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. 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. 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.

Boxy-shaped bulges

NGC 1375 and NGC 1175 are examples for lenticular galaxies that have so-called boxy-shaped bulges. They are classified as SB0 pec. Boxy-shaped bulges are seen in edgewise galaxies, mostly spiral, but rarely lenticular.

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 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. Of course, 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

Hubble sequence Galaxy morphological classification scheme invented by Edwin Hubble

The Hubble sequence is a morphological classification scheme for galaxies invented 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.

Elliptical galaxy Galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile

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.

Spiral galaxy Class of galaxy having a number of arms of younger stars

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.

Galactic bulge 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.

NGC 5866 Lenticular galaxy in the constellation Draco

NGC 5866 is a relatively bright lenticular galaxy in the constellation Draco. NGC 5866 was most likely discovered by Pierre Méchain or Charles Messier in 1781, and independently found by William Herschel in 1788. Measured orbital velocities of its globular cluster system imply that dark matter makes up only 34±45% of the mass within 5 effective radii; a notable paucity.

Messier 59 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.

Messier 84 Elliptical or lenticular 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.

NGC 1316 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.

NGC 2787 Barred lenticular galaxy in the constellation Ursa Major

NGC 2787 is a barred lenticular galaxy approximately 24 million light-years away in the northern 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 and an apparent visual magnitude of 11.8.

NGC 7217 Unbarred spiral galaxy in the constellation Pegasus

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

NGC 1553 lenticular 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.

NGC 4293 Lenticular 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.

NGC 4203 Lenticular galaxy in the constellation Coma Berenices

NGC 4203 is the New General Catalogue identifier for a lenticular galaxy in the northern constellation of Coma Berenices. It was discovered on March 20, 1787 by English astronomer William Herschel, and is situated 5.5° to the northwest of the 4th magnitude star Gamma Comae Berenices and can be viewed with a small telescope. The morphological classification of NGC 4203 is SAB0−, indicating that it has a lenticular form with tightly wound spiral arms and a weak bar structure at the nucleus.

NGC 2681 Lenticular 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.

NGC 7013 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.

NGC 3941 Barred lenticular 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.

NGC 4608 Barred lenticular galaxy in the constellation Virgo

NGC 4608 is a barred lenticular galaxy located in the constellation of Virgo. The galaxy was discovered by astronomer William Herschel on March 15, 1784. At about 56 million light-years away, it is a member of the Virgo Cluster.

NGC 6907 Spiral galaxy in the constellation Capricornus

NGC 6907 is a spiral galaxy located in the constellation Capricornus. It is located at a distance of circa 120 million light years from Earth, which, given its apparent dimensions, means that NGC 6907 is about 115,000 light years across. It was discovered by William Herschel on July 12, 1784. The total infrared luminosity of the galaxy is 1011.03 L, and thus it is categorised as a luminous infrared galaxy.

NGC 4278 Elliptical 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.

NGC 3945 Barred lenticular galaxy in the constellation Ursa Major

NGC 3945 is a barred lenticular galaxy in the constellation Ursa Major. It was discovered on March 19, 1790 by the astronomer William Herschel.

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