Draco Dwarf
Encyclopedia
The Draco Dwarf is a spheroidal galaxy
which was discovered by Albert George Wilson
of Lowell Observatory
in 1954 on photographic plates of the National Geographic Society's
Palomar Observatory Sky Survey (POSS). It is part of the local group
and a satellite galaxy
of the Milky Way galaxy. The Draco Dwarf is situated in the direction of the Draco
Constellation
at 34.6° above the galactic plane
.
analyzed the distribution of its stars in 1964 and concluded that its ellipticity was 0.29 ± 0.04.
Recent studies have indicated that the galaxy may potentially hold large amounts of dark matter
. Having an absolute magnitude of -8.6 and a total luminosity of only 2 L☉, it is one of the faintest companions to our Milky Way.
Draco Dwarf contains many red giant branch (RGB) stars; five carbon star
s have been identified in Draco Dwarf and four likely asymptotic giant branch
(AGB) stars have been detected.
The Draco Dwarf is estimated to be about 80 ± 10 kpc from earth and span a distance of 830 ± 100 × 570 ± 70 pc.
and Henrietta H. Swope studied Draco Dwarf and discovered over 260 variables
, of the 138 in the cluster's center, all but five were determined to be RR Lyrae
. From this work a RR Lyrae derived distance modulus
of 19.55 is found which implies a distance of 81 kpc.
of stars and insignificant amounts of interstellar matter (being basically dust free). From 75% to 90% of its stars formed more than ~10 Gyr ago followed by a low rate of formation with a small burst of star formation around 2-3 Gyr ago. It has a single Gaussian distribution with average metallicity of [Fe/H] = −1.74 dex with a standard deviation
(sigma/σ) of 0.24 dex and a small tail of metal-rich stars. The central region of Draco Dwarf exhibits a concentration of more metal-rich stars there being more centrally concentrated red horizontal branch
stars than blue horizontal branch stars.
. The Draco Dwarf is one which has received specific attention. Radial velocity
computations of Draco have revealed a large internal velocity dispersion giving a mass to luminosity ratio of up to 440 M☉
/L☉, suggesting large amounts of dark matter. It has been hypothesized that large velocity dispersions could be explained as tidal dwarfs (virtually unbound stellar streams from dwarf galaxies tidally disrupted in the Milky Way potential). However, Draco Dwarf's narrow horizontal branch
width does not support this model. This only leaves the dark matter explanation and makes Draco Dwarf the most dark matter dominated object known as of 2007. The dark matter distribution within Draco Dwarf is at least nearly isothermal.
At large radii, radial velocity dispersion exhibit strange behavior. One possible explanation for this would be the presence of more than one stellar population. This suggests the need for further study of Draco Dwarf population's metallicity and ages and of dwarf spheroidals in general.
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...
which was discovered by Albert George Wilson
Albert George Wilson
Albert George Wilson is an American astronomer.He was born in Houston, Texas. He received his Ph.D. in mathematics from Caltech in 1947; his thesis title was Axially Symmetric Thermal Stresses in a Semi-Infinite Solid....
of Lowell Observatory
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark in 1965....
in 1954 on photographic plates of the National Geographic Society's
National Geographic Society
The National Geographic Society , headquartered in Washington, D.C. in the United States, is one of the largest non-profit scientific and educational institutions in the world. Its interests include geography, archaeology and natural science, the promotion of environmental and historical...
Palomar Observatory Sky Survey (POSS). It is part of the local group
Local Group
The Local Group is the group of galaxies that includes Earth's galaxy, the Milky Way. The group comprises more than 30 galaxies , with its gravitational center located somewhere between the Milky Way and the Andromeda Galaxy...
and a satellite galaxy
Satellite galaxy
A satellite galaxy orbits a larger galaxy due to gravitational attraction. Although a galaxy is made of a large number of objects which are not connected to each other, it has a center of mass, which represents a weighted average of the positions of each component object...
of the Milky Way galaxy. The Draco Dwarf is situated in the direction of the Draco
Draco (constellation)
Draco is a constellation in the far northern sky. Its name is Latin for dragon. Draco is circumpolar for many observers in the northern hemisphere...
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....
at 34.6° above the galactic plane
Galactic plane
The galactic plane is the plane in which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles...
.
Characteristics
Paul W. HodgePaul W. Hodge
Paul W. Hodge is an American astronomer.He was born in Washington. He earned an undergraduate degree in Physics from Yale and a doctorate in Astronomy from Harvard. In 1965 he and George Wallerstein created the astronomy department at the University of Washington, and he has been a professor...
analyzed the distribution of its stars in 1964 and concluded that its ellipticity was 0.29 ± 0.04.
