Galaxies are star factories. But for some, such as massive elliptical galaxies, their star-forming days are now over. All of their available gas has already been turned into more than a hundred billion stars.
Collectively, these galaxies contain about half of all the stars that have ever existed in the universe.
In a new study, published in Science, astronomers have tracked down how the fire of star formation burnt out within these galaxies.
It appears that most of the time, the star formation is first quenched within the heart of the galaxy, and it’s not until a few billion years later that the outer regions run out of gas and stop producing stars.
This discovery that giant galaxies die from the inside-out is useful for astronomers trying to understand the mechanisms that promote and hinder star formation. And, ultimately, it’s about unravelling the history of the universe and understanding why we see the kinds of galaxies that exist today.
Our universe, the time machine
It’s one of the mind-boggling things about astronomy that every time we look into space, we are looking back in time. The universe is so vast, that even though light hurtles across space at 300,000 km/s, its journey can last billions of years.
The light emitted from the 22 galaxies investigated in this study has taken more than 10 billion years to reach us. We are seeing these galaxies from a time when the universe was around three billion years old.
It was the heyday of the universe, when galaxies were vigorously forming stars at a rate about 20 times faster than occurs today. Each year, massive galaxies were producing the equivalent of hundreds of sun-like stars. In comparison, our Milky Way Galaxy creates just four sun-like stars a year.
And when the universe was young, it was the most massive galaxies that were undergoing the fastest growth, as confirmed by the Galaxy and Mass Assembly (GAMA) project, a major study being undertaken in Australia with the Anglo-Australian Telescope.
So how does the growth slow down?
Steady as she goes
To answer this, PhD student Sandro Tacchella (ETH Zurich, Switzerland) and colleagues used observations from both the Hubble Space Telescope (HST) and the European Southern Observatory’s Very Large Telescope (VLT). Images from the HST, were used to trace the distribution of older stars within the galaxies – the locations where star formation had previously occurred.
Using an instrument known as SINFONI on the VLT, the astronomers looked for regions where star formation was actively occurring. This is done by searching for hydrogen gas that has been excited or ionised by the radiation emitted from young, hot stars.
To be useful, both sets of observations needed to track changes across small spatial scales. The HST can do this because it’s in space and doesn’t have to deal with the blurring effects of the Earth’s atmosphere.
It’s harder for the VLT to resolve small parts of the distant galaxies because the telescope sits on Earth’s surface, in Chile, and must cancel out Earth’s atmospheric effects. The VLT does this by monitoring an artificial star (produced by a laser), then keeps the starlight steady by deforming its mirrors, making the results even more impressive.
Red and dead
Together, the observations showed that among the most massive galaxies of the sample, stars built-up quickly in the central regions of the galaxy but then stopped. While slow and steady star formation lingered in the galaxy’s outskirts.
A comparison was made with galaxies of similar mass that are much closer and therefore have been around for a far longer time since the Big Bang.
These are the “red and dead” galaxies, so called because they contain only red stars (which are cooler and more evolved, than young, hot blue stars) and the star formation has all dried up. A galaxy that is no longer forming stars is no longer growing, and therefore “dead”.
The central bulges of the massive galaxies – both the young (or distant) and the old (or nearby) – are very similar. These nearby galaxies are modern-day equivalents; they reveal to us that their central bulges were already in their current form 10 billion years ago.
What can kill a galaxy?
The key factor in understanding the shut-off of star formation in a galaxy is to know where its gas lives. Gas is the fuel from which new stars form, like the logs added to a campfire to keep it burning. Not only must the logs be present in order to burn, they have to be thrown on to the fire in order to ignite.
There are a number of mechanisms that could potentially strip gas from a galaxy or prevent the gas from getting onto the galaxy’s disk where is where it needs to be to create stars. Some of these processes are internal to the galaxy itself, while others depend on how crowded the neighbourhood is where the galaxy lives.
Many galaxies, especially ellipticals, are found in dense cities of galaxies called clusters. It has been suggested that within these dense environments gas can be stripped away from a galaxy via processes such as ram pressure stripping and galaxy strangulation.
But such processes remove gas from the outer regions of a galaxy and are at odds with Tacchella’s discovery, that star-forming gas survives the longest at a galaxy’s periphery.
A GAMA study of nearby galaxies with slightly less mass than our Milky Way Galaxy also showed no evidence that star formation is quenched by a galaxy’s local environment. The galaxies in that study continue to form stars at a rate of a few sun-like stars per year and could indicate that an environmental shut down of star formation is a very rapid process.
It’s not possible to rule out external factors completely – mechanisms that could stop the flow of gas into a galaxy and starve it of new fuel for stars. But with the kind of inside-out quenching that is observed, Tacchella and colleagues suggest that the process is likely internal to the galaxy.
Bring in the black hole
What’s going on in a galaxy’s core? A big black hole, that’s what. Astronomer’s call them supermassive, because they typically harbour as much mass as a few million or even a billion suns.
Black holes are known for their ability to draw material in, a result of their strong gravitational pull. But they also stir up powerful winds and jets that can push gas out of a galaxy.
In addition, it is only within the most dense galaxy cores that star-formation is being quenched, which may suggest that once a critical density is reached, the galaxy stabilises, no longer lets fresh gas into its centre and self-quenches star formation.
It’s an amazing thing to be able to observe a set of galaxies that are 10 billion light years away and dissect them to figure out what regions are still active, compared to other areas where galaxy growth has ended. This result provides new information on the lasting question of why and how massive galaxies stop forming new stars.