Elliptical galaxy

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The giant elliptical galaxy ESO 325-G004 Abell S740, cropped to ESO 325-G004.jpg
The giant elliptical galaxy ESO 325-G004

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, [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.

Contents

Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium, and they tend to be surrounded by large numbers of globular clusters. Star formation activity in elliptical galaxies is typically minimal; they may, however, undergo brief periods of star formation when merging with other galaxies. [5] 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. [6] They are preferentially found close to the centers of galaxy clusters. [7]

Elliptical galaxies range in size from dwarf ellipticals with tens of millions of stars, to supergiants of over one hundred trillion stars that dominate their galaxy clusters. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which was later discovered to be false, [8] although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure. [9] [10] Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. [8]

Examples

General characteristics

Elliptical galaxy IC 2006 Elliptical galaxy IC 2006.jpg
Elliptical galaxy IC 2006

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. 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. They generally have two distinct populations of globular clusters: one that is redder and metal-rich, and another that is bluer and metal-poor. [14]

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. [15]

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. [16] The mass of the black hole is tightly correlated with the mass of the galaxy, [17] 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.

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.

Sizes and shapes

Hercules A, a supergiant elliptical galaxy and also a radio galaxy. The radio lobes shown here in pink are over a million light-years across. A Multi-Wavelength View of Radio Galaxy Hercules A.jpg
Hercules A, a supergiant elliptical galaxy and also a radio galaxy. The radio lobes shown here in pink are over a million light-years across.

The largest galaxies are supergiant ellipticals, or type-cD galaxies. Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 light years to more than 700,000 light years, 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 brilliant central object is the supergiant elliptical galaxy SDSS J142347.87+240442.4, the dominant member of the galaxy cluster MACS J1423.8+2404. It has a diameter of 380,000 light-years. Note the gravitational lensing. MACSJ1423.8+2404.jpg
The brilliant central object is the supergiant elliptical galaxy SDSS J142347.87+240442.4, the dominant member of the galaxy cluster MACS J1423.8+2404. It has a diameter of 380,000 light-years. Note the gravitational lensing.

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. [20] [21] 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. [22] [23] This is, however, an abuse of the nomenclature, as there are 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.

NGC 3597 is the product of a collision between two galaxies. It is evolving into a giant elliptical galaxy. A galactic mega-merger.jpg
NGC 3597 is the product of a collision between two galaxies. It is evolving into a giant elliptical galaxy.

Star formation

In recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs [24] and low level star-formation. [25]

Herschel Space Observatory researchers have speculated that the central black holes in elliptical galaxies keep the gas from cooling enough for star formation. [26]

See also

Related Research Articles

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

<span class="mw-page-title-main">Hubble sequence</span> Galaxy morphological classification scheme advocated by Edwin Hubble

The Hubble sequence is a morphological classification scheme for galaxies published by Edwin Hubble in 1926. It is often colloquially known as the Hubble tuning-fork diagram because the shape in which it is traditionally represented resembles a tuning fork. It was invented by John Henry Reynolds and Sir James Jeans.

<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">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 bulge</span> Tightly packed group of stars within a larger formation

In astronomy, a galactic 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. 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.

<span class="mw-page-title-main">Centaurus A</span> Radio galaxy in the constellation Centaurus

Centaurus A is a galaxy in the constellation of Centaurus. It was discovered in 1826 by Scottish astronomer James Dunlop from his home in Parramatta, in New South Wales, Australia. There is considerable debate in the literature regarding the galaxy's fundamental properties such as its Hubble type and distance. NGC 5128 is one of the closest radio galaxies to Earth, so its active galactic nucleus has been extensively studied by professional astronomers. The galaxy is also the fifth-brightest in the sky, making it an ideal amateur astronomy target. It is only visible from the southern hemisphere and low northern latitudes.

