Galaxy morphological classification

Short description: System for categorizing galaxies based on appearance

Tuning-fork-style diagram of the Hubble sequence

Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. There are several schemes in use by which galaxies can be classified according to their morphologies, the most famous being the Hubble sequence, devised by Edwin Hubble and later expanded by Gérard de Vaucouleurs and Allan Sandage. However, galaxy classification and morphology are now largely done using computational methods and physical morphology.

## Contents

* 1 Hubble sequence
* 2 De Vaucouleurs system
* 2.1 Numerical Hubble stage
* 3 Yerkes (or Morgan) scheme
* 4 See also
* 5 References
* 6 External links

## Hubble sequence

Main article: Hubble sequence

Spiral galaxy UGC 12591 is classified as an S0/Sa galaxy.[1]

The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926.[2][3] It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented. Hubble's scheme divides galaxies into three broad classes based on their visual appearance (originally on photographic plates):[4]

* Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter "E", followed by an integer n representing their degree of ellipticity on the sky.[5] The specific ellipticity rating depends on ratio of the major (a) to minor axes (b), thus:[6]

E=10×(1−ba)

* Spiral galaxies consist of a flattened disk, with stars forming a (usually two-armed) spiral structure, and a central concentration of stars known as the bulge, which is similar in appearance to an elliptical galaxy. They are given the symbol "S". Roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol "SB".
* Lenticular galaxies (designated S0) also consist of a bright central bulge surrounded by an extended, disk-like structure but, unlike spiral galaxies, the disks of lenticular galaxies have no visible spiral structure and are not actively forming stars in any significant quantity.[7]

The Hubble sequence throughout the universe's history[8]

These broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure (either disk-like or ellipsoidal).[4]

The Hubble sequence is often represented in the form of a two-pronged fork, with the ellipticals on the left (with the degree of ellipticity increasing from left to right) and the barred and unbarred spirals forming the two parallel prongs of the fork on the right. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two prongs meet the “handle”.[9]

To this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy.[10] Nonetheless, in June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification, particularly concerning spiral galaxies, may not be supported, and may need updating.[11][12]

## De Vaucouleurs system

Hubble – de Vaucouleurs Galaxy Morphology Diagram NGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar. NGC 7793: a spiral galaxy of type SA(s)d. The Large Magellanic Cloud: a type SBm galaxy.

The de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959.[13] De Vaucouleurs argued that Hubble's two-dimensional classification of spiral galaxies—based on the tightness of the spiral arms and the presence or absence of a bar—did not adequately describe the full range of observed galaxy morphologies. In particular, he argued that rings and lenses are important structural components of spiral galaxies.[14]

The de Vaucouleurs system retains Hubble's basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. To complement Hubble's scheme, de Vaucouleurs introduced a more elaborate classification system for spiral galaxies, based on three morphological characteristics:[15]

* Bars. Galaxies are divided on the basis of the presence or absence of a nuclear bar. De Vaucouleurs introduced the notation SA to denote spiral galaxies without bars, complementing Hubble’s use of SB for barred spirals. He also allowed for an intermediate class, denoted SAB, containing weakly barred spirals.[16] Lenticular galaxies are also classified as unbarred (SA0) or barred (SB0), with the notation S0 reserved for those galaxies for which it is impossible to tell if a bar is present or not (usually because they are edge-on to the line-of-sight).
* Rings. Galaxies are divided into those possessing ring-like structures (denoted ‘(r)’) and those without rings (denoted ‘(s)’). So-called ‘transition’ galaxies are given the symbol (rs).[16]
* Spiral arms. As in Hubble’s original scheme, spiral galaxies are assigned to a class based primarily on the tightness of their spiral arms. The de Vaucouleurs scheme extends the arms of Hubble’s tuning fork to include several additional spiral classes:

* Sd (SBd) – diffuse, broken arms made up of individual stellar clusters and nebulae; very faint central bulge
* Sm (SBm) – irregular in appearance; no bulge component
* Im – highly irregular galaxy

Most galaxies in these three classes were classified as Irr I in Hubble’s original scheme. In addition, the Sd class contains some galaxies from Hubble’s Sc class. Galaxies in the classes Sm and Im are termed the “Magellanic” spirals and irregulars, respectively, after the Magellanic Clouds. The Large Magellanic Cloud is of type SBm, while the Small Magellanic Cloud is an irregular (Im).

The different elements of the classification scheme are combined — in the order in which they are listed — to give the complete classification of a galaxy. For example, a weakly barred spiral galaxy with loosely wound arms and a ring is denoted SAB(r)c.

