NGC 3109 is a small barred Magellanic type

spiral In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Helices Two major definitions of "spiral" in the American Heritage Dictionary are:irregular galaxy around 4.34

Mly A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

away in the direction of the constellation of Hydra. NGC 3109 is believed to be tidally interacting with the

dwarf elliptical galaxy Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are smaller than ordinary elliptical galaxies. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. Examples "Dwarf elliptical" galaxie ...

Antlia Dwarf The Antlia Dwarf is a dwarf spheroidal/ irregular galaxy. It lies about 1.3 Mpc (4.3 million light-years) from Earth in the constellation Antlia. It is the fourth and faintest member of the nearby Antlia-Sextans Group of galaxies. The ga ...

. It was discovered by

John Herschel Sir John Frederick William Herschel, 1st Baronet (; 7 March 1792 – 11 May 1871) was an English polymath active as a mathematician, astronomer, chemist, inventor, experimental photographer who invented the blueprint and did botanical wo ...

on March 24, 1835 while he was in what is now

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

.''Irregular Galaxy NGC 3109''

Size and morphology

NGC 3109 is classified as a Magellanic type irregular galaxy, but it may in fact be a small

spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae'' NGC 3109 has a mass of about 2.3×10⁹ times the mass of the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

, of which 20% is in the form of neutral

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-to ...

. It is oriented edge-on from our point of view, and may contain a disk and a

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * ''Halo'' (franch ...

. The disk appears to be composed of stars of all ages, whereas the halo contains only very old and

metal-poor In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

stars. NGC 3109 does not appear to possess a galactic nucleus. From measurements of the neutral atomic

in the galaxy, it has been found that the disk of NGC 3109 is warped. The warp has the same

radial velocity The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection ...

as gas in the

galaxy, indicating that the two galaxies had a close encounter approximately one billion years ago.

Composition

Based on spectroscopy of blue supergiants in NGC 3109, it is known that the galaxy has a low metallicity, similar to that to the

Small Magellanic Cloud The Small Magellanic Cloud (SMC), or Nubecula Minor, is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about , and contains several hundred million stars. It has a total mass of ...

. It is one of the most metal-poor galaxies in the Local group. NGC 3109 seems to contain an unusually large number of

planetary nebula A planetary nebula (PN, plural PNe) is a type of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from red giant stars late in their lives. The term "planetary nebula" is a misnomer because they are unrelate ...

e for its luminosity. It also contains a substantial amount of

dark matter Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ...

Location

NGC 3109 is located about away, in the constellation Hydra. This puts it at the very outskirts of the Local Group. Its membership of the Local Group has been questioned, because it seems to be receding faster than estimates of the Local Group's

escape velocity In celestial mechanics, escape velocity or escape speed is the minimum speed needed for a free, non- propelled object to escape from the gravitational influence of a primary body, thus reaching an infinite distance from it. It is typically ...

. It is distant enough from the largest members of the Local Group that it has not been tidally influenced by them.

NGC 3109 and ΛCDM

The high radial velocity of NGC 3109 for its position poses a severe challenge to the

Lambda-CDM model The ΛCDM (Lambda cold dark matter) or Lambda-CDM model is a parameterization of the Big Bang cosmological model in which the universe contains three major components: first, a cosmological constant denoted by Lambda ( Greek Λ) associated ...

of cosmology. It is too massive and distant from the Local Group for it to have been flung out in a three-body interaction involving the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

Andromeda Galaxy The Andromeda Galaxy (IPA: ), also known as Messier 31, M31, or NGC 224 and originally the Andromeda Nebula, is a barred spiral galaxy with the diameter of about approximately from Earth and the nearest large galaxy to the Milky Way. The gal ...

, unless more exotic scenarios like

MOND Modified Newtonian dynamics (MOND) is a hypothesis that proposes a modification of Newton's law of universal gravitation to account for observed properties of galaxies. It is an alternative to the hypothesis of dark matter in terms of explaining ...

are invoked.

Notes

References

*Grebel, Gallagher, Harbeck (2003
''The Progenitors of Dwarf Spheroidal Galaxies''
ArXiv.org. retrieved November 2007

External links

* {{DEFAULTSORT:NGC 3109 Irregular galaxies Barred spiral galaxies NGC 3109 subgroup Hydra (constellation) 3109 29128 Local Group 18350324 UGCA objects