<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">Messier 59</span> Elliptical galaxy in the constellation Virgo

Messier 59 or M59, also known as NGC 4621, is an elliptical galaxy in the equatorial constellation of Virgo. It is a member of the Virgo Cluster, with the nearest fellow member 8′ away and around 5 magnitudes fainter. The nearest cluster member of comparable brightness is the lenticular galaxy NGC 4638, which is around 17′ away. It and the angularly nearby elliptical galaxy Messier 60 were both discovered by Johann Gottfried Koehler in April 1779 when observing comet seeming close by. Charles Messier listed both in the Messier Catalogue about three days after Koehler's discovery.

<span class="mw-page-title-main">Messier 60</span> Elliptical galaxy in the constellation Virgo

Messier 60 or M60, also known as NGC 4649, is an elliptical galaxy approximately 57 million light-years away in the equatorial constellation of Virgo. Together with NGC 4647, it forms a pair known as Arp 116. Messier 60 and nearby elliptical galaxy Messier 59 were discovered by Johann Gottfried Koehler in April 1779, observing a comet in the same part of the sky. Charles Messier added both to his catalogue about three days after this.

<span class="mw-page-title-main">Dwarf galaxy</span> Small galaxy composed of up to several billion stars

A dwarf galaxy is a small galaxy composed of about 1000 up to several billion stars, as compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which closely orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy; others consider it a full-fledged galaxy. Dwarf galaxies' formation and activity are thought to be heavily influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their shape and composition.

<span class="mw-page-title-main">Messier 84</span> Galaxy in the constellation Virgo

Messier 84 or M84, also known as NGC 4374, is a giant elliptical or lenticular galaxy in the constellation Virgo. Charles Messier discovered the object in 1781 in a systematic search for "nebulous objects" in the night sky. It is the 84th object in the Messier Catalogue and in the heavily populated core of the Virgo Cluster of galaxies, part of the local supercluster.

<span class="mw-page-title-main">Messier 85</span> Elliptical galaxy in the constellation Coma Berenices

Messier 85 is a lenticular galaxy, or elliptical galaxy for other authors, in the Coma Berenices constellation. It is 60 million light-years away, and it is estimated to be 125,000 light-years across.

<span class="mw-page-title-main">NGC 1316</span> Lenticular radio galaxy in the constellation Fornax

NGC 1316 is a lenticular galaxy about 60 million light-years away in the constellation Fornax. It is a radio galaxy and at 1400 MHz is the fourth-brightest radio source in the sky.

<span class="mw-page-title-main">NGC 4526</span> Lenticular galaxy in the constellation Virgo

NGC 4526 is a lenticular galaxy with an embedded dusty disc, located approximately 55 million light-years from the Solar System in the Virgo constellation and discovered on 13 April 1784 by William Herschel.

<span class="mw-page-title-main">Eyes Galaxies</span> Pair of galaxies in the constellation Virgo

The Eyes Galaxies are a pair of galaxies about 52 million light-years away in the constellation Virgo. The pair are members of the string of galaxies known as Markarian's Chain.

<span class="mw-page-title-main">NGC 4293</span> Galaxy in the constellation Coma Berenices

NGC 4293 is a lenticular galaxy in the northern constellation of Coma Berenices. It was discovered by English astronomer William Herschel on March 14, 1784, who described it as "large, extended, resolvable, 6 or 7′ long". This galaxy is positioned to the north-northwest of the star 11 Comae Berenices and is a member of the Virgo Cluster of galaxies. It is assumed to lie at the same distance as the Virgo Cluster itself: around 54 million light years away. The galaxy spans an apparent area of 5.3 × 3.1 arc minutes.

<span class="mw-page-title-main">NGC 541</span> Galaxy in the constellation Cetus

NGC 541 is a lenticular galaxy located in the constellation Cetus. It is located at a distance of about 230 million light years from Earth, which, given its apparent dimensions, means that NGC 541 is about 130,000 light years across. It was discovered by Heinrich d'Arrest on October 30, 1864. It is a member of the Abell 194 galaxy cluster and is included in the Atlas of Peculiar Galaxies in the category galaxies with nearby fragments. NGC 541 is a radio galaxy of Fanaroff–Riley class I, also known as 3C 40A.

