IC 10, a dwarf galaxy in Cassiopeia

IC 10, a dwarf galaxy in Cassiopeia

IC 10 is an irregular dwarf galaxy with an H II nucleus of about 5,000 light-years across, located some 2.2 million light-years away in the northern constellation Cassiopeia and it is approaching the Milky Way at approximately 350 kilometers per second. IC 10 is an outlying member of the Local Group of galaxies and belongs to the M31 subgroup.

Compared to other Local Group galaxies, IC 10 has a large population of newly formed stars that are massive and intrinsically very bright, including a luminous X-ray binary star system that contains a black hole. Although considerably smaller the Small Magellanic Cloud (SMC) IC 10 has a similar luminosity.

IC 10 is the only known starburst galaxy in the Local Group of galaxies, and compared to both of the Magellanic Clouds it has many more Wolf-Rayet stars. The green emissions are from older stars while the red filaments are H-alpha regions with active star formations.

Its higher metallicity compared to the SMC suggests that star formation activity has continued for a longer time period. The evolutionary status of the Wolf-Rayet stars suggests that they all formed in a relatively short timespan. Currently the galaxy produces stars at the rate of 0.04–0.08 solar masses per year, which means that the gas supply in the galaxy can last for only a few billion years longer.

The galaxy has a huge envelope of hydrogen gas, which is far larger than the apparent size of the galaxy in visible light. IC 10 is also unusual in the respect that the visible part of the galaxy seems to rotate in a different direction than the outer envelope.

Astronomers have found the most massive known stellar black hole within IC 10. It is orbited by a companion star, which passes in front of the black hole, periodically blocking the hole’s X-rays. By observing the periodic dimming, scientists were able to determine the orbit of the companion, and the mass of the two bodies.

With a mass of 24-33 times the Sun, the black hole smashes all known records for stellar black holes. These black holes form during the death of a star. Usually, it is expected that a dying star will throw off much of its mass before a black hole forms. How this hole managed to retain so much mass is something of a mystery.

One theory involves a possible scarcity of heavy elements in the star from which the hole was born. Heavy elements get more of a push on their electrons from radiation pressure inside a star. The outgoing light literally carries away some of the mass. If the star did not have much of the heavy elements, the light would not have been able to push out much mass, and such a heavy black hole could form. Because the entire galaxy is lacking in heavy elements, this theory definitely fits the observations.

Despite its closeness, the galaxy is rather difficult to study because it lies near the plane of the Milky Way and is therefore heavily obscured by interstellar matter.

Image Credit: The Survey Team of the Lowell Observatory

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