Lenticular galaxy

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The Spindle Galaxy (NGC 5866), a lenticular galaxy in the constellation Draco. This image shows that lenticular galaxies may retain a considerable amount of dust in their disk. However, there is little to no gas, and thus they are considered deficient in interstellar matter. Ngc5866 hst big.png
The Spindle Galaxy (NGC 5866), a lenticular galaxy in the constellation Draco. This image shows that lenticular galaxies may retain a considerable amount of dust in their disk. However, there is little to no gas, and thus they are considered deficient in interstellar matter.

A lenticular galaxy (denoted S0) is a type of galaxy intermediate between an elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. [1] 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. [2] They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars (like elliptical galaxies). 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. [3]

Contents

Morphology and structure

Classification

NGC 2787 is an example of a lenticular galaxy with visible dust absorption. While this galaxy has been classified as an S0 galaxy, one can see the difficulty in differentiating among spirals, ellipticals, and lenticulars. Credit: HST NGC 2787.jpg
NGC 2787 is an example of a lenticular galaxy with visible dust absorption. While this galaxy has been classified as an S0 galaxy, one can see the difficulty in differentiating among spirals, ellipticals, and lenticulars. Credit: HST
NGC 1387 has a large nuclear ring. This galaxy is a member of the Fornax Cluster. NGC1387 - hst 10217R850GB475.png
NGC 1387 has a large nuclear ring. This galaxy is a member of the Fornax Cluster.
Grid showing the location of early-type galaxies (including the lenticular S0 galaxies) relative to the late-type spiral galaxies. The horizontal axis shows the morphological type, primarily dictated by the nature of the spiral arms. Hubble-Grid.jpg
Grid showing the location of early-type galaxies (including the lenticular S0 galaxies) relative to the late-type spiral galaxies. The horizontal axis shows the morphological type, primarily dictated by the nature of the spiral arms.
The percentage of galaxies with a particular axis ratio (minor/major) for a sample of lenticular and spiral galaxies. The inset is a visual representation of the profile of either at the specified minor (b) to major (a) axis ratios. Wikifigure.png
The percentage of galaxies with a particular axis ratio (minor/major) for a sample of lenticular and spiral galaxies. The inset is a visual representation of the profile of either at the specified minor (b) to major (a) axis ratios.

Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component. They have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type [note 1] galaxies, yet may exhibit a central bar. [4] This bulge dominance can be seen in the axis ratio (i.e. the ratio between the observed minor and major axial of a disk galaxy) distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially flat in that same range. [5] Larger axial ratios can be explained by observing face-on disk galaxies or by having a sample of spheroidal (bulge-dominated) galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge. The galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of disk galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the lenticular galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. [4]

Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their intermediate placement on the Hubble sequence. This results from lenticulars having both prominent disk and bulge components. The disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is usually spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are thus divided into subclasses based upon either the amount of dust present or the prominence of a central bar. The classes of lenticular galaxies with no bar are S01, S02, and S03 where the subscripted numbers indicate the amount of dust absorption in the disk component; the corresponding classes for lenticulars with a central bar are SB01, SB02, and SB03. [4]

Sérsic decomposition

The surface brightness profiles of lenticular galaxies are well described by the sum of a Sérsic model for the spheroidal component plus an exponentially declining model (Sérsic index of n ≈ 1) for the disk, and often a third component for the bar. [6] Sometimes there is an observed truncation in the surface brightness profiles of lenticular galaxies at ~ 4 disk scalelengths. [7] These features are consistent with the general structure of spiral galaxies. However, the bulge component of lenticulars is more closely related to elliptical galaxies in terms of morphological classification. This spheroidal region, which dominates the inner structure of lenticular galaxies, has a steeper surface brightness profile (Sérsic index typically ranging from n = 1 to 4) [8] [9] than the disk component. Lenticular galaxy samples are distinguishable from the diskless (excluding small nuclear disks) elliptical galaxy population through analysis of their surface brightness profiles. [10]

Bars

Like spiral galaxies, lenticular galaxies can possess a central bar structure. While the classification system for normal lenticulars depends on dust content, barred lenticular galaxies are classified by the prominence of the central bar. SB01 galaxies have the least defined bar structure and are only classified as having slightly enhanced surface brightness along opposite sides of the central bulge. The prominence of the bar increases with index number, thus SB03 galaxies, like the NGC 1460 have very well defined bars that can extend through the transition region between the bulge and disk. [4] NGC 1460 is actually the galaxy with one of the largest bars seen among lenticular galaxies. Unfortunately, the properties of bars in lenticular galaxies have not been researched in great detail. Understanding these properties, as well as understanding the formation mechanism for bars, would help clarify the formation or evolution history of lenticular galaxies. [7]