Recent studies have indicated that the galaxy may potentially hold large 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...
. Having an absolute magnitude of -8.6 and a total luminosity of only 2 L☉, it is one of the faintest companions to our Milky Way.
Draco Dwarf contains many red giant branch (RGB) stars; five carbon star
Carbon star
A carbon star is a late-type star similar to a red giant whose atmosphere contains more carbon than oxygen; the two elements combine in the upper layers of the star, forming carbon monoxide, which consumes all the oxygen in the atmosphere, leaving carbon atoms free to form other carbon compounds,...
s have been identified in Draco Dwarf and four likely asymptotic giant branch
Asymptotic Giant Branch
The asymptotic giant branch is the region of the Hertzsprung-Russell diagram populated by evolving low to medium-mass stars. This is a period of stellar evolution undertaken by all low to intermediate mass stars late in their lives....
(AGB) stars have been detected.
The Draco Dwarf is estimated to be about 80 ± 10 kpc from earth and span a distance of 830 ± 100 × 570 ± 70 pc.
RR Lyrae
In 1961, Walter BaadeWalter Baade
Wilhelm Heinrich Walter Baade was a German astronomer who worked in the USA from 1931 to 1959.-Biography:He took advantage of wartime blackout conditions during World War II, which reduced light pollution at Mount Wilson Observatory, to resolve stars in the center of the Andromeda galaxy for the...
and Henrietta H. Swope studied Draco Dwarf and discovered over 260 variables
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, of the 138 in the cluster's center, all but five were determined to be RR Lyrae
RR Lyrae
RR Lyrae is a variable star in the Lyra constellation. It is the prototype of the RR Lyrae variable class of stars. It has a period of about 13 hours, and oscillates between apparent magnitudes 7 and 8. Its variable nature was discovered by the Scottish astronomer Williamina Fleming at Harvard...
. From this work a RR Lyrae derived distance modulus
Distance modulus
-Definition:The distance modulus \mu=m-M is the difference between the apparent magnitude m and the absolute magnitude M of an astronomical object...
of 19.55 is found which implies a distance of 81 kpc.
Metallicity
The Draco Dwarf contains primarily an old populationMetallicity
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...
of stars and insignificant amounts of interstellar matter (being basically dust free). From 75% to 90% of its stars formed more than ~10 Gyr ago followed by a low rate of formation with a small burst of star formation around 2-3 Gyr ago. It has a single Gaussian distribution with average metallicity of [Fe/H] = −1.74 dex with a standard deviation
Standard deviation
Standard deviation is a widely used measure of variability or diversity used in statistics and probability theory. It shows how much variation or "dispersion" there is from the average...
(sigma/σ) of 0.24 dex and a small tail of metal-rich stars. The central region of Draco Dwarf exhibits a concentration of more metal-rich stars there being more centrally concentrated red horizontal branch
Horizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
stars than blue horizontal branch stars.
Dark matter
Recently, dwarf spheroidal galaxies have become key objects for the study of 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...
. The Draco Dwarf is one which has received specific attention. Radial velocity
Radial velocity
Radial velocity is the velocity of an object in the direction of the line of sight . In astronomy, radial velocity most commonly refers to the spectroscopic radial velocity...
computations of Draco have revealed a large internal velocity dispersion giving a mass to luminosity ratio of up to 440 M☉
Solar mass
The solar mass , , is a standard unit of mass in astronomy, used to indicate the masses of other stars and galaxies...
/L☉, suggesting large amounts of dark matter. It has been hypothesized that large velocity dispersions could be explained as tidal dwarfs (virtually unbound stellar streams from dwarf galaxies tidally disrupted in the Milky Way potential). However, Draco Dwarf's narrow horizontal branch
Horizontal branch
The horizontal branch is a stage of stellar evolution which immediately follows the red giant branch in stars whose masses are similar to the Sun's...
width does not support this model. This only leaves the dark matter explanation and makes Draco Dwarf the most dark matter dominated object known as of 2007. The dark matter distribution within Draco Dwarf is at least nearly isothermal.
At large radii, radial velocity dispersion exhibit strange behavior. One possible explanation for this would be the presence of more than one stellar population. This suggests the need for further study of Draco Dwarf population's metallicity and ages and of dwarf spheroidals in general.