Visually, the de Vaucouleurs system can be represented as a three-dimensional version of Hubble's tuning fork, with stage (spiralness) on the x-axis, family (barredness) on the y-axis, and variety (ringedness) on the z-axis.[17]

### Numerical Hubble stage

De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme. Values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies (ellipticals and lenticulars) and positive numbers to late types (spirals and irregulars).[18] Thus, as a rough rule, lower values of T correspond to a larger fraction of the stellar mass contained in a spheroid/bulge relative to the disk. The approximate mapping between the spheroid-to-total stellar mass ratio (MB/MT) and the Hubble stage is MB/MT=(10−T)2/256 based on local galaxies.[19]

Elliptical galaxies are divided into three 'stages': compact ellipticals (cE), normal ellipticals (E) and late types (E+). Lenticulars are similarly subdivided into early (S−), intermediate (S0) and late (S+) types. Irregular galaxies can be of type magellanic irregulars (T = 10) or 'compact' (T = 11).

Numerical Hubble stage  Hubble stage T | −6  | −5  | −4  | −3  | −2  | −1  | 0  | 1  | 2  | 3  | 4  | 5  | 6  | 7  | 8  | 9  | 9-10  | 10  | 11  | 12  | 13

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de Vaucouleurs class[17] | cE  | E  | E+ | S0− | S00 | S0+ | S0/a  | Sa  | Sab  | Sb  | Sbc  | Sc  | Scd  | Sd  | Sdm  | Sm  | Ir  | Im  | Irp  | d Trans  | dSph
approximate Hubble class[20] | E  | S0  | S0/a  | Sa  | Sa-b  | Sb  | Sb-c  | Sc  | Sc-Irr  | Irr I  | Irr Il  | d E  | dSph

The use of numerical stages allows for more quantitative studies of galaxy morphology.

## Yerkes (or Morgan) scheme

The Yerkes scheme was created by American astronomer William Wilson Morgan. Together with Philip Keenan, Morgan also developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy; the shape, real and apparent; and the degree of the central concentration to classify galaxies.[21]

Spectral Type  | Explanation

|

a | Prominent A stars
af | Prominent A–F stars
f | Prominent F stars
fg | Prominent F–G stars
g | Prominent G stars
gk | Prominent G–K stars
k | Prominent K stars
Galactic Shape  | Explanation

|

B | Barred spiral
D | Rotational symmetry without pronounced spiral or elliptical structure
E | Elliptical
Ep | Elliptical with dust absorption
I | Irregular
L | Low surface brightness
N | Small bright nucleus
S | Spiral
Inclination | Explanation

|

1 | Galaxy is "face-on"
2 |
3 |
4 |
5 |
6 |
7 | Galaxy is "edge-on"

Thus, for example, the Andromeda Galaxy is classified as kS5.[22]

## See also

* Astronomy:Morphological Catalogue of Galaxies - Russian catalogue of 30,642 galaxies
* Astronomy:Galaxy color–magnitude diagram - Chart depicting the relationship between brightness and mass of large star systems
* Astronomy:Galaxy Zoo - Crowdsourced astronomy project
* Biography:Fritz Zwicky

