Australasian Science: Australia's authority on science since 1938

The Monster Galaxy that Grew Up Too Fast

An international team of astronomers has spotted a massive inactive galaxy from a time when the universe was only 1.65 billion years old.

Astronomers expect most galaxies from this epoch to be low-mass minnows, busily forming stars. However, team leader Prof Karl Glazebrook of Swinburne University’s Centre for Astrophysics and Supercomputing says this galaxy is an inactive “monster”.

The researchers found that within a short time period this massive galaxy, known as ZF-COSMOS-20115, formed three times more stars than the Milky Way during an extreme starburst event. However, it stopped forming stars only a billion years after the Big Bang to become a “red and dead” galaxy – common in our universe today, but not expected to exist at this ancient epoch.

The galaxy is also small and extremely dense, with 300 billion stars crammed into a region of space about the same size as the distance from the Sun to the nearby Orion Nebula.

Astrophysicists are still debating just how galaxies stop forming stars. Until recently, models suggested that dead galaxies such as this should only exist from around three billion years after the Big Bang. “This discovery sets a new record for the earliest massive red galaxy,” Glazebrook said. “It is an incredibly rare find that poses a new challenge to galaxy evolution models to accommodate the existence of such galaxies much earlier in the universe.”

The research, published in Nature, took deep spectra at near-infrared wavelengths to seek out the definitive features signifying the presence of old stars and a lack of active star formation. “By collecting enough light to measure this galaxy’s spectrum, we decipher the cosmic narrative of what stars and elements are present in these galaxies and construct a timeline of when they formed their stars,” said co-author Prof Vy Tran of Texas A&M University.

“This huge galaxy formed like a firecracker in less than 100 million years, right at the start of cosmic history,” Glazebrook adds. “It quickly made a monstrous object, then just as suddenly it quenched and turned itself off. As to how it did this, we can only speculate. This fast life and death so early in the universe is not predicted by our modern galaxy-formation theories.”

Co-author Dr Corentin Schreiber of Leiden University speculates that these early firecrackers are obscured behind a veil of dust, and that future observations using sub-millimetre wave telescopes will spot them. “Sub-millimetre waves are emitted by the hot dust which blocks other light, and will tell us when these firecrackers exploded and how big a role they played in developing the primordial universe,” Schreiber said.

With the launch of the James Webb Space Telescope in 2018, astronomers will be able to build up large samples of these dead galaxies due to its high sensitivity, large mirror, and the advantage of no atmosphere in space.