NGC 4555
Encyclopedia
NGC 4555 is a solitary elliptical galaxy
about 40,000 parsec
s (125,000 light-year
s) across. Observations by the Chandra X-ray Observatory
have shown it to be surrounded by a halo
of hot gas about 120,000 parsecs across. The hot gas has a temperature of around 10,000,000 kelvin
s. The galaxy is one of the few elliptical galaxies proven to have significant amounts of dark matter
. Large amounts of dark matter are necessary to prevent the gas from escaping the galaxy; the visible mass clearly is not large enough to hold such an extensive gas halo. The dark matter halo is estimated to have 10 times the mass of the stars in the galaxy.
NGC 4555 is important because of its isolation. Most elliptical galaxies are found in the cores of groups and clusters of galaxies, and almost all those for which dark matter estimates are available are located in the centres of these larger systems. In these circumstances it impossible to know whether the dark matter is associated with the galaxy or the surrounding cluster. NGC 4555, as a field galaxy
is not part of any group or cluster, and therefore provides strong evidence that dark matter can be associated with individual ellipticals.
Elliptical galaxy
An elliptical galaxy is a galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. They range in shape from nearly spherical to highly flat and in size from hundreds of millions to over one trillion stars...
about 40,000 parsec
Parsec
The parsec is a unit of length used in astronomy. It is about 3.26 light-years, or just under 31 trillion kilometres ....
s (125,000 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s) across. Observations by the Chandra X-ray Observatory
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
have shown it to be surrounded by a 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...
of hot gas about 120,000 parsecs across. The hot gas has a temperature of around 10,000,000 kelvin
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
s. The galaxy is one of the few elliptical galaxies proven to have significant amounts of 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...
. Large amounts of dark matter are necessary to prevent the gas from escaping the galaxy; the visible mass clearly is not large enough to hold such an extensive gas halo. The dark matter halo is estimated to have 10 times the mass of the stars in the galaxy.
NGC 4555 is important because of its isolation. Most elliptical galaxies are found in the cores of groups and clusters of galaxies, and almost all those for which dark matter estimates are available are located in the centres of these larger systems. In these circumstances it impossible to know whether the dark matter is associated with the galaxy or the surrounding cluster. NGC 4555, as a field galaxy
Field galaxy
A field galaxy is a galaxy that does not belong to a larger cluster of galaxies, but is gravitationally alone. The vast majority of galaxies exist outside of clusters.Most low surface brightness galaxies are field galaxies....
is not part of any group or cluster, and therefore provides strong evidence that dark matter can be associated with individual ellipticals.