Ultra compact galaxies weigh heavy on early Universe
BY DR EMILY BALDWIN
ASTRONOMY NOW
Posted: April 29, 2008

Astronomers looking at the Universe’s distant past have discovered nine young, ultra compact galaxies, each weighing in at 200 billion times the mass of the Sun but measuring only 5,000 light years across and containing the same number of stars as today’s adult galaxies.

Illustration showing the comparative sizes of our Milky Way galaxy and an ultra compact galaxy, which existed in the early Universe. The compact galaxy contains as many stars as the Milky Way and could fit inside the central hub of our galaxy. Image: NASA, ESA, A. Feild (STScI), P. van Dokkum (Yale University).

The astronomers peered back in time to a 3 billion year old Universe using NASA’s Hubble Space Telescope and the W.M. Keck Observatory in Hawaii, and say that no massive galaxy at this distance has ever been observed to be so compact. The ultra dense galaxies may even comprise half of all galaxies of that mass 11 billion years ago, forming the building blocks of today’s largest galaxies. “It is not yet clear how they would build themselves up to become the large galaxies we see today,” says Pieter G. van Dokkum of Yale University, who lead the study. “They would have to change a lot over 11 billion years, growing five times bigger. They could get larger by colliding with other galaxies, but such collisions may not be the complete answer."

Small star-forming galaxies were also revealed in the Hubble Deep Field, but these galaxies were also very low in mass. “Our study surveyed a much larger area than the Hubble Deep Field,” says Marijn Franx of Leiden University, “and surprisingly shows that galaxies with the same weight as our Milky Way were also very small in the past. All galaxies look really different in early times, even massive ones that formed their stars early.”

Images taken by Hubble of nine ultra compact galaxies as they appeared 11 billion years ago. Image: NASA, ESA, P. van Dokkum (Yale University), M. Franx (Leiden University), G. Illingworth (University of California, Lick Observatory).

Speculation surrounding the formation mechanism of these compact galaxies still remains, but according to the recent research, one way involves the interaction of dark matter and hydrogen gas in the nascent Universe. It is theorised that shortly after the Big Bang, hydrogen gas became trapped in whirlpools of dark matter and began spinning wildly, sparking an intense period of star formation.

The team also used the Gemini South Telescope to measure the galaxies’ distances and ages, and found that the stars are just half a billion to a billion years old and that the most massive stars had already exploded as supernovae. The team also revealed the stars to be spinning around their galactic discs at around 500 kilometres per second, almost double the speed of today’s stars.

Further observations will be crucial in attempting to understand the evolution of galaxies in the early life of the Universe, and the nine compact galaxies at the centre of this research will provide the perfect target for the Wide Field Camera 3, which will be installed on Hubble in the next servicing mission scheduled for later this year.

Illustration from the surface of a hypothetical planet in an ultra dense galaxy. There would be 200 times more stars in the sky than in the Earth's night sky. Image: NASA, ESA, G. Bacon (STScI), P. van Dokkum (Yale University).