Segue 1
Encyclopedia
Segue 1 is a dwarf spheroidal galaxy
or globular cluster
situated in the Leo constellation and discovered in 2006 by Sloan Digital Sky Survey
. It is located at the distance of about 23 kpc from the Sun and moves away from the Sun with the velocity of about 206 km/s. Segue 1 has a noticeably elongated (ratio of axes ~ 2:1) shape with the half-light radius
of about 30 pc. This elongation may be caused by the tidal forces acting from the Milky Way galaxy if Segue 1 is being tidally disrupted now.
Segue 1 is one of the smallest and faintest satellites of the Milky Way
—its integrated luminosity is about 300 times that of the Sun (absolute visible magnitude of about −1.5), which is much smaller than the luminosity of a typical globular cluster
. Observations indicate its mass is about 0.6 million solar mass
es, which means that Segue's 1 mass to light ratio
is around 3400. Segue 1 had the highest known mass-to-light ratio of any observed galaxy as of 2011. A high mass to light ratio implies that Segue 1 may be dominated by dark matter
. It is difficult, however, to estimate the mass of such faint objects due to significant foreground contamination, which inflates the velocity dispersion. In addition, any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.
The stellar population of Segue 1 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 300 times less heavy elements than the Sun. Currently there is no star formation in Segue 1. Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 13 solar masses.
Segue 1 is located in the middle of the Sagittarius stream
and at approximately the same distance from the Sun. It may once have been a globular cluster of the Sagittarius Dwarf Elliptical Galaxy
, which was latter stripped from it by the tidal force
s acting from the Milky Way galaxy. However, more recent studies concluded that Segue 1 is not actually associated with the Sagittarius stream and that it is not being tidally disrupted. If Segue 1 is a galaxy it may have been a satellite of Sagittarius Dwarf Elliptical Galaxy in the past.
Dwarf spheroidal galaxy
Dwarf spheroidal galaxy is a term in astronomy applied to low luminosity galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31...
or globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
situated in the Leo constellation and discovered in 2006 by Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
. It is located at the distance of about 23 kpc from the Sun and moves away from the Sun with the velocity of about 206 km/s. Segue 1 has a noticeably elongated (ratio of axes ~ 2:1) shape with the half-light radius
Effective radius
The effective radius of a galaxy is the radius at which one half of the total light of the system is emitted interior to this radius. This assumes the galaxy is circularly symmetric...
of about 30 pc. This elongation may be caused by the tidal forces acting from the Milky Way galaxy if Segue 1 is being tidally disrupted now.
Segue 1 is one of the smallest and faintest satellites of 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...
—its integrated luminosity is about 300 times that of the Sun (absolute visible magnitude of about −1.5), which is much smaller than the luminosity of a typical globular cluster
Globular cluster
A globular cluster is a spherical collection of stars that orbits a galactic core as a satellite. Globular clusters are very tightly bound by gravity, which gives them their spherical shapes and relatively high stellar densities toward their centers. The name of this category of star cluster is...
. Observations indicate its mass is about 0.6 million solar mass
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
es, which means that Segue's 1 mass to light ratio
Mass to light ratio
In astrophysics and physical cosmology the mass to light ratio, normally designated with the symbol \Upsilon is the quotient between the total mass of a spatial volume and its luminosity...
is around 3400. Segue 1 had the highest known mass-to-light ratio of any observed galaxy as of 2011. A high mass to light ratio implies that Segue 1 may be dominated by dark matter
Dark 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...
. It is difficult, however, to estimate the mass of such faint objects due to significant foreground contamination, which inflates the velocity dispersion. In addition, any mass estimate is based on an implicit assumption that an object is gravitationally bound, which may not be true if the object is in a process of disruption.
The stellar population of Segue 1 consists mainly of old stars formed more than 12 billion years ago. The metallicity of these old stars is also very low at , which means that they contain 300 times less heavy elements than the Sun. Currently there is no star formation in Segue 1. Measurements have so far failed to detect neutral hydrogen in it—the upper limit is 13 solar masses.
Segue 1 is located in the middle of the Sagittarius stream
Field of Streams
The Field of Streams is a patch of sky where several stellar streams are visible and crisscross. The analysis of the streams of the zone determined that the dark matter configuration of the Milky Way Galaxy in the inner halo is round, at variance with expectations that it would be oblate.It was...
and at approximately the same distance from the Sun. It may once have been a globular cluster of the Sagittarius Dwarf Elliptical Galaxy
Sagittarius Dwarf Elliptical Galaxy
The Sagittarius Dwarf Elliptical Galaxy is an elliptical loop-shaped satellite galaxy of the Milky Way Galaxy. It consists of four globular clusters, the main cluster being discovered in 1994...
, which was latter stripped from it by the tidal force
Tidal force
The tidal force is a secondary effect of the force of gravity and is responsible for the tides. It arises because the gravitational force per unit mass exerted on one body by a second body is not constant across its diameter, the side nearest to the second being more attracted by it than the side...
s acting from the Milky Way galaxy. However, more recent studies concluded that Segue 1 is not actually associated with the Sagittarius stream and that it is not being tidally disrupted. If Segue 1 is a galaxy it may have been a satellite of Sagittarius Dwarf Elliptical Galaxy in the past.