<span class="mw-page-title-main">NGC 4278</span> Galaxy in the constellation Coma Berenices

NGC 4278 is an elliptical galaxy located in the constellation Coma Berenices. It is located at a distance of circa 55 million light years from Earth, which, given its apparent dimensions, means that NGC 4278 is about 65,000 light years across. It was discovered by William Herschel on March 13, 1785. NGC 4278 is part of the Herschel 400 Catalogue and can be found about one and 3/4 of a degree northwest of Gamma Comae Berenices even with a small telescope.

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. Alt URL(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. ↑ Pearson, W. J.; Wang, L.; Alpaslan, M.; Baldry, I.; Bilicki, M.; Brown, M. J. I.; Grootes, M. W.; Holwerda, B. W.; Kitching, T. D.; Kruk, S.; van der Tak, F. F. S. (November 2019). "Effect of galaxy mergers on star-formation rates". Astronomy & Astrophysics. 631: A51. arXiv: 1908.10115 . doi:10.1051/0004-6361/201936337.
  6. ↑ 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.
  7. ↑ 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 .
  8. 1 2 John, D. (2006). Astronomy: The definitive guide to the universe. Bath, UK: Parragon Publishing., p. 224-225
  9. ↑ Dekel, A., et al. (2009), Cold streams in early massive hot haloes as the main mode of galaxy formation
  10. ↑ Stewart, Kyle R., et al. (2013), Angular Momentum Acquisition in Galaxy Halos
  11. ↑ The Event Horizon Telescope Collaboration (2019). "First M87 Event Horizon Telescope results. IV. Imaging the central supermassive black hole" (PDF). The Astrophysical Journal. 875 (1): L4. arXiv: 1906.11241 . Bibcode:2019ApJ...875L...4E. doi: 10.3847/2041-8213/ab0e85 . ISSN   2041-8213. S2CID   146068771.
  12. ↑ Jeffrey, K. (10 April 2019). "These are the first pictures of a black hole — And that's a big, even supermassive, deal". Time . Retrieved 10 April 2019.
  13. ↑ "Elliptical galaxy IC 2006". www.spacetelescope.org. ESA/Hubble. Retrieved 21 April 2015.
  14. ↑ Hixenbaugh, Kyle; Chandar, Rupali; Mok, Angus (2022). "The Ancient Globular Clusters of NGC 1291". The Astronomical Journal. 163 (6): 271. arXiv: 2205.14047 . Bibcode:2022AJ....163..271H. doi: 10.3847/1538-3881/ac680d . S2CID   248853311.
  15. ↑ Graham, A.W. (2013), Elliptical and Disk Galaxy Structure and Modern Scaling Laws
  16. ↑ Kormendy, John; Ho, Luis C. (18 August 2013). "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. S2CID   118172025.
  17. ↑ Graham, A.W. (2016), Galaxy Bulges and Their Massive Black Holes: A Review
  18. ↑ Fraknoi, Andrew; Morrison, David; Wolf, Sidney C. (13 January 2017). Open Stax Astronomy . Retrieved 2 February 2017.
  19. ↑ "Your NED Search Results". ned.ipac.caltech.edu. Retrieved 8 May 2023.
  20. ↑ Graham, A.W. et al. (1998), Extended stellar kinematics of elliptical galaxies in the Fornax cluster
  21. ↑ 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
  22. ↑ Pedraz, S. et al. (2002), Evidence of fast rotation in dwarf elliptical galaxies
  23. ↑ 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
  24. ↑ 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. S2CID   119267988.
  25. ↑ 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. S2CID   54764264.
  26. ↑ "Red And Dead Galaxies Have Beating Black Hole 'Hearts', Preventing Star Formation | HNGN - Headlines & Global News" . Retrieved 8 May 2023.

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