## References

1. ↑ "A remarkable galactic hybrid". https://www.spacetelescope.org/images/potw1709a/.
2. ↑ Hubble, E. P. (1926). "Extra-galactic nebulae". Contributions from the Mount Wilson Observatory / Carnegie Institution of Washington 324: 1–49. Bibcode: 1926CMWCI.324....1H.
3. ↑ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. https://archive.org/details/in.ernet.dli.2015.212163.
4. ↑ 4.0 4.1 "The Hubble tuning fork – classification of galaxies" (in en). https://www.spacetelescope.org/images/heic9902o/.
5. ↑ Binney, James (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
6. ↑ "Elliptical Galaxy". https://astronomy.swin.edu.au/cosmos/E/elliptical+galaxy.
7. ↑ "Lenticular Galaxies". http://cas.sdss.org/dr2/en/proj/basic/galaxies/lenticular.asp.
8. ↑ "Hubble explores the origins of modern galaxies". ESA/Hubble Press Release. http://www.spacetelescope.org/news/heic1315/.
9. ↑ "Galaxies". http://www.jb.man.ac.uk/distance/exploring/course/strobel/galaxy/galaxya.htm.
10. ↑ Iafrate, G.. "THE HUBBLE SEQUENCE". http://svn.ari.uni-heidelberg.de/svn/edu/trunk/aida_03_galaxies/en/en_aida_03_galaxies.pdf.
11. ↑ Royal Astronomical Society (11 June 2019). "Citizen scientists re-tune Hubble's galaxy classification". EurekAlert!. https://www.eurekalert.org/pub_releases/2019-06/ras-csr061119.php.
12. ↑ Masters, Karen L. (30 April 2019). "Galaxy Zoo: unwinding the winding problem – observations of spiral bulge prominence and arm pitch angles suggest local spiral galaxies are winding". Monthly Notices of the Royal Astronomical Society 487 (2): 1808–1820. doi:10.1093/mnras/stz1153. Bibcode: 2019MNRAS.487.1808M. https://academic.oup.com/mnras/article-abstract/487/2/1808/5482087. Retrieved 12 June 2019.
13. ↑ De Vaucouleurs, G. (1959). "Classification and Morphology of External Galaxies". Astrophysik IV: Sternsysteme / Astrophysics IV: Stellar Systems. Handbuch der Physik / Encyclopedia of Physics. 53. pp. 275–310. doi:10.1007/978-3-642-45932-0_7. ISBN 978-3-642-45934-4. Bibcode: 1959HDP....53..275D.
14. ↑ Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
15. ↑ "Galaxy – Types of galaxies" (in en). https://www.britannica.com/science/galaxy.
16. ↑ 16.0 16.1 de Vaucouleurs, Gérard (April 1963). "Revised Classification of 1500 Bright Galaxies". Astrophysical Journal Supplement 8: 31. doi:10.1086/190084. Bibcode: 1963ApJS....8...31D.
17. ↑ 17.0 17.1 De Vaucouleurs, G. (1994). "Global Physical Parameters of Galaxies" (PostScript). Proceedings of "Quantifying Galaxy Morphology at High Redshift", A Workshop Held at the Space Telescope Science Institute, Baltimore MD, April 27-29 1994. https://www.stsci.edu/stsci/meetings/galaxy-morphology/proceedings/devaucouleurs.ps. Retrieved 2008-01-02.
18. ↑ "Qualitative and Quantitative Classifications of Galaxies". https://ned.ipac.caltech.edu/level5/Tinsley/Tinsley1.html.
19. ↑ Obreschkow, D.; Croton, D.; De Lucia, G.; Khochfar, S.; Rawlings, S. (2009). "Simulation of the Cosmic Evolution of Atomic and Molecular Hydrogen in Galaxies". The Astrophysical Journal 698 (2): Equation (18). doi:10.1088/0004-637X/698/2/1467. Bibcode: 2009ApJ...698.1467O.
20. ↑ Binney, J.; Merrifield, M. (1998). Galactic Astronomy. Princeton: Princeton University Press. ISBN 978-0-691-02565-0.
21. ↑ "The Yerkes Classification". https://ned.ipac.caltech.edu/level5/CLASSIFICATION/yc.html.
22. ↑ Darling, David. "galaxy classification". http://www.daviddarling.info/encyclopedia/G/galaxy_classification.html.

## External links

* Galaxies and the Universe – an introduction to galaxy classification
* Near-Infrared Galaxy Morphology Atlas, T.H. Jarrett
* The Spitzer Infrared Nearby Galaxies Survey (SINGS) Hubble Tuning-Fork, SINGS Spitzer Space Telescope Legacy Science Project
* Go to GalaxyZoo.org to try your hand at classifying galaxies as part of an Oxford University open community project

* v
* t
* e

Galaxies

Morphology|

* Disc
* Lenticular
* barred
* unbarred
* Spiral
* anemic
* barred
* flocculent
* grand design
* intermediate
* Magellanic
* unbarred
* Dwarf galaxy
* elliptical
* irregular
* spheroidal
* spiral
* Elliptical galaxy
* cD
* Irregular
* barred
* Peculiar
* Ring
* Polar

Structure|

* Active galactic nucleus
* Bar
* Bulge
* Dark matter halo
* Disc
* Disc galaxy
* Halo
* corona
* Galactic center
* Galactic plane
* Galactic ridge
* Interstellar medium
* Protogalaxy
* Spiral arm
* Supermassive black hole

Active nuclei|

* Blazar
* LINER
* Markarian
* Quasar
* Radio
* X-shaped
* Relativistic jet
* Seyfert

Energetic galaxies|

* Lyman-alpha emitter
* Luminous infrared
* Starburst
* blue compact dwarf
* pea
* faint blue
* Hot dust-obscured

Low activity|

* Low surface brightness
* Ultra diffuse
* Dark galaxy

Interaction|

* Field
* Galactic tide
* Cloud
* Groups and clusters
* group
* cluster
* Brightest cluster galaxy
* fossile group
* Interacting
* merger
* Jellyfish
* Satellite
* Stellar stream
* Superclusters
* Walls
* Voids and supervoids
* void galaxy

Lists|

* Galaxies
* Galaxies named after people
* Largest
* Nearest
* Polar-ring
* Ring
* Spiral
* Groups and clusters
* Large quasar groups
* Quasars

* Superclusters
* Voids

See also|

* Extragalactic astronomy
* Galactic astronomy
* Galactic coordinate system
* Galactic empire
* Galactic habitable zone
* Galactic magnetic fields
* Galactic orientation
* Galactic quadrant
* Galaxy color–magnitude diagram
* Galaxy formation and evolution
* Galaxy rotation curve
* Illustris project
* Intergalactic dust
* Intergalactic stars
* Intergalactic travel
* Population III stars

* Book
* Category
* Portal

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