NGC 2787.jpg
SB01 (NGC 2787)
NGC 1533 .jpg
SB02 (NGC 1533)
NGC 1460 -HST10217 18-R850GB475.png
SB03 (NGC 1460)
Barred lenticular galaxies by classification

Box-shaped bulges

NGC 1375 and NGC 1175 are examples of lenticular galaxies that have so-called box-shaped bulges. They are classified as SB0 pec. Box-shaped bulges are seen in edge-on galaxies, mostly spiral, but rarely lenticular.[ citation needed ]

Content

Hubble image of ESO 381-12 Hubble image of ESO 381-12.jpg
Hubble image of ESO 381-12

In many respects the composition of lenticular galaxies is like that of ellipticals. For example, they both consist of predominately older, hence redder, stars. All of their stars are thought to be older than about a billion years, in agreement with their offset from the Tully–Fisher relation (see below). In addition to these general stellar attributes, globular clusters are found more frequently in lenticular galaxies than in spiral galaxies of similar mass and luminosity. They also have little to no molecular gas (hence the lack of star formation) and no significant hydrogen α or 21-cm emission. Finally, unlike ellipticals, they may still possess significant dust. [4]

Kinematics

Measurement difficulties and techniques

NGC 4866 is a lenticular galaxy located in the constellation of Virgo. Potw1328a.tif
NGC 4866 is a lenticular galaxy located in the constellation of Virgo.

Lenticular galaxies share kinematic properties with both spiral and elliptical galaxies. [13] This is due to the significant bulge and disk nature of lenticulars. The bulge component is similar to elliptical galaxies in that it is pressure supported by a central velocity dispersion. This situation is analogous to a balloon, where the motions of the air particles (stars in a bulge's case) are dominated by random motions. However, the kinematics of lenticular galaxies are dominated by the rotationally supported disk. Rotation support implies the average circular motion of stars in the disk is responsible for the stability of the galaxy. Thus, kinematics are often used to distinguish lenticular galaxies from elliptical galaxies. Determining the distinction between elliptical galaxies and lenticular galaxies often relies on the measurements of velocity dispersion (σ), rotational velocity (v), and ellipticity (ε). [13] In order to differentiate between lenticulars and ellipticals, one typically looks at the v/σ ratio for a fixed ε. For example, a rough criterion for distinguishing between lenticular and elliptical galaxies is that elliptical galaxies have v/σ < 0.5 for ε = 0.3. [13] The motivation behind this criterion is that lenticular galaxies do have prominent bulge and disk components whereas elliptical galaxies have no disk structure. Thus, lenticulars have much larger v/σ ratios than ellipticals due to their non-negligible rotational velocities (due to the disk component) in addition to not having as prominent of a bulge component compared to elliptical galaxies. However, this approach using a single ratio for each galaxy is problematic due to the dependence of the v/σ ratio on the radius out to which it is measured in some early-type galaxies. For example, the ES galaxies that bridge the E and S0 galaxies, with their intermediate-scale disks, have a high v/σ ratio at intermediate radii that then drops to a low ratio at large radii. [14] [15]

The kinematics of disk galaxies are usually determined by Hα or 21-cm emission lines, which are typically not present in lenticular galaxies due to their general lack of cool gas. [7] Thus kinematic information and rough mass estimates for lenticular galaxies often comes from stellar absorption lines, which are less reliable than emission line measurements. There is also a considerable amount of difficulty in deriving accurate rotational velocities for lenticular galaxies. This is a combined effect from lenticulars having difficult inclination measurements, projection effects in the bulge-disk interface region, and the random motions of stars affecting the true rotational velocities. [16] These effects make kinematic measurements of lenticular galaxies considerably more difficult compared to normal disk galaxies.

Offset Tully–Fisher relation

This plot illustrates the Tully-Fisher relation for a spiral galaxy sample (black) as well as a lenticular galaxy sample (blue). One can see how the best-fit line for spiral galaxies differs from the best-fit line for lenticular galaxies. OffsetTF.png
This plot illustrates the Tully–Fisher relation for a spiral galaxy sample (black) as well as a lenticular galaxy sample (blue). One can see how the best-fit line for spiral galaxies differs from the best-fit line for lenticular galaxies.

