It’s part of being human to want to make sense of the world around us.
These days, expanding our knowledge involves looking further and further out into space, grappling with some of the mind-bogglingly vast questions about the existence of the universe and trying to understand the creation of everything in it.
That’s the purpose of Galaxy Explorer, ABC Science’s citizen science project asking regular people to help scientists classify galaxies to help out astronomers.
The research project behind Galaxy Explorer is the Galaxy and Mass Assembly (GAMA) project –a global research project led by Professor Simon Driver from the International Centre for Radio Astronomy Research (ICRAR) in Western Australia.
“GAMA is blue sky,” says Driver, “we want to understand the evolution of energy, the evolution of mass and the evolution of structure.”
Most of us are more concerned about whether our energy is renewable or not, and how much it costs! But the astronomers involved in Galaxy Explorer want to understand the origin of energy in the universe.
“We want to understand all the processes in the universe that generate energy, and we want to understand how this has evolved,” says Driver.
And that’s just one of the big questions they’re interested in.
Far-off galaxies are the keys to the past
There are over 200,000 images of galaxies between 800 million to 4 billion light-years away to be classified by citizen scientists in Galaxy Explorer.
Comparing distant galaxies will help scientists understand inconsistencies with what’s observed in the universe and what’s predicted by Einstein’s equations, and as a result may change our fundamental understanding of dark matter and dark energy. They will also help astronomers understand how galaxy evolution has changed through time, which will provide insight into how the processes in the universe have evolved.
“We’re drilling a hole right through the universe, collecting samples of galaxies,” says Driver.
“Because it takes so long for light to travel when we’re looking at something further away we’re looking at something as it was in the past.”
It’s very analogous to drilling a core sample in Antarctic, he says, where every layer tells you something about what conditions on Earth were like at different times.
Similarly, astronomers can look at examples of nearby galaxies and far-away galaxies and work out how they might have changed.
Galaxy evolution has changed through the history of the universe
Astronomers believe the process that drives galaxy growth has changed from the time of the early universe.
“So far we think that right after the Big Bang, gravity started to pull galaxies together, and then they went through a period when there was lots of merging, lots of collisions, and violent episodes leading to distorted looking galaxies,” says Driver.
Galaxies undergoing collisions tend to be highly distorted and have an asymmetrical shape.
While mergers may have been the dominant process early on, gas accretion is much more common now, he says. This occurs when a galaxy swallows gas and results in a symmetrical flattened rotating disk of stars, often with spiral arms.
“That’s when we start to see the beautiful spiral arms and those sort or ordered symmetrical structures,” says Driver.
Comparing galaxies of differing age should allow astronomers to confirm this theory of galaxy evolution.
“It will also provide detailed information about when mergers were taking place, when the gas accretion began, and when galaxies first started to develop spiral arms and other features of more ordered systems,” says Driver.
What does the shape and size of the galaxy tell us?
Each galaxy carries a record of how it formed and its evolutionary history is encoded in its shape, colour, and features.
If a galaxy has a central bulge then it’s probably the result of a merger that has had time to re-organise itself into a spherical shape. If it’s a thin disc then it’s grown by slowly swallowing gas.
“A lot of galaxies we see have a central bulge and a thin disk. Which suggests that the galaxy first formed by merging and then later on it formed a disc through gas accretion,” says Driver.
If a galaxy looks messy or irregular then it’s undergoing a significant evolutionary event – either merging with another galaxy or accreting gas very fast. These galaxies aren’t in equilibrium.
Alternatively, if a galaxy is left alone and hasn’t undergone any major mergers for a long time then a bar may start forming. This begins if there’s a region where there are slightly more stars. With time, these stars tend to pull others towards them.
“An over-density of stars rotating round a galaxy’s centre will pull on the ones in front, slowing them down, and accelerate the ones behind,” says Driver.
“And over time, you go from a flat Frisbee-like structure to a galaxy with a bar. But it only happens if a galaxy is left alone. If another galaxy goes by, it gives enough of a kick that disrupts that bar process.”
When it comes to spiral arms, the process that forms them is not fully understood.
“They’re believed to be a shock-wave or density-wave that permeates out from the centre of the galaxy,” says Driver.
Many galaxies have all three features – bulges, bars and spiral arms – telling a complex story of evolution.
Why we need citizen scientists to help
Some of the galaxies that will be classified by citizen scientists in Galaxy Explorer.
With the help of citizen scientists, the astronomers will be able to very quickly build up statistics as to how many galaxies have bars, how many have spiral arms, how many have bulges, how many are in a state of merging, and how many look very smooth.
“These statistics can be used to build a model of how the entire galaxy population in the universe has evolved,” says Driver.
“It would take an enormous amount of time for us to go through every galaxy one by one – we’re just a team of five or six here.”
The astronomers understand that the process of classifying galaxies will be challenging for some people. It’s a process they themselves often have trouble with.
“That’s why we have to make sure that every galaxy is looked at by five people,” says Driver.
“We should then be able to look at the average of those five. If they all agree it’s a dead certainty. If all five disagree then it’s probably one of those ones that’s really hard to classify and we need to have a look at it ourselves. And if four agree and one doesn’t then we can probably weed that one person out, and go with the majority.”
Answering the big questions
Besides the evolution of galaxies and the origin of energy, Driver and his team are also interested in other big questions like how mass built up in the universe and the processes that created gravity.
“So maybe if we carry on studying the distribution of galaxies, carrying on studying the motions of galaxies we’ll start to get some insight.
“We’re just trying to understand this strange force called gravity in all its glory, and then one day we may find a way to harness it, just like we harnessed electromagnetism and use it to our benefit.”
Written by Kylie Andrews, ABC Science.