Ring galaxy

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Hoag's Object, a ring galaxy. Another red ring galaxy can be seen behind it. Hoag's object.jpg
Hoag's Object, a ring galaxy. Another red ring galaxy can be seen behind it.

A ring galaxy is a galaxy with a circle-like appearance. Hoag's Object, discovered by Arthur Hoag in 1950, is an example of a ring galaxy. [1] The ring contains many massive, relatively young blue stars, which are extremely bright. The central region contains relatively little luminous matter. Some astronomers believe that ring galaxies are formed when a smaller galaxy passes through the center of a larger galaxy. Because most of a galaxy consists of empty space, this "collision" rarely results in any actual collisions between stars. However, the gravitational disruptions caused by such an event could cause a wave of star formation to move through the larger galaxy. [2] Other astronomers think that rings are formed around some galaxies when external accretion takes place. Star formation would then take place in the accreted material because of the shocks and compressions of the accreted material. [3] [4]

Contents

Formation theories

Ring galaxies are theorized to be formed through various methods including, but not limited to, the following scenarios:

Bar instability

A phenomenon where the rotational velocity of the bar in a barred spiral galaxy increases to the point of spiral spin-out. Under typical conditions, gravitational density waves would favor the creation of spiral arms. When the bar is unstable, these density waves are instead migrated out into a ring-structure by the pressure, force, and gravitational influence of the baryonic and dark matter furiously orbiting about the bar. This migration forces the stars, gas and dust found within the former arms into a torus-like region, forming a ring, and often igniting star formation.

Galaxies with this structure have been found where the bar dominates, and essentially "carves out" the ring of the disc as it rotates. Oppositely, ring galaxies have been found where the bar has collapsed or disintegrated into a highly-flattened bulge.

Despite this, observations suggest that bars, rings and spiral arms have the ability to fall apart and reform over the span of hundreds of millions of years, particularly in dense intergalactic environments, such as galaxy groups and clusters, where gravitational influences are more likely to play a role in the morphological and physical evolution of a galaxy without the influence of collisions and mergers.

Galactic collisions

Another observed way that ring galaxies can form is through the process of two or more galaxies colliding. The Cartwheel Galaxy, galaxy pair AM 2026-424, and Arp 147 are all examples of ring galaxies thought to be formed by this process.

In pass-through galactic collisions, or bullseye collisions, an often smaller galaxy will pass directly through the disc of an often larger spiral, causing an outward push of the arms from the gravity of the smaller galaxy, as if dropping a rock into a pond of still water. These collisions can either launch the bulge and core away from the main disk, creating an almost empty ring appearance as the shockwave pushes the spiral arms out, or shove the core out towards the disk, often creating an oval-shaped ring with the bulge still somewhat intact. In side-swipe and head-on collisions, the appearance of a perfect ring are less likely, with chaotic and warped appearances dominating.

Rings formed through collision processes are believed to be transient features of the affected galaxies, lasting only a few ten to hundred million years (a relatively short timeframe considering some mergers can take over a billion years to complete) before disintegrating, reforming into spiral arms, or succumbing to further disturbance from gravitational influence.

Intergalactic medium accretion

This method has been inferred through the existence of Hoag's object, along with UV observations of several other large and ultra-large super spiral galaxies and current formation theories of spiral galaxies.

UV-light observations show several cases of faint, ring-like and spiral structures of hot young stars that have formed along the network of cooled inflowing gas, extending far from the visible luminous galactic disc. If conditions are favorable, a ring can form in the place of a spiral structure.

Since some spiral galaxies are theorized to have formed from massive clouds of intergalactic gas collapsing and then rotationally forming into a disc structure, one could assume that a ring disc could form in place of a spiral disc if, as mentioned before, conditions are favorable. This holds true for protogalaxies, or galaxies just throughout to be forming, and old galaxies that have migrated into a section of space with a higher gas content than its previous locations.