The kinematic connection between spiral and lenticular galaxies is most clear when analyzing the Tully–Fisher relation for spiral and lenticular samples. If lenticular galaxies are an evolved stage of spiral galaxies then they should have a similar Tully–Fisher relation with spirals, but with an offset in the luminosity / absolute magnitude axis. This would result from brighter, redder stars dominating the stellar populations of lenticulars. An example of this effect can be seen in the adjacent plot. [7] One can clearly see that the best-fit lines for the spiral galaxy data and the lenticular galaxy have the same slope (and thus follow the same Tully–Fisher relation), but are offset by ΔI ≈ 1.5. This implies that lenticular galaxies were once spiral galaxies but are now dominated by old, red stars.

Formation theories

The morphology and kinematics of lenticular galaxies each, to a degree, suggest a mode of galaxy formation. Their disk-like, possibly dusty, appearance suggests they come from faded spiral galaxies, whose arm features disappeared. However, some lenticular galaxies are more luminous than spiral galaxies, which suggests that they are not merely the faded remnants of spiral galaxies. Lenticular galaxies might result from a galaxy merger, which increase the total stellar mass and might give the newly merged galaxy a disk-like, arm-less appearance. [7] Alternatively, it has been proposed [19] that they grew their disks via (gas and minor merger) accretion events. It had previously been suggested that the evolution of luminous lenticular galaxies may be closely linked to that of elliptical galaxies, whereas fainter lenticulars might be more closely associated with ram-pressure stripped spiral galaxies, [20] although this latter galaxy harassment scenario has since been queried due to the existence [21] of extremely isolated, low-luminosity lenticular galaxies such as LEDA 2108986.

Faded spirals

The absence of gas, presence of dust, lack of recent star formation, and rotational support are all attributes one might expect of a spiral galaxy which had used up all of its gas in the formation of stars. [7] This possibility is further enhanced by the existence of gas poor, or "anemic", spiral galaxies. If the spiral pattern then dissipated the resulting galaxy would be similar to many lenticulars. [22] Moore et al. also document that tidal harassment – the gravitational effects from other, near-by galaxies – could aid this process in dense regions. [23] The clearest support for this theory, however, is their adherence to slightly shifted version of Tully–Fisher relation, discussed above.

A 2012 paper that suggests a new classification system, first proposed by the Canadian astronomer Sidney van den Bergh, for lenticular and dwarf spheroidal galaxies (S0a-S0b-S0c-dSph) that parallels the Hubble sequence for spirals and irregulars (Sa-Sb-Sc-Im) reinforces this idea showing how the spiral–irregular sequence is very similar to this new one for lenticulars and dwarf ellipticals. [24]

Mergers

Messier 85 is a merged galaxy. Messier 85 Hubble WikiSky.jpg
Messier 85 is a merged galaxy.

The analyses of Burstein [25] and Sandage [26] showed that lenticular galaxies typically have surface brightness much greater than other spiral classes. It is also thought that lenticular galaxies exhibit a larger bulge-to-disk ratio than spiral galaxies and this may be inconsistent with simple fading from a spiral. [27] [28] If S0s were formed by mergers of other spirals these observations would be fitting and it would also account for the increased frequency of globular clusters. It should be mentioned, however, that advanced models of the central bulge which include both a general Sersic profile and bar indicate a smaller bulge, [29] and thus a lessened inconsistency. Mergers are also unable to account for the offset from the Tully–Fisher relation without assuming that the merged galaxies were quite different from those we see today.

Disk growth via accretion

The creation of disks in, at least some, lenticular galaxies via the accretion of gas, and small galaxies, around a pre-existing spheroidal structure was first suggested as an explanation to match the high-redshift compact massive spheroidal-shaped galaxies with the equally compact massive bulges seen in nearby massive lenticular galaxies. [30] In a "downsizing" scenario, bigger lenticular galaxies may have been built first – in a younger universe when more gas was available – and the lower-mass galaxies may have been slower to attract their disk-building material, as in the case of the isolated early-type galaxy LEDA 2108986. Within galaxy clusters, ram-pressure stripping removes gas and prevents the accretion of new gas that might be capable of furthering the development of the disk.

