I Zwicky 18
| I Zw 18 | |
|---|---|
|
|
| Observation data (J2000 epoch) | |
| Constellation | Ursa Major |
| Right ascension | 09h 34m 02.0s |
| Declination | +55° 14′ 28″ |
| Redshift | 751 ± 2 km/s |
| Distance | 0.059 Gly |
| Apparent magnitude (V) | 16.0 |
| Characteristics | |
| Type | I, Starburst Galaxy |
| Apparent size (V) | 0′.3 × 0′.2 |
| Other designations | |
| UGCA 166, Mrk 116 PGC 027182 | |
I Zwicky 18 is a dwarf irregular galaxy located about 59 million light years away in the constellation Ursa Major. The galaxy was first identified by Swiss astronomer Fritz Zwicky in a 1930s photographic survey of galaxies.
Studies at the Palomar Observatory some 40 years ago led astronomers to believe that the galaxy erupted with star formation billions of years after its galactic neighbors. Galaxies resembling I Zwicky 18's youthful appearance are typically found only in the early universe. Early observations with the Hubble Space Telescope suggested an age of 500 million years old for I Zwicky 18. The Hubble Space Telescope, however, later found faint, older stars contained within the galaxy, suggesting its star formation started at least one billion years ago and possibly as much as ten billion years ago. The galaxy, therefore, may have formed at the same time as most other galaxies.
Spectroscopic observations with ground-based telescopes have shown that I Zwicky 18 is almost exclusively composed of hydrogen and helium, the main ingredients created in the Big Bang. The galaxy's primordial makeup suggests that its rate of star formation has been much lower than that of other galaxies of similar age. The galaxy has been studied with most of NASA's telescopes, including the Spitzer Space Telescope, the Chandra X-ray Observatory, and the Far Ultraviolet Spectroscopic Explorer (FUSE). However, it remains a mystery why I Zwicky 18 formed so few stars in the past, and why it is forming so many new stars right now.
In 2015 a study found a very large region of ionised helium in this small galaxy, which tends to be more frequent in very distant galaxies with low presence of metals. The ionization of helium implies the presence of objects emitting a radiation intense enough to knock electrons off the helium atoms. Scientists therefore conclude that I Zwicky 18 must contain Population III stars, which are almost absent in all other galaxies in the local universe.
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