5 mins read 12 Nov 2021

Galactic murder mystery solved

An international team of astronomers, including researchers from ICRAR-UWA, has found that galaxies are being killed by extreme environments reaching into galaxies, robbing them of their vital molecular gas.

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).

Astronomers are turning their eyes to the Virgo Cluster to try to solve a long-standing murder mystery - what's killing the galaxies? 

An international collaboration of 36 astronomers, including researchers from the University of Western Australia (UWA) node of the International Centre for Radio Astronomy Research (ICRAR), has released a paper providing evidence that extreme environments surrounding galaxies can reach far within them and steal the molecular gas needed to fuel the birth of new stars and keep the galaxy alive. 

This is the first paper released by VERTICO (the 'Virgo Environment Traced in Carbon Monoxide Survey) which was undertaken using the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile. This is the first 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. 

The paper co-author Dr Claudia Lagos is the ASTRO 3D Senior Research Fellow at ICRAR-UWA.

"We know that galaxies in the most extreme environments suffer tremendously, losing their gas reservoirs and eventually being unable to form any more stars. For a galaxy, that's the equivalent of dying. 

"VERTICO offers us an unprecedented window on how the molecular gas, the direct fuel for new stars, is affected, allowing us to effectively diagnose what's killing those galaxies," said Dr Lagos.

The survey is mapping the molecular hydrogen gas that fuels the birth of new stars across 51 galaxies in the Virgo Cluster. While Virgo has been studied at almost every wavelength of cosmic light, this is the first extensive survey of molecular gas with high sensitivity and resolution in a galaxy cluster.

Galaxy clusters are one of the most extreme environments in the universe, and the Virgo Cluster is no exception. Seven million light-years across and containing thousands of galaxies hurtling through superheated plasma at speeds of up to several million kilometres per hour, combined with its relative closeness, make it a perfect study subject of extreme cluster environments. 

Lead author Dr. Toby Brown, ICRAR alumnus and Plaskett Fellow at the National Research Council of Canada, said that the Virgo Cluster is an ideal location for detailed studies of the environment.

"It is our nearest massive galaxy cluster and is in the process of forming, which means that we can get a snapshot of galaxies in different stages of their lifecycle. This allows us to understand how star formation is shut off in cluster galaxies."

"The Virgo Cluster is a bit unusual in that it has a relatively large population of galaxies that are still forming stars," said Prof. Christine Wilson, McMaster University and co-principal investigator on the VERTICO project. "Many galaxy clusters in the Universe are dominated by red galaxies with little gas and star formation." 

NGC 4567 and NGC 4568 are 2 of the 2,000 galaxies in the Virgo Cluster, located roughly 65 million light-years from Earth. 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).

The observations by VERTIGO showed that in the majority of the galaxies studied, external environmental mechanisms reached into galaxies to disturb and perturb their molecular gas which in turn impacts their star formation. 

“With VERTICO, we looked at the gas reservoir of 51 galaxies in the Virgo Cluster, one of the most extreme environments we know of. We were able to create among the most detailed maps of the distribution of gas in cluster galaxies ever observed. These images provide the missing pieces of the puzzle of how the environment affects the gas content of galaxies, specifically the dense and cold gas, and hence their ability to form stars.” said Dr Lagos.

“The Virgo Cluster is the most extreme region of the local Universe, filled with million-degree plasma, extreme galaxy speeds, violent interactions between galaxies and their surroundings, a galaxy retirement village, and accordingly, a galaxy graveyard,” said Dr Brown.

“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. 

“Gas stripping is one of the most spectacular and violent external mechanisms that can shut down star formation in galaxies.”

Once the extreme environment has stripped the gas reservoirs from a galaxy it can stop entire galaxies from forming stars in a process known as galaxy quenching

The project was aided by ALMA’s Band 6 receiver—developed at the National Radio Astronomy Observatory’s Central Development Laboratory (CDL)—which provides high sensitivity and high resolution while minimizing required observing time.

The high quality and quantity of data will give astronomers more clues about how environments impact galaxies and how galaxies die. 

“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,” said Prof. Wilson. 

“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.”

The paper has been published in the Astrophysical Journal Supplement Series.