Examples

See also

Notes

  1. ↑ Galaxies to the left side of the Hubble classification scheme are sometimes referred to as "early-type", while those to the right are "late-type".

Related Research Articles

<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">Galaxy rotation curve</span> Observed discrepancy in galactic angular momenta

The rotation curve of a disc galaxy is a plot of the orbital speeds of visible stars or gas in that galaxy versus their radial distance from that galaxy's centre. It is typically rendered graphically as a plot, and the data observed from each side of a spiral galaxy are generally asymmetric, so that data from each side are averaged to create the curve. A significant discrepancy exists between the experimental curves observed, and a curve derived by applying gravity theory to the matter observed in a galaxy. Theories involving dark matter are the main postulated solutions to account for the variance.

<span class="mw-page-title-main">Elliptical galaxy</span> Spherical or ovoid mass of stars

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, along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population.

<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">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">Galaxy morphological classification</span> System for categorizing galaxies based on appearance

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.

<span class="mw-page-title-main">NGC 404</span> Galaxy in the constellation Andromeda

NGC 404 is a field galaxy located about 10 million light years away in the constellation Andromeda. It was discovered by William Herschel in 1784, and is visible through small telescopes. NGC 404 lies just beyond the Local Group and does not appear gravitationally bound to it. It is located within 7 arc-minutes of second magnitude star Mirach, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".

<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">NGC 3115</span> Galaxy in the constellation Sextans

NGC 3115 is a field lenticular (S0) galaxy in the constellation Sextans. The galaxy was discovered by William Herschel on February 22, 1787. At about 32 million light-years away from Earth, it is several times bigger than the Milky Way. It is a lenticular (S0) galaxy because it contains a disk and a central bulge of stars, but without a detectable spiral pattern. NGC 3115 is seen almost exactly edge-on, but was nevertheless mis-classified as elliptical. There is some speculation that NGC 3115, in its youth, was a quasar.

<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 2787</span> Galaxy in the constellation Ursa Major

NGC 2787 is a barred lenticular galaxy approximately 24 million light-years away in the northern constellation of Ursa Major. It was discovered on December 3, 1788 by German-born astronomer William Herschel. J. L. E. Dreyer described it as, "bright, pretty large, a little extended 90°, much brighter middle, mottled but not resolved, very small (faint) star involved to the southeast". The visible galaxy has an angular size of 2.5 × 1.5 arcminutes or 3.24 × 1.81 arcminutes and an apparent visual magnitude of 11.8.

<span class="mw-page-title-main">NGC 7217</span> Galaxy in the constellation Pegasus

NGC 7217 is an unbarred spiral galaxy in the constellation Pegasus.

<span class="mw-page-title-main">NGC 6340</span> Galaxy in the constellation Draco

NGC 6340 is an unbarred spiral galaxy in the northern constellation of Draco. It was discovered by German-British astronomer William Herschel on June 6, 1788. The galaxy is located approximately 55 million light-years (17 Mpc) away, and is receding with a heliocentric radial velocity of 1,217 km/s. It is the largest member of a triplet of galaxies known as the NGC 6340 group.

<span class="mw-page-title-main">NGC 1553</span> Galaxy in the constellation Dorado

NGC 1553 is a prototypical lenticular galaxy in the constellation Dorado. It is the second brightest member of the Dorado Group of galaxies. British astronomer John Herschel discovered NGC 1553 on December 5, 1834 using an 18.7 inch reflector.

<span class="mw-page-title-main">NGC 4036</span> Galaxy in the constellation Ursa Major

NGC 4036 is the New General Catalogue identifier for a lenticular galaxy in the northern circumpolar constellation of Ursa Major. In the Carnegie Atlas of Galaxies, it is described as being "characterized by an irregular pattern of dust lanes threaded through the disc in an 'embryonic' spiral pattern indicating a mixed S0/Sa form." It is located near the Big Dipper, a little to the north of the mid-way point between the stars Alpha Ursae Majoris and Delta Ursae Majoris. With a visual magnitude of 10.7, it can be dimly viewed using a 4 in (10 cm) aperture telescope.

<span class="mw-page-title-main">NGC 2681</span> Galaxy in the constellation Ursa Major

NGC 2681 is a lenticular galaxy in the constellation Ursa Major. The galaxy lies 50 million light years away from Earth, which means, given its apparent dimensions, that NGC 2681 is approximately 55,000 light years across. NGC 2681 has an active galactic nucleus and it is a type 3 Seyfert galaxy. Its nucleus is also a low-ionization nuclear emission-line region.

