What’s Snuffing Out Galaxies Before Their Time?

In the Milky Way, the formation rate of stars is about one solar mass every year. About 10 billion years ago, it was ten solar masses every year. What happened?

Stars are born in giant molecular clouds (GMCs), and astronomers think that the environment in galaxies affects these clouds and their ability to spawn new stars. Sometimes the environment is so extreme that entire galaxies stop forming new stars.

Astronomers call this “quenching,” and they want to know what causes it.

Galactic quenching is one of astrophysics’ long-standing mysteries. Astronomers and astrophysicists have studied the phenomenon in detail and come up with different possible explanations.

One idea is called black hole feedback. In that scenario, as a supermassive black hole in a galactic center draws in material, it releases energy back into the galaxy. That energetic feedback disrupts the galaxy’s gas supply. Gas from the halo no longer flows into the galaxy to fuel star formation.

Other researchers have pointed to galaxy mergers and harassment as the culprit. Some have focused on how long the quenching process takes. Some have found evidence that it happens in only 100 million years. Others have found that it may take billions of years.

There are lots of questions about the process of galactic quenching.

A new survey will help astrophysicists tackle the question. Its title is “VERTICO: The Virgo Environment Traced In CO Survey,” and it’s available on the arxiv.org preprint server. The principal investigator is Toby Brown, an astrophysicist and Plaskett Fellow at the National Research Council of Canada.

This survey takes a different tack than others. Instead of focusing on individual galaxies that experience quenching, it looks at galaxy clusters. Galaxy clusters can be very extreme environments, and they’re a focus of study in astrophysics.

VERTICO is the Virgo Environment Traced in Carbon Monoxide Survey. It used ALMA to map the star-forming molecular gas in 51 separate galaxies in the Virgo cluster. The Virgo cluster is our closest neighbouring galaxy cluster and contains about two thousand galaxies.

This deep image of the Virgo Cluster obtained by Chris Mihos and his colleagues using the Burrell Schmidt telescope shows the diffuse light between the galaxies belonging to the cluster. North is up, east to the left. The dark spots indicate where bright foreground stars were removed from the image. Messier 87 is the largest galaxy in the picture (lower left). Image Credit: By Chris Mihos (Case Western Reserve University)/ESO – CC BY 4.0

“What VERTICO reveals better than ever before is which physical processes affect molecular gas and how they dictate the life and death of the galaxy,” said VERTICO leader Toby Brown in a press release.

The Virgo cluster is huge, and it’s also close to us. This makes it a natural target for a survey like VERTICO. “VERTICO is an ideal resource for studying the fate of molecular gas in cluster galaxies and the physics of environment-driven processes that perturb the star formation cycle,” the authors write in their paper.

The Virgo cluster is unusual compared to other clusters: there’s lots of star formation going on.

“The Virgo Cluster is a bit unusual in that it has a relatively large population of galaxies that are still forming stars,” said Christine Wilson, Distinguished University Professor at 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.”

This high-resolution revealed a wild and crazy population of gas reservoirs in the 51 galaxies it studied. The environment is so extreme that it can quench star formation in galaxies that reside in the cluster.

Some labelled galaxies in the Virgo Cluster. Image Credit: Wikisky

“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 Brown.

Brown explained that all these factors create gas-stripping, which starves galaxies of the gas needed to form new stars.

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

VERTICO in itself is not an answer to the question of galactic quenching. Instead, it constitutes a high-resolution dataset for study. “Our intent with this work is to provide an overview of the VERTICO survey and highlight its potential as a resource for revealing the role environment plays in galaxy evolution. To this end, VERTICO will be used to study the fate of molecular gas in cluster galaxies and the physics of environment-driven processes that perturb the star formation cycle,” the authors write.

In that sense, it’s similar to other surveys like GAIA and the SLOAN Digital Sky Survey (SDSS), for example. GAIA created a precise three-dimensional map of the Milky Way by cataloguing over one billion objects. And the SDSS created the most detailed map of the Universe, with the spectra of more than three million objects.

The creators of VERTICO hope that their work can make the same kind of contribution that surveys like GAIA and the SDSS have.

“It is our hope that VERTICO advances our understanding and provides a valuable legacy resource that serves the community for years to come,” they write.

“There have been a lot of questions over the years on whether and how the cluster environment affects the molecular gas in galaxies, and how exactly those environments may contribute to their deaths. We still have work to do, but I’m confident VERTICO will allow us to answer these questions once and for all,” said co-principal investigator Wilson.

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Evan Gough

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