Terzan 7
Encyclopedia
Terzan 7 is a sparse and young globular cluster
that is believed to have originated in the Sagittarius Dwarf Elliptical Galaxy
(Sag DEG) and is physically associated with it. It is relatively metal rich
with[ Fe
/H
] = -0.6 and an estimated age of 7.5 Gyr
. Terzan 7 has low levels of nickel
([Ni/Fe]=-0.2) which supports its membership in the Sag DEG system since it has a similar chemical signature. It has a rich population of blue straggler
s that are strongly concentrated toward the center of Terzan 7. It has an average luminosity distribution of Mv = -5.05. It has a half-light radius
(Rh) = 6.5pc.
in 1968.
's galactic halo
formed at the same time (12-15 Gyr). Even the far situated NGC 2419
(~100 kpc from galactic center) has a similar age. This trend also applies to the age of globulars found in the Large Magellanic Cloud
and Fornax Dwarf
(~140 kpc from galactic center). However, a few globulars seem to be significantly younger than the rest; these include Palomar 1
, Palomar 3, Palomar 4
, Palomar 12
, Palomar 14, Ruprecht 106, IC 4499, Arp 2, Eridanus, Fornax 4, and Terzan 7. In particular, the ones associated with the Sag DEG appear to have formed more recently. The data suggests that all the present outer halo globulars may have originally formed in dwarf spheroidals.
of other dwarf spheroidals
like Sag DEG. The best candidate for this idea is Palomar 12
.
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...
that is believed to have originated in 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...
(Sag DEG) and is physically associated with it. It is relatively metal rich
Metallicity
In astronomy and physical cosmology, the metallicity of an object is the proportion of its matter made up of chemical elements other than hydrogen and helium...
with
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
/H
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
] = -0.6 and an estimated age of 7.5 Gyr
Gyr
Gyr or gyr can mean:* Phoenix Goodyear Airport , an airport in Arizona, United States* Gyrfalcon, a species of bird of prey* Byr or Gyr, abbreviations for gigayear, i.e. a billion years...
. Terzan 7 has low levels of nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
([Ni/Fe]=-0.2) which supports its membership in the Sag DEG system since it has a similar chemical signature. It has a rich population of blue straggler
Blue straggler
Blue stragglers are main sequence stars in open or globular clusters that are more luminous and bluer than stars at the main sequence turn-off point for the cluster. Blue stragglers were first discovered by Allan Sandage in 1953 while performing photometry of the stars in the globular cluster M3...
s that are strongly concentrated toward the center of Terzan 7. It has an average luminosity distribution of Mv = -5.05. It has a 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...
(Rh) = 6.5pc.
History
Terzan 7 was the brightest of six globulars discovered by the French astronomer Agop TerzanAgop Terzan
Agop Terzan is a French–Armenian astronomer. Born in Istanbul, he received his BA in Mathematical and Astronomical Science in 1945. He received a Master's degree in Astronomy in 1949, and a PhD from Lyon University in 1965....
in 1968.
Young globular
Nearly all globular clusters of the Milky WayMilky 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...
's galactic halo
Galactic halo
The term galactic halo is used to denote an extended, roughly spherical component of a galaxy, which extends beyond the main, visible component. It can refer to any of several distinct components which share these properties:* the galactic spheroid...
formed at the same time (12-15 Gyr). Even the far situated NGC 2419
NGC 2419
NGC 2419 is a globular cluster in the constellation Lynx. It was discovered by William Herschel on December 31, 1788...
(~100 kpc from galactic center) has a similar age. This trend also applies to the age of globulars found in the Large Magellanic Cloud
Large Magellanic Cloud
The Large Magellanic Cloud is a nearby irregular galaxy, and is a satellite of the Milky Way. At a distance of slightly less than 50 kiloparsecs , the LMC is the third closest galaxy to the Milky Way, with the Sagittarius Dwarf Spheroidal and Canis Major Dwarf Galaxy lying closer to the center...
and Fornax Dwarf
Fornax Dwarf
The Fornax Dwarf Spheroidal is an elliptical dwarf galaxy in the constellation Fornax that was discovered in 1938 by Harlow Shapley. He discovered it while he was in South Africa on photographic plates taken by a 24 inch reflecting telescope at Boyden Observatory, shortly after he discovered...
(~140 kpc from galactic center). However, a few globulars seem to be significantly younger than the rest; these include Palomar 1
Palomar 1
Palomar 1 is a globular cluster in the constellation Cepheus in thehalo or Outer Arm of the Milky Way galaxy. First discovered discovered by George O. Abell in 1954 on the Palomar Survey Sky plates,it was catalogued as a globular cluster...
, Palomar 3, Palomar 4
Palomar 4
Palomar 4 is a globular cluster of the Milky Way galaxy. It was discovered in 1949 by Edwin Hubble and again in 1955 by A. G. Wilson. It is 356,000 light years away.This star cluster is further away than the Magellanic Clouds and SagDEG satellite galaxies....
, Palomar 12
Palomar 12
Palomar 12 is a globular cluster in the constellation Capricornus that belongs to the halo of the Milky Way galaxy. First discovered on the Palomar Survey Sky plates by Robert G. Harrington and Fritz Zwicky,...
, Palomar 14, Ruprecht 106, IC 4499, Arp 2, Eridanus, Fornax 4, and Terzan 7. In particular, the ones associated with the Sag DEG appear to have formed more recently. The data suggests that all the present outer halo globulars may have originally formed in dwarf spheroidals.
Hierarchical galaxy formation
Alternatively, a hierarchical galaxy formation model is hypothesized under which a portion, possibly large, of the Milky Way's globular clusters would have originated in the accretionAccretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
of other dwarf spheroidals
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...
like Sag DEG. The best candidate for this idea is Palomar 12
Palomar 12
Palomar 12 is a globular cluster in the constellation Capricornus that belongs to the halo of the Milky Way galaxy. First discovered on the Palomar Survey Sky plates by Robert G. Harrington and Fritz Zwicky,...
.