NGC 404 is a

field galaxy A field galaxy is a galaxy that does not belong to a larger galaxy group or cluster and hence is gravitationally alone. Roughly 80% of all galaxies located within of the Milky Way are in groups or clusters of galaxies. Most low-surface-bright ...

located about 10 million

light year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distance, astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by t ...

s away in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

Andromeda. It was discovered by

William Herschel Frederick William Herschel ( ; ; 15 November 1738 – 25 August 1822) was a German-British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline Herschel. Born in the Electorate of Hanover ...

in 1784, and is visible through small telescopes.Mirach's Ghost (NGC 404)
''The Internet Encyclopedia of Science'', David Darling. Accessed on line August 15, 2008. NGC 404 lies just beyond the

Local Group The Local Group is the galaxy group that includes the Milky Way, where Earth is located. It has a total diameter of roughly , and a total mass of the order of . It consists of two collections of galaxies in a " dumbbell" shape; the Milky Way ...

and does not appear gravitationally bound to it. It is located within 7

arc-minute A minute of arc, arcminute (abbreviated as arcmin), arc minute, or minute arc, denoted by the symbol , is a unit of angular measurement equal to of a degree. Since one degree is of a turn, or complete rotation, one arcminute is of a tu ...

s of second magnitude star

Mirach Mirach is a prominent star in the northern constellation of Andromeda. It is pronounced and has the Bayer designation Beta Andromedae, which is Latinized from β Andromedae. This star is positioned northeast of the ''Great Square'' o ...

, making it a difficult target to observe or photograph and granting it the nickname "Mirach's Ghost".

Physical characteristics

NGC NGC may refer to: Companies * NGC Corporation, the name of US electric company Dynegy, Inc. from 1995 to 1998 * National Gas Company of Trinidad and Tobago, a state-owned natural gas company in Trinidad and Tobago * National Grid plc, a former nam ...

404 is a very isolated dwarf

lenticular galaxy A lenticular galaxy (denoted S0) is a Galaxy morphological classification, type of galaxy intermediate between an elliptical galaxy, elliptical (denoted E) and a spiral galaxy in galaxy morphological classification schemes. It contains a large-s ...

, a bit more luminous and smaller than the

Small Magellanic Cloud The Small Magellanic Cloud (SMC) 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 approximately 7 bill ...

. Unlike many other early-type galaxies, it is very rich in

neutral hydrogen The hydrogen line, 21 centimeter line, or H I line is a spectral line that is created by a change in the energy state of solitary, electrically neutral hydrogen atoms. It is produced by a spin-flip transition, which means the directio ...

, most of it concentrated in a pair of large rings around it. It also has

star formation Star formation is the process by which dense regions within molecular clouds in interstellar space—sometimes referred to as "stellar nurseries" or "star-forming regions"—Jeans instability, collapse and form stars. As a branch of astronomy, sta ...

both in its center and in its outermost regions, albeit at a low level. Both the outer gas disk and its star formation are assumed to have been triggered by one or several

mergers Mergers and acquisitions (M&A) are business transactions in which the ownership of a company, business organization, or one of their operating units is transferred to or consolidated with another entity. They may happen through direct absorpt ...

with smaller galaxies roughly 1 billion years ago and it has been proposed

404 is a former

spiral galaxy Spiral galaxies form a galaxy morphological classification, class of galaxy originally described by Edwin Hubble in his 1936 work ''The Realm of the Nebulae''

that was transformed into a lenticular one by that event.

LINER emission

NGC 404 contains a

low-ionization nuclear emission-line region A low-ionization nuclear emission-line region (LINER) is a type of galactic nucleus that is defined by its spectral line emission. The spectra typically include line emission from weakly ionized or neutral atoms, such as O, O+, N+, and S ...

(LINER), a type of region that is characterized by

spectral line A spectral line is a weaker or stronger region in an otherwise uniform and continuous spectrum. It may result from emission (electromagnetic radiation), emission or absorption (electromagnetic radiation), absorption of light in a narrow frequency ...

emission from weakly

ion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by convent ...

ized atoms. A

nuclear star cluster A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of most galaxies. NSCs are the central massive objects of fainter, l ...

is also present as well as (likely) a

supermassive black hole A supermassive black hole (SMBH or sometimes SBH) is the largest type of black hole, with its mass being on the order of hundreds of thousands, or millions to billions, of times the mass of the Sun (). Black holes are a class of astronomical ...

, with a mass of several tens of thousands

solar mass The solar mass () is a frequently used unit of mass in astronomy, equal to approximately . It is approximately equal to the mass of the Sun. It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxie ...

es.

Distance measurements

At least two techniques have been used to measure distances to NGC 404. The infrared

surface brightness fluctuation Surface brightness fluctuation (SBF) is a secondary standard candle, distance indicator used to estimate distances to galaxies. It is useful to 100 Mpc (parsec). The method measures the variance in a galaxy's light distribution arising from fluc ...

s distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to NGC 404 using this technique in 2003 is 9.9 ± 0.5 Mly (3.03 ± 0.15 Mpc). However, NGC 404 is close enough that

red supergiants Red supergiants (RSGs) are stars with a supergiant luminosity class (Stellar classification#Yerkes spectral classification, Yerkes class I) and a stellar classification K or M. They are the List of largest stars, largest stars in the universe in t ...

can be imaged as individual stars. The light from these stars and knowledge of how they should compare to nearby stars within the

Milky Way The Milky Way or Milky Way Galaxy is the galaxy that includes the Solar System, with the name describing the #Appearance, galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars in other arms of the galax ...

galaxy allows for direct measurement of the distance to the galaxy. This method is referred to as the

tip of the red giant branch Tip of the red-giant branch (TRGB) is a primary distance indicator used in astronomy. It uses the luminosity of the brightest red-giant-branch stars in a galaxy as a standard candle to gauge the distance to that galaxy. It has been used in conjun ...

(TRGB) method. The estimated distance to NGC 404 using this technique is 10.0 ± 1.2 Mly (3.1 ± 0.4 Mpc). Averaged together, these distance measurements give a distance estimate of 10.0 ± 0.7 Mly (3.07 ± 0.21 Mpc).

Possible satellite

In 2018 a possible satellite designated Donatiello I was identified. Donatiello I is a

dwarf spheroidal galaxy A dwarf spheroidal galaxy (dSph) is a term in astronomy applied to small, low-luminosity galaxies with very little dust and an older stellar population. They are found in the Local Group as companions to the Milky Way and as systems that are c ...

with little recent star formation. Difficulty in establishing the exact distance to the galaxy leaves its status as a satellite unconfirmed.

Gallery

File:NGC 404 PanS.jpg,

Pan-STARRS The Panoramic Survey Telescope and Rapid Response System (Pan-STARRS1; List of observatory codes, obs. code: IAU code#F51, F51 and Pan-STARRS2 obs. code: IAU code#F52, F52) located at Haleakala Observatory, Hawaii, US, consists of astronomical ...

image of NGC 404

Notes

average(3.03 ± 0.15, 3.1 ± 0.4) = ((3.03 + 3.1) / 2) ± ((0.15² + 0.4²)^0.5 / 2) = 3.07 ± 0.21

References

External links

Encyclopedia of Astrobiology, Astronomy & Spaceflight Entry

* * {{DEFAULTSORT:NGC 0404 Lenticular galaxies Field galaxies LINER galaxies Andromeda (constellation) 0404 00718 04126 17840913 Virgo Supercluster Discoveries by William Herschel