<span class="mw-page-title-main">NGC 7013</span> Spiral or lenticular galaxy in the constellation Cygnus

NGC 7013 is a relatively nearby spiral or lenticular galaxy estimated to be around 37 to 41.4 million light-years away from Earth in the constellation of Cygnus. NGC 7013 was discovered by English astronomer William Herschel on July 17, 1784 and was also observed by his son, astronomer John Herschel on September 15, 1828.

<span class="mw-page-title-main">NGC 3941</span> Galaxy in the constellation Ursa Major

NGC 3941 is a barred lenticular galaxy located in the constellation Ursa Major. It is located at a distance of circa 40 million light years from Earth, which, given its apparent dimensions, means that NGC 3941 is about 40,000 light years across. It was discovered by William Herschel in 1787.

<span class="mw-page-title-main">IC 1459</span> Elliptical galaxy in the constellation of Grus

IC 1459 is an elliptical galaxy located in the constellation Grus. It is located at a distance of circa 85 million light-years from Earth, which, given its apparent dimensions, means that IC 1459 is about 130,000 light-years across. It was discovered by Edward Emerson Barnard in 1892.

<span class="mw-page-title-main">NGC 4324</span> Galaxy in the constellation of Virgo

NGC 4324 is a lenticular galaxy located about 85 million light-years away in the constellation Virgo. It was discovered by astronomer Heinrich d'Arrest on March 4, 1862. NGC 4324 has a stellar mass of 5.62 × 1010M☉, and a baryonic mass of 5.88 × 1010M☉. The galaxy's total mass is around 5.25 × 1011M☉. NGC 4324 is notable for having a ring of star formation surrounding its nucleus. It was considered a member of the Virgo II Groups until 1999, when its distance was recalculated and it was placed in the Virgo W Group.

References

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  4. 1 2 3 4 5 6 Binney & Merrifield (1998). Galactic Astronomy. Princeton University Press. ISBN   0-691-02565-7.
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  6. ↑ Laurikainen, Eija; Salo, Heikki; Buta, Ronald (2005), Multicomponent decompositions for a sample of S0 galaxies
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  26. ↑ Sandage, A (2005). "THE CLASSIFICATION OF GALAXIES: Early History and Ongoing Developments". Annual Review of Astronomy and Astrophysics. 43 (1): 581–624. Bibcode:2005ARA&A..43..581S. doi:10.1146/annurev.astro.43.112904.104839.
  27. ↑ Dressler, A; Gilmore, Diane M. (1980). "On the interpretation of the morphology-density relation for galaxies in clusters". The Astrophysical Journal. 236: 351–65. Bibcode:1991ApJ...367...64W. doi:10.1086/169602.
  28. ↑ Christlein, D; Zabludoff AI (2004). "Can Early-Type Galaxies Evolve from the Fading of the Disks of Late-Type Galaxies?". The Astrophysical Journal. 616 (1): 192–98. arXiv: astro-ph/0408036 . Bibcode:2004ApJ...616..192C. doi:10.1086/424909. S2CID   13813083.
  29. ↑ Laurikainen, Eija; Heikki Salo; Ronald Buta (October 2005). "Multicomponent decompositions for a sample of S0 galaxies". MNRAS. 362 (4): 1319–1347. arXiv: astro-ph/0508097 . Bibcode:2005MNRAS.362.1319L. doi: 10.1111/j.1365-2966.2005.09404.x . S2CID   15159305.
  30. ↑ Graham, Alister W. (2013), Elliptical and Disk Galaxy Structure and Modern Scaling Laws
  31. ↑ "A greedy giant". www.spacetelescope.org. Retrieved 7 December 2016.
  32. ↑ "Standing out from the crowd". www.spacetelescope.org. Retrieved 12 September 2016.
  33. ↑ "Busy bees" . Retrieved 16 May 2016.
  34. ↑ "Elegance conceals an eventful past" . Retrieved 18 April 2016.
  35. ↑ "At the centre of the tuning fork" . Retrieved 2 November 2015.
  36. ↑ "A fascinating core" . Retrieved 8 June 2015.
  37. ↑ "The third way of galaxies". www.spacetelescope.org. ESA/Hubble. Retrieved 12 January 2015.
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