See also

Related Research Articles

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<span class="mw-page-title-main">Galactic astronomy</span> Study of the Milky Way galaxy and its contents

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<span class="mw-page-title-main">Hoag's Object</span> Unusual galaxy in the constellation Serpens

Hoag's Object is an unusual ring galaxy in the constellation of Serpens Caput. It is named after Arthur Hoag, who discovered it in 1950 and identified it as either a planetary nebula or a peculiar galaxy. The galaxy has a D25 isophotal diameter of 45.41 kiloparsecs (148,000 light-years).

<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">Lenticular galaxy</span> Class of galaxy between an elliptical galaxy and a spiral galaxy

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<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">Messier 94</span> Galaxy in the constellation Canes Venatici

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<span class="mw-page-title-main">Milky Way</span> Galaxy containing the Solar System

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<span class="mw-page-title-main">Cartwheel Galaxy</span> Lenticular galaxy and ring galaxy in the constellation Sculptor

The Cartwheel Galaxy (also known as ESO 350-40 or PGC 2248) is a lenticular ring galaxy about 500 million light-years away in the constellation Sculptor. It has a D25 isophotal diameter of 44.23 kiloparsecs (144,300 light-years), and a mass of about 2.9–4.8 × 109 solar masses; its outer ring has a circular velocity of 217 km/s.

<span class="mw-page-title-main">Interacting galaxy</span> Galaxies with interacting gravitational fields

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<span class="mw-page-title-main">NGC 1232</span> Galaxy in the constellation Eridanus

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

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<span class="mw-page-title-main">NGC 922</span> Peculiar galaxy in the constellation Fornax

NGC 922 is a peculiar galaxy in the southern constellation of Fornax, located at a distance of 142 Mly from the Milky Way. It is one of the nearest known collisional galaxies. This object was described by the Herschels as "considerably faint, pretty large, round, gradually pretty much brighter middle." The general form is described by the morphological classification of SB(s)cd pec, which indicates a peculiar (pec) barred spiral galaxy (SB) with no inner ring system around the bar (s) and loosely-wound spiral arms (cd).

<span class="mw-page-title-main">UGC 2885</span> Large barred spiral galaxy in the constellation Perseus

UGC 2885 is a large barred spiral galaxy of type SA(rs)c in the constellation Perseus. It is 232 million light-years (71 Mpc) from Earth and measures 463,000 ly (142,000 pc) across, making it one of the largest known spiral galaxies. It is also a possible member of the Pisces-Perseus supercluster.

<span class="mw-page-title-main">NGC 7727</span> Peculiar galaxy in the constellation Aquarius

NGC 7727 is a peculiar galaxy in the constellation Aquarius. It harbors two galactic nuclei, each containing a supermassive black hole, separated 1,600 light years apart.

The following outline is provided as an overview of and topical guide to galaxies:

<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 6028</span> Galaxy in the constellation Hercules

NGC 6028 is a barred lenticular galaxy and a ring galaxy located about 200 million light-years away in the constellation Hercules. Ring galaxies such as NGC 6028 are also known as Hoag-type galaxies as they may have a resemblance to the prototype, Hoag's Object. NGC 6028 was discovered by astronomer William Herschel on March 14, 1784. It was then rediscovered by astronomer Guillaume Bigourdan on May 4, 1886.

References

  1. ↑ Nemiroff, R.; Bonnell, J., eds. (September 9, 2002). "Hoag's Object: A Strange Ring Galaxy". Astronomy Picture of the Day . NASA . Retrieved March 31, 2012.
  2. ↑ Fernandez, Julia; Alonso, Sol; Mesa, Valeria; Duplancic, Fernanda; Coldwell, Georgina (2021-09-01). "Properties of galaxies with ring structures". Astronomy & Astrophysics. 653: A71. doi:10.1051/0004-6361/202141208. ISSN   0004-6361.
  3. ↑ Appleton, P.N.; Struck-Marcell, Curtis (1996). "Collisional Ring Galaxies" . Retrieved March 31, 2012.
  4. ↑ "Ring galaxies, the rarest in the Universe, finally explained". Big Think. 2022-04-20. Retrieved 2024-09-13.
  5. ↑ "A Cosmic Hit and Run" . Retrieved 9 May 2016.
  6. ↑ "One ring to rule them all". ESA/Hubble. Retrieved 2 April 2013.
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