Messier 94
Encyclopedia
Messier 94 is a spiral galaxy
in the constellation
Canes Venatici
. It was discovered by Pierre Méchain
in 1781,
and catalogued by Charles Messier
two days later. Although some references describe M94 as a barred spiral galaxy
, the "bar" structure appears to be more oval-shaped. The galaxy is also notable in that it has two ring structures.
that reveal that ion
ized gas is present but the gas is only weakly ionized (i.e. the atoms are missing relatively few electrons).
locations within the disk of the galaxy. The inner ring is the site of strong star formation
activity and is sometimes referred to as a starburst
ring. This star formation is fueled by gas that is dynamically driven into the ring by the inner oval-shaped bar-like structure.
A 2009 study conducted by an international team of astrophysicists revealed that the outer ring of M94 is not a closed stellar ring, as historically attributed in the literature, but a complex structure of spiral arms when viewed in mid-IR and UV. The study found that the outer disk of this galaxy is active. It contains approximately 23% of the galaxy's total stellar mass and contributes about 10% of the galaxy's new stars. In fact, the star formation rate of the outer disk is approximately two times greater than the inner disk because it is more efficient per unit of stellar mass.
There are several possible external events that could have led to the origin of M94's outer disk including the accretion of a satellite galaxy or the gravitational interaction with a nearby star system. However, further research found problems with each of these scenarios. Therefore, the report concludes that the inner disk of M94 (NGC4736) is an oval distortion which led to the creation of this galaxy's peripheral disk.
argued that M94 contains a prototypical pseudobulge
. A classical spiral galaxy
consists of a disk of gas and young stars that intersects a large sphere (or bulge) of older stars. In contrast, a galaxy with a pseudobulge does not have a large bulge of old stars but instead contain a bright central structure with intense star formation that looks like a bulge when the galaxy is viewed face-on. In the case of M94, this pseudobulge takes the form of a ring around a central oval-shaped region.
s distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to M94 using this technique is 17.0 ± 1.4 Mly
(5.2 ± 0.4 Mpc
). However, M94 is close enough that the Hubble Space Telescope
can be used to resolve and measure the fluxes of the brightest individual stars within the galaxy. These measured fluxes can then be compared to the measured fluxes of similar stars within the Milky Way
to measure the distance. The estimated distance to M94 using this technique is 15 ± 2 Mly
(4.7 ± 0.6 Mpc
). Averaged together, these distance measurements give a distance estimate of 16.0 ± 1.3 Mly
(4.9 ± 0.4 Mpc
).
present. The study analyzed the rotation curves of the galaxy's stars and the density of hydrogen gas and found that ordinary luminous matter appeared to account for all of the galaxy's mass. This result was unusual and somewhat controversial, as current models don't indicate how a galaxy could form without a dark matter halo or how a galaxy could lose its dark matter. Other explanations for galactic rotation curves, such as MOND
, also have difficulty explaining this galaxy. This result has yet to be confirmed or accepted by other research groups, however, and has not actually been tested against the predictions of standard galaxy formation models.
, a group of galaxies that contains between 16 and 24 galaxies. This group is one of many that lies within the Virgo Supercluster
(i.e. the Local Supercluster). Although a large number of galaxies may be associated with M94, only a few galaxies near M94 appear to form a gravitationally bound system. Most of the other nearby galaxies appear to be moving with the expansion of the universe.
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
Canes Venatici
Canes Venatici
Canes Venatici is one of the 88 official modern constellations. It is a small northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for "hunting dogs", and the constellation is often depicted in illustrations as representing the dogs of Boötes the...
. It was discovered by Pierre Méchain
Pierre Méchain
Pierre François André Méchain was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep sky objects and comets.-Life:...
in 1781,
and catalogued by Charles Messier
Charles Messier
Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of deep sky objects such as nebulae and star clusters that came to be known as the 110 "Messier objects"...
two days later. Although some references describe M94 as a barred spiral galaxy
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies...
, the "bar" structure appears to be more oval-shaped. The galaxy is also notable in that it has two ring structures.
Nucleus
M94 is classified as having a low ionization nuclear emission region (LINER) nucleus. LINERs in general are characterized by optical spectraSpectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
that reveal that ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
ized gas is present but the gas is only weakly ionized (i.e. the atoms are missing relatively few electrons).
Inner and outer rings
M94 contains both an inner ring with a diameter of 70" and an outer ring with a diameter of 600". These rings appear to form at resonanceResonance
In physics, resonance is the tendency of a system to oscillate at a greater amplitude at some frequencies than at others. These are known as the system's resonant frequencies...
locations within the disk of the galaxy. The inner ring is the site of strong star formation
Star formation
Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. As a branch of astronomy star formation includes the study of the interstellar medium and giant molecular clouds as precursors to the star formation process and the study of young...
activity and is sometimes referred to as a starburst
Starburst (astronomy)
In astronomy, starburst is a generic term to describe a region of space with an abnormally high rate of star formation. It is reserved for truly unusual objects....
ring. This star formation is fueled by gas that is dynamically driven into the ring by the inner oval-shaped bar-like structure.
