the growth of elliptical galaxies in the early universe by inhibiting star formation.

Examples

See also

References

  1. ↑ Hubble, E.P. (1936). The realm of the nebulae. Mrs. Hepsa Ely Silliman Memorial Lectures, 25. New Haven: Yale University Press. ISBN 9780300025002. OCLC 611263346. Archived from the original on 29 September 2012. (pp. 124–151)
  2. 1 2 Liller, M.H. (1966), The Distribution of Intensity in Elliptical Galaxies of the Virgo Cluster. II
  3. ↑ Nieto, J.-L. et al. (1988), More isotropic oblate rotators in elliptical galaxies
  4. ↑ Graham, A.W. et al. (2016), Disky Elliptical Galaxies and the Allegedly Over-massive Black Hole in the Compact “ES“ Galaxy NGC 1271 (see their Fig.7).
  5. 1 2 John, D. (2006). Astronomy: The definitive guide to the universe. Bath, UK: Parragon Publishing., p. 224-225
  6. ↑ Dekel, A., et al. (2009), Cold streams in early massive hot haloes as the main mode of galaxy formation
  7. ↑ Stewart, Kyle R., et al. (2013), Angular Momentum Acquisition in Galaxy Halos
  8. ↑ Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue". Monthly Notices of the Royal Astronomical Society. 278 (4): 1025–1048. arXiv:astro-ph/9603040. Bibcode:1996MNRAS.278.1025L. doi:10.1093/mnras/278.4.1025.
  9. ↑ Dressler, A. (March 1980). "Galaxy morphology in rich clusters – Implications for the formation and evolution of galaxies". The Astrophysical Journal. 236: 351–365. Bibcode:1980ApJ...236..351D. doi:10.1086/157753.
  10. ↑ "Elliptical galaxy IC 2006". www.spacetelescope.org. ESA/Hubble. Retrieved 21 April 2015.
  11. ↑ Graham, A.W. (2013), Elliptical and Disk Galaxy Structure and Modern Scaling Laws
  12. ↑ Kormendy, John; Ho, Luis C. (2013-08-18). "Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies". Annual Review of Astronomy and Astrophysics. 51 (1): 511–653. arXiv:1304.7762. Bibcode:2013ARA&A..51..511K. doi:10.1146/annurev-astro-082708-101811. ISSN 0066-4146.
  13. ↑ Graham, A.W. (2016), Galaxy Bulges and Their Massive Black Holes: A Review
  14. ↑ Young, L. M.; et al. (June 2011). "The Atlas3D project – IV: the molecular gas content of early-type galaxies". Monthly Notices of the Royal Astronomical Society. 414 (2): 940–967. arXiv:1102.4633. Bibcode:2011MNRAS.414..940Y. doi:10.1111/j.1365-2966.2011.18561.x.
  15. ↑ Crocker, A. F.; et al. (January 2011). "Molecular gas and star formation in early-type galaxies". Monthly Notices of the Royal Astronomical Society. 410 (2): 1197–1222. arXiv:1007.4147. Bibcode:2011MNRAS.410.1197C. doi:10.1111/j.1365-2966.2010.17537.x.
  16. ↑ "Red And Dead Galaxies Have Beating Black Hole 'Hearts', Preventing Star Formation."
  17. ↑ "Galactic fireflies". ESA/Hubble Picture of the Week. Retrieved 13 February 2013.
  18. ↑ Fraknoi, Andrew; Morrison, David; Wolf, Sidney C. (13 January 2017). Open Stax Astronomy. Retrieved 2 February 2017.
  19. ↑ Graham, A.W. et al. (1998), Extended stellar kinematics of elliptical galaxies in the Fornax cluster
  20. ↑ Emsellem, E. (2011), The ATLAS3D project - III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of fast and slow rotators
  21. ↑ Pedraz, S. et al. (2002), Evidence of fast rotation in dwarf elliptical galaxies
  22. ↑ Toloba, E. et al. (2015), Stellar Kinematics and Structural Properties of Virgo Cluster Dwarf Early-type Galaxies from the SMAKCED Project. III. Angular Momentum and Constraints on Formation Scenarios
  23. ↑ Nagamine, Kentaro; Loeb, Abraham (2003), Future evolution of nearby large-scale structures in a universe dominated by a cosmological constant

Further reading

  • Mo, Houjun; van den Bosch, Frank; White, Simon (June 2010), Galaxy Formation and Evolution (1 ed.), Cambridge University Press, ISBN 978-0521857932
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