Astronomers have discovered large galaxies that have not changed much in size for the last 9 billion years -- a finding that conflicts with widely-held views of when and how galaxies form.
The Conventional Model
The most popular model for galaxy formation is a bottom-up or hierarchical model in which small galaxies gradually develop into larger ones over a long period of time. The model predicts that smaller galaxies would merge to form larger ones, growing to 90% of their size about 11 billion years after the Big Bang. This means that smaller galaxies would be older and larger ones, younger.
A team of astrophysicists, led by Professor Chris Collins from Liverpool John Moores University (LJMU), used observations from the Subaru Telescope (one of the world’s largest optical-infrared telescopes) as well as the XMM-Newton (an x-ray observatory in space) to collect data about galaxies that existed two-thirds of the way back in time since the Universe began. By being able to extend the baseline of their data so far back in time, they have a foundation for concluding that large galaxies were almost fully formed 4-5 billion years after the Big Bang, 6-7 billion years earlier than the hierarchical model would forecast. Dr. John Stott, who carried out the photometry and data analysis, stressed the significance of the research: “We were surprised to find that the largest and brightest galaxies in the universe have remained essentially unchanged for the last 9 billion years, having grown rapidly soon after the Big Bang.”
Candles for Studying the Universe
The galaxies examined in the observations were brightest cluster galaxies (BCGs), located at the centers of galaxy clusters. Because they are so bright and uniform, they are often used as beacons for studying the universe.
At the beginning of their research, the scientists examined the ages of the stars in the galaxies. Their findings indicated a consistent epoch of formation for the majority of the stars -- about 2-3 billion years after the Big Bang. Since the galaxies they studied were almost fully formed 4-5 billion years after the Big Bang, they concluded that they probably grew rapidly and formed much sooner than the hierarchical model of galaxy evolution suggests.
Questions May Enrich or Change Models for Galaxy Formation
These findings add fuel to the longstanding debate about when galaxies form. Although there is evidence that some merging of galaxies is occurring, as the hierarchical model predicts, it does not seem to be the explanation for the earlier birth of large galaxies that this research discovered.
The breakthrough findings of the LJMU astronomers’ research may alter the way that simulation models are constructed in the future. Simulation models provide a useful framework to compare theory with data. In light of these findings, future simulation models may need to take into account the rapid growth of some galaxies. Professor Chris Collins commented, “Current predictions using simulations run on supercomputers suggest that at such a young age these galaxies should be only 20% of their final weight, so to find galaxies so large
suggests that galaxy formation is a much more rapid process than we previously thought and perhaps the theories are missing some important physics.”
The surprising findings of this research also give scientists new questions to think about: How do these large, Bright Cluster Galaxies form sooner than expected after the Big Bang? And, how have the BCGs remained almost unchanged in size and weight for 9 billion years?