Leo I is a dwarf spheroidal galaxy in the constellation

Leo Leo or Léo may refer to: Acronyms * Law enforcement officer * Law enforcement organisation * ''Louisville Eccentric Observer'', a free weekly newspaper in Louisville, Kentucky * Michigan Department of Labor and Economic Opportunity Arts an ...

. At about 820,000

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s distant, it is a member 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 ...

of galaxies and is thought to be one of the most distant satellites 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. It was discovered in 1950 by Albert George Wilson on

photographic plate Photographic plates preceded photographic film as a capture medium in photography, and were still used in some communities up until the late 20th century. The light-sensitive emulsion of silver salts was coated on a glass plate, typically thi ...

s of the

National Geographic Society – Palomar Observatory Sky Survey The National Geographic Society – Palomar Observatory Sky Survey (NGS-POSS, or just POSS, also POSS I) was a major astronomical survey, that took almost 2,000 photographic plates of the night sky. It was conducted at Palomar Observatory, Californ ...

, which were taken with the 48-inch Schmidt camera at Palomar Observatory.

Visibility

Leo I is located only 12

arc minutes ARC may refer to: Business * Aircraft Radio Corporation, a major avionics manufacturer from the 1920s to the '50s * Airlines Reporting Corporation, an airline-owned company that provides ticket distribution, reporting, and settlement services * ...

from Regulus, the brightest star in the constellation. For that reason, the galaxy is sometimes called the ''Regulus Dwarf''. Scattered light from the star makes studying the galaxy more difficult, and it was not until the 1990s that it was detected visually. The proximity of Regulus and the low surface brightness make it a real challenge to observe. Medium-sized amateur telescopes (15 cm or more) and a dark sky appear to be required for a sighting. But some reports of April 2013 tell that one observer with an 11 cm mini Dobson and even a refractor as small as 7 cm f/10 has sighted Leo I under very dark sky conditions.

Mass

The measurement of radial velocities of some bright red giants in Leo I have made possible to measure its mass. It was found to be at least (2.0 ± 1.0) × 10⁷ . The results are not conclusive, and do not exclude or confirm the existence of a large

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 ...

halo around the galaxy. However, it seems to be certain that the galaxy does not rotate. A kinematic study of Leo I could not place much constraints on dark matter, but found that there is almost certainly a

black hole A black hole is a region of spacetime where gravity is so strong that nothing, including light or other electromagnetic waves, has enough energy to escape it. The theory of general relativity predicts that a sufficiently compact mass can defo ...

of three million

solar mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass o ...

es in the center of the galaxy. This is significant, as it is the first time this has been done with a dwarf spheroidal galaxy. Leo I's black hole has a mass comparable to the mass of the Milky Way's black hole,

Sagittarius A* Sagittarius A* ( ), abbreviated Sgr A* ( ), is the supermassive black hole at the Galactic Center of the Milky Way. It is located near the border of the constellations Sagittarius and Scorpius, about 5.6° south of the ecliptic, ...

. It has been suggested that Leo I is a tidal debris stream in the outer

halo Halo, halos or haloes usually refer to: * Halo (optical phenomenon) * Halo (religious iconography), a ring of light around the image of a head HALO, halo, halos or haloes may also refer to: Arts and entertainment Video games * Halo (franchise), ...

of the Milky Way. This hypothesis has not been confirmed, however.

Star formation

Typical to a dwarf galaxy, the

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as ...

of Leo I is very low, only one percent that of the Sun. Gallart et al. (1999) deduce from

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most ver ...

observations that the galaxy experienced a major increase (accounting for 70% to 80% of its population) in its star formation rate between 6 Ga and 2 Ga (billion years ago). There is no significant evidence of any stars that are more than 10 Ga old. About 1 Ga ago, star formation in Leo I appears to have dropped suddenly to an almost negligible rate, roughly coinciding with its latest periastron passage of the Milky Way.

Ram pressure Ram pressure is a pressure exerted on a body moving through a fluid medium, caused by relative bulk motion of the fluid rather than random thermal motion. It causes a drag force to be exerted on the body. Ram pressure is given in tensor form as ...

stripping would have removed its gas, decreasing its star formation rate. Some low-level activity may have continued until 200-500 Ma. Therefore, it is thought to be the youngest dwarf spheroidal satellite galaxy of the Milky Way. In addition, the galaxy may be embedded in a cloud of ionized gas with a mass similar to that of the whole galaxy.

References

External links

* *
SEDS page on Leo I
{{Sky, 10, 08, 27.4, +, 12, 18, 27, 820000 Dwarf spheroidal galaxies Local Group Milky Way Subgroup Leo (constellation) 05470 29488 ?