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Something is killing galaxies — and science is on the case

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In the most extreme regions of the universe, galaxies are being killed. Their star formation is being shut down and astronomers want to know why.

The first ever Canadian-led large project on one of the world’s leading telescopes is hoping to do just that. The new program, called the Virgo Environment Traced in Carbon Monoxide survey (VERTICO), is investigating, in brilliant detail, how galaxies are killed by their environment.

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As VERTICO’s principal investigator, I lead a team of 30 experts that are using the Atacama Large Millimeter Array (ALMA) to map the molecular hydrogen gas, the fuel from which new stars are made, at high resolution across 51 galaxies in our nearest galaxy cluster, called the Virgo Cluster.

Commissioned in 2013 at a cost of US$1.4 billion, ALMA is an array of connected radio dishes at an altitude of 5,000 metres in the Atacama Desert of northern Chile. It is an international partnership between Europe, the United States, Canada, Japan, South Korea, Taiwan and Chile. The largest ground-based astronomical project in existence, ALMA is the most advanced millimetre wavelength telescope ever built and ideal for studying the clouds of dense cold gas from which new stars form, which cannot be seen using visible light.

Large ALMA research programs such as VERTICO are designed to address strategic scientific issues that will lead to a major advance or breakthrough in the field.

Galaxy clusters

Where galaxies live in the universe and how they interact with their surroundings (the intergalactic medium that surrounds them) and each other are major influences on their ability to form stars. But precisely how this so-called environment dictates the life and death of galaxies remains a mystery.

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Galaxy clusters are the most massive and most extreme environments in the universe, containing many hundreds or even thousands of galaxies. Where you have mass, you also have gravity and the huge gravitational forces present in clusters accelerates galaxies to great speeds, often thousands of kilometres-per-second, and superheats the plasma in between galaxies to temperatures so high that it glows with X-ray light.

In the dense, inhospitable interiors of these clusters, galaxies interact strongly with their surroundings and with each other. It is these interactions that can kill off — or quench — their star formation.

Understanding which quenching mechanisms shut off star formation and how they do it is main the focus of the VERTICO collaboration’s research.

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The life cycle of galaxies

As galaxies fall through clusters, the intergalactic plasma can rapidly remove their gas in a violent process called ram pressure stripping. When you remove the fuel for star formation, you effectively kill the galaxy, turning it into a dead object in which no new stars are formed.

In addition, the high temperature of clusters can stop hot gas cooling and condensing onto galaxies. In this case, the gas in the galaxy isn’t actively removed by the environment but is consumed as it forms stars. This process leads to a slow, inexorable shut down in star formation known, somewhat morbidly, as starvation or strangulation.

An image of spiral galaxy NGC 4330 in the Virgo Cluster. Ram pressure stripped hot gas is shown in red and a blue overlay shows star-forming gas.
Fossatie et al. (2018), Author provided

While these processes vary considerably, each leaves a unique, identifiable imprint on the galaxy’s star-forming gas. Piecing these imprints together to form a picture of how clusters drive changes in galaxies is a major focus of the VERTICO collaboration. Building on decades of work to provide insight into how environment drives galaxy evolution, we aim to add a critical new piece of the puzzle.

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An ideal case study

The Virgo Cluster is an ideal location for such a detailed study of 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 life cycles. This allows us to build up a detailed picture of how star formation is shut off in cluster galaxies.

Galaxies in the Virgo cluster have been observed at almost every wavelength in the electromagnetic spectrum (for example, radio, optical and ultra-violet light), but observations of star-forming gas (made at at millimetre wavelengths) with the required sensitivity and resolution do not exist yet. As one of the largest galaxy surveys on ALMA to date, VERTICO will provide high resolution maps of molecular hydrogen gas — the raw fuel for star formation — for 51 galaxies.

With ALMA data for this large sample of galaxies, it will be possible to reveal exactly which quenching mechanisms, ram pressure stripping or starvation, are killing galaxies in extreme environments and how.

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By mapping the star-forming gas in galaxies that are the smoking gun examples of environment-driven quenching, VERTICO will advance our current understanding of how galaxies evolve in the densest regions of the Universe.

[ Deep knowledge, daily. Sign up for The Conversation’s newsletter. ]The Conversation

Toby Brown, Post Doctorate Fellow in Astrophysics, McMaster University

This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Trump waves off deaths in Syria: The Kurds are ‘no angels’

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President Donald Trump on Wednesday defended his decision to leave Syria by saying the Kurds are "no angels."

Trump made the remarks during a White House meeting.

"They are no angels, by the way," he was quoted as saying about the Kurds.

Republicans have accused Trump of betraying a U.S. ally by abandoning the Kurds to be crushed by Turkish forces in Syria.

Read some of the reports below.

Per pool, Trump described the Kurds as "no angels." Your regular reminder that the Kurds were our allies in the fight against ISIS -- which Trump has taken full credit for eradicating.

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Trump’s tax inconsistencies look like ‘bank fraud’: Ex-prosecutor

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On Wednesday, former federal prosecutor Joyce Vance argued that the inconsistencies in Trump's tax information resemble outright "bank fraud," and must be subject to further investigation:

No surprise that Trump’s NY state tax filings are inconsistent with bank loan applications. Perhaps there’s an explanation but it looks like either bank fraud or tax fraud & at a minimum, merits further investigation. https://t.co/czxdpNjrvA

— Joyce Alene (@JoyceWhiteVance) October 16, 2019

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‘Too much for him’: Pompeo’s senior adviser plans to deliver damning testimony on ouster of Ukraine ambassador

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Michael McKinley, a former senior adviser to Secretary of State Mike Pompeo, is set to deliver damning testimony to Congress about the ouster of former Ukrainian ambassador Marie Yovanovitch.

The Washington Post reports that McKinley will tell investigators on Wednesday that he resigned from the State Department because he was disgusted by its unwillingness to stick up for Yovanovitch, who was removed from her post after pressure from both President Donald Trump and personal attorney Rudy Giuliani.

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