A 2009 study conducted by an international team of astrophysicists revealed that the outer ring of M94 is not a closed stellar ring, as historically attributed in the literature, but a complex structure of spiral arms when viewed in mid-IR and UV. The study found that the outer disk of this galaxy is active. It contains approximately 23% of the galaxy's total stellar mass and contributes about 10% of the galaxy's new stars. In fact, the star formation rate of the outer disk is approximately two times greater than the inner disk because it is more efficient per unit of stellar mass.
There are several possible external events that could have led to the origin of M94's outer disk including the accretion of a satellite galaxy or the gravitational interaction with a nearby star system. However, further research found problems with each of these scenarios. Therefore, the report concludes that the inner disk of M94 (NGC4736) is an oval distortion which led to the creation of this galaxy's peripheral disk.
Pseudobulge
In a paper published in 2004, John Kormendy and Robert KennicuttRobert Kennicutt
Robert Charles Kennicutt, Jr. FRS is an American astronomer. He is the Plumian Professor of Astronomy at the Institute of Astronomy in the University of Cambridge. He was formerly Editor-in-Chief of the Astrophysical Journal...
argued that M94 contains a prototypical pseudobulge
Bulge (astronomy)
In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies...
. A classical spiral galaxy
Spiral galaxy
A spiral galaxy is a certain kind of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as...
consists of a disk of gas and young stars that intersects a large sphere (or bulge) of older stars. In contrast, a galaxy with a pseudobulge does not have a large bulge of old stars but instead contain a bright central structure with intense star formation that looks like a bulge when the galaxy is viewed face-on. In the case of M94, this pseudobulge takes the form of a ring around a central oval-shaped region.
Distance measurements
At least two techniques have been used to measure distances to M94. The surface brightness fluctuationSurface brightness fluctuation
Surface brightness fluctuation is a secondary distance indicator used to estimate distances to galaxies. The technique uses the fact that galaxies are made up of a finite number of stars. The number of stars in any small patch of the galaxy will vary from point to point, creating a noise-like...
s distance measurement technique estimates distances to spiral galaxies based on the graininess of the appearance of their bulges. The distance measured to M94 using this technique is 17.0 ± 1.4 Mly
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
(5.2 ± 0.4 Mpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
). However, M94 is close enough that the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
can be used to resolve and measure the fluxes of the brightest individual stars within the galaxy. These measured fluxes can then be compared to the measured fluxes of similar stars within the Milky Way
Milky Way
The Milky Way is the galaxy that contains the Solar System. This name derives from its appearance as a dim un-resolved "milky" glowing band arching across the night sky...
to measure the distance. The estimated distance to M94 using this technique is 15 ± 2 Mly
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
(4.7 ± 0.6 Mpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
). Averaged together, these distance measurements give a distance estimate of 16.0 ± 1.3 Mly
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
(4.9 ± 0.4 Mpc
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
).
Dark matter
In 2008 a study was published that appeared to show that M94 had very little or no dark matterDark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
present. The study analyzed the rotation curves of the galaxy's stars and the density of hydrogen gas and found that ordinary luminous matter appeared to account for all of the galaxy's mass. This result was unusual and somewhat controversial, as current models don't indicate how a galaxy could form without a dark matter halo or how a galaxy could lose its dark matter. Other explanations for galactic rotation curves, such as MOND
Mond
Mond may refer to:* MOND - Modified Newtonian Dynamics. A proposed adjustment to the classical inverse-square law of gravity.* Der Mond, an opera in one act* Mond Nickel Company, a defunct mining company...
, also have difficulty explaining this galaxy. This result has yet to be confirmed or accepted by other research groups, however, and has not actually been tested against the predictions of standard galaxy formation models.
Galaxy group information
M94 is one of the brightest galaxies within the M94 GroupM94 Group
The M94 Group is a loose, extended group of galaxies located about 13 million light-years away in the constellations Canes Venatici and Coma Berenices. The group is one of many groups that lies within the Virgo Supercluster The M94 Group (Canes Venatici I Group) is a loose, extended group of...
, a group of galaxies that contains between 16 and 24 galaxies. This group is one of many that lies within the Virgo Supercluster
Virgo Supercluster
The Virgo Supercluster or Local Supercluster is the irregular supercluster that contains the Virgo Cluster in addition to the Local Group, which in turn contains the Milky Way and Andromeda galaxies. At least 100 galaxy groups and clusters are located within its diameter of 33 megaparsecs...
(i.e. the Local Supercluster). Although a large number of galaxies may be associated with M94, only a few galaxies near M94 appear to form a gravitationally bound system. Most of the other nearby galaxies appear to be moving with the expansion of the universe.