What's killing galaxies? Large survey reveals how star formation is shut down in extreme regions of the Universe


- Victoria, British Columbia

A Canadian-led team of 36 astronomers has just completed the largest imaging survey of molecular gas—the fuel needed for star formation—in one of the most extreme regions of the Universe, the Virgo Cluster.

What's killing galaxies in extreme regions of the nearby Universe? - Transcript

[On screen: Toby Brown, Plaskett Fellow at the Herzberg Astronomy and Astrophysics Research Centre]

[00:00:05] "How do galaxies evolve" is one of the grandest questions in all of science. Galaxies are defined as vast collections of stars that are born from huge collapsing clouds of cold molecular gas. The physical processes that govern the evolution and the life cycle of these beautiful systems, is closely related to the region of space in which the galaxy resides—known as the galaxy's environment.

[00:00:29] We want to understand the physical connections between galaxies and their environment. Our team has used the world's most advanced ground-based telescope, ALMA (Atacama Large Millimetre/submillimetre Array), in order to complete the largest-ever imaging survey of cold molecular gas of galaxies in one of the nearby universe's most extreme environments—a massive cluster of galaxies known as the Virgo cluster.

[00:00:54] Virgo contains thousands of galaxies whizzing by one another at millions of kilometers per hour through superheated million degree plasma. We already know that when galaxies fall into a galaxy cluster, they can be robbed of their gas. When gas is removed, this shuts down star formation, effectively killing a galaxy and turning into a so-called red and dead object.

[00:01:19] What VERTICO (Virgo Environment Traced in Carbon Monoxide Survey) reveals better than ever before is the physical processes that affect the molecular gas and therefore the star formation cycle of galaxies in the Virgo cluster. VERTICO is surveying the gas in 51 Virgo cluster galaxies, providing some of the most detailed images of gas disks in cluster galaxies ever observed. What these new images are incredibly useful for is revealing how star formation in galaxies, arguably one of the universe's most important physical processes, is affected by the environment in which the galaxy resides.

[00:01:56] What we see with VERTICO is the fingerprints of environment all over the gas disks of these galaxies, so, when the environment is capable of reaching far into galaxies to perturb the cycle of star formation.

[On screen: Official signature, National Research Council of Canada / Conseil national de recherches Canada]

[On screen: Government of Canada wordmark]

Galaxies are being killed in one of the most extreme regions of the nearby Universe and astronomers want to know why. A recently published paper, which will be featured in the December edition of the Astrophysical Journal Supplement Series, provides the clearest evidence yet, that the environments surrounding galaxies can reach far within the galaxies and have a lethal impact on the fuel needed to birth new stars: their molecular gas.

The Canadian-led paper from a collaboration of 36 international astronomers presents state-of-the-art observations of molecular gas in 51 galaxies belonging to the Virgo Cluster. The paper is the first to be released from the Virgo Environment Traced in Carbon Monoxide (VERTICO) Survey, undertaken using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile.

"We know that galaxies are being robbed of their gas. If enough gas is destroyed or removed, star formation is shut down, effectively killing the galaxy and turning it into a dead object. What VERTICO reveals better than ever before is which physical processes affect the molecular gas and how they dictate the life and death of the galaxy."

Dr. Toby Brown, Plaskett Fellow at the National Research Council of Canada and lead author on the paper

A galaxy's ability to form stars is influenced by where the galaxy lives in the Universe and how it interacts with its surroundings. Of the many different environments in the Universe, galaxy clusters are among the most massive, hottest, and most extreme, making them the perfect cosmic laboratory for observations such as VERTICO. The nearby Virgo Cluster is 7 million light-years across and contains thousands of galaxies hurtling through superheated plasma at speeds of up to several million kilometres per hour. It is an environment so extreme and inhospitable that entire galaxies can be stopped from forming stars in a process known as galaxy quenching. Not to worry though, our Milky Way galaxy is nowhere near the Virgo Cluster or any cluster and thus not in danger.

"With VERTICO, we looked at 51 galaxy gas reservoirs in the Virgo Cluster; which are the direct fuel supply for new stars, and provided many of the most detailed images of gas disks in cluster galaxies ever observed. These new images are incredibly useful for revealing how star formation in galaxies—arguably the Universe's most important physical process—is shut down by external influence," explains Dr. Brown.


The VERTICO—Virgo Environment Traced in Carbon Monoxide—Survey observed the gas reservoirs in 51 galaxies in the nearby Virgo Cluster and found that the extreme environment in the cluster was killing galaxies by robbing them of their star-forming fuel. In this composite image, ALMA's radio wavelength observations of the VERTICO galaxies' molecular gas disks are magnified by a factor of 20. They are overlaid on the X-ray image of the hot plasma within the Virgo Cluster.
Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO)/Böhringer et al. (ROSAT All-Sky Survey)

In the new results, the majority of VERTICO galaxies show evidence that the cluster environment left an imprint on their gas disks, providing the clearest evidence yet that external environmental mechanisms reach far into galaxies to disturb and perturb their molecular gas, and impact their star formation.

"Gas stripping is one of the most spectacular and violent external mechanisms that can shut down star formation in galaxies," Dr. Brown explains. "Gas stripping occurs when galaxies are moving so fast through hot plasma in the cluster, that vast quantities of cold molecular gas are stripped from the galaxy—as though the gas is being swept away by a huge cosmic broom. The exquisite quality of VERTICO's observations allows us to better see and understand such mechanisms."


NGC 4567 and NGC 4568 are 2 of the 2,000 galaxies in the Virgo Cluster, located roughly 65 million light-years from Earth. Observed by the VERTICO—Virgo Environment Traced in Carbon Monoxide—Survey, the 2 galaxies are among those in the galaxy cluster impacted by extreme physical processes that can lead to the death of galaxies. The galaxies are shown here in composite radio data from ALMA with molecular gas in red/orange and optical data from the Hubble Space Telescope with stars in white/blue.
Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO)

VERTICO is the first-ever Canadian-led ALMA Large Program, a specific subset of ALMA observations designed to address strategic scientific issues that will lead to a major advance or breakthrough. While Virgo has been studied at almost every wavelength of cosmic light, this is the first large survey of molecular gas with high sensitivity and resolution in a galaxy cluster.

"The first study of molecular gas in the Virgo cluster was published over 30 years ago, and astronomers have been debating about the influence of the cluster environment on this star-forming gas ever since. I'm confident that the VERTICO data will allow us to answer this long-standing question, as well as to understand exactly how these various environmental effects cause cluster galaxies to shut down their star formation production line."

Dr. Christine Wilson, Distinguished University Professor at McMaster University and co-principal investigator on the VERTICO project

VERTICO observations were taken between July 2019 and April 2021. Beyond this first paper, this unprecedented detail of galaxies in the highly active Virgo Cluster will provide astronomers with the data needed to study and better understand how star formation and galaxy evolution proceed in the most extreme environments in the Universe, in an effort to better understand our own.

Learn more about the NRC's Herzberg Astronomy and Astrophysics Research Centre.

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