The Draco Dwarf is a
spheroidal galaxy which was discovered by
Albert George Wilson of
Lowell Observatory
Lowell Observatory is an astronomical observatory in Flagstaff, Arizona, United States. Lowell Observatory was established in 1894, placing it among the oldest observatories in the United States, and was designated a National Historic Landmark ...
in 1954 on photographic plates of the
National Geographic Society's Palomar Observatory Sky Survey
Palomar may refer to:
Places
* Any of several locations in San Diego County, California:
** Palomar Mountain
** Palomar Observatory, located on Palomar Mountain
** Palomar College in San Marcos, California
** Palomar Medical Center in Escondido, ...
(POSS).
It is part of the
Local Group
The Local Group is the galaxy group that includes the Milky Way.
It has a total diameter of roughly , and a total mass of the order of .
It consists of two collections of galaxies in a "dumbbell" shape: the Milky Way and its satellites form ...
and a
satellite galaxy
A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy (also known as the primary galaxy). Satellite galaxies and their constituents are bound t ...
of the
Milky Way
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked ey ...
galaxy. The Draco Dwarf is situated in the direction of the
Draco Constellation at 34.6°
above the
galactic plane
The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms ''galactic plane'' and ''galactic poles'' usual ...
.
Characteristics
Paul W. Hodge 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 is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
.
Having an absolute magnitude of -8.6 and a total luminosity of only , it is one of the faintest companions to our Milky Way.
Draco Dwarf contains many
red giant branch
The red-giant branch (RGB), sometimes called the first giant branch, is the portion of the giant branch before helium ignition occurs in the course of stellar evolution. It is a stage that follows the main sequence for low- to intermediate-mass st ...
(RGB) stars; five
carbon star
A carbon star (C-type star) is typically an asymptotic giant branch star, a luminous 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 mo ...
s have been identified in Draco Dwarf and four likely
asymptotic giant branch
The asymptotic giant branch (AGB) is a region of the Hertzsprung–Russell diagram populated by evolved cool luminous stars. This is a period of stellar evolution undertaken by all low- to intermediate-mass stars (about 0.5 to 8 solar masses) lat ...
(AGB) stars have been detected.
The Draco Dwarf is estimated to be 80 ± 10 kpc
from
Earth
Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surf ...
and span a distance of 830 ± 100 × 570 ± 70 pc.
RR Lyrae
In 1961,
Walter Baade
Wilhelm Heinrich Walter Baade (March 24, 1893 – June 25, 1960) was a German astronomer who worked in the United States from 1931 to 1959.
Biography
The son of a teacher, Baade finished school in 1912. He then studied maths, physics and astro ...
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 variables
RR Lyrae variables are periodic variable stars, commonly found in globular clusters. They are used as standard candles to measure (extra) galactic distances, assisting with the cosmic distance ladder. This class is named after the prototype and ...
.
From this work a RR Lyrae derived
distance modulus of 19.55 is found which implies a distance of 81 kpc.
Metallicity
The Draco Dwarf contains primarily an
old population of stars and insignificant amounts of
interstellar matter
In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstellar ...
(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
In statistics, a normal distribution or Gaussian distribution is a type of continuous probability distribution for a real-valued random variable. The general form of its probability density function is
:
f(x) = \frac e^
The parameter \mu i ...
with average metallicity of
e/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
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proce ...
stars than blue horizontal branch stars.
Dark matter
Recently, dwarf spheroidal galaxies have become key objects for the study of
dark matter
Dark matter is a hypothetical form of matter thought to account for approximately 85% of the matter in the universe. Dark matter is called "dark" because it does not appear to interact with the electromagnetic field, which means it does not ab ...
. The Draco Dwarf is one which has received specific attention.
Radial velocity
The radial velocity or line-of-sight velocity, also known as radial speed or range rate, of a target with respect to an observer is the rate of change of the distance or range between the two points. It is equivalent to the vector projection ...
computations of Draco have revealed a large internal velocity dispersion giving a mass to luminosity ratio of up to /, 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
The horizontal branch (HB) is a stage of stellar evolution that immediately follows the red-giant branch in stars whose masses are similar to the Sun's. Horizontal-branch stars are powered by helium fusion in the core (via the triple-alpha proce ...
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
In thermodynamics, an isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and ...
.
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.
Notes
- Assuming an absolute magnitude of +0.5 V for RR Lyrae the apparent modulus of the Draco Dwarf is 19.58 m - M.
Using a reddening value towards Draco Dwarf of 0.03 ± 0.01 we get a true distance modulus of 19.55.
- Using the distance modulus formula of 1 we get an RR Lyrae estimated distance of 81 kpc.
- Apparent Magnitude of 10.9
- distance modulus of 19.52 (80 kpc) = −8.6
- distance 80 ± 10 kpc × tan( diameter_angle = 35′.5 × 24′.5
) = 830 ± 100 × 570 ± 70 pc diameter
References
External links
*
{{Sky, 17, 20, 12.4, +, 57, 54, 55, 260000
Dwarf spheroidal galaxies
Peculiar galaxies
Local Group
Milky Way Subgroup
Draco (constellation)
10822
60095
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