Astronomers have completed the first in-depth census of molecular clouds in the nearby Universe. The study has revealed that these star-forming regions not only look different but also behave differently. This finding runs in opposition to previous scientific consensus, which considered these clouds of dust and gas to be fairly uniform.

The project–Physics at High Angular Resolution in Nearby GalaxieS (PHANGS)–consisted of a systematic survey of 100,000 molecular clouds in 90 galaxies in the local Universe. The primary aim of the PHANGS was to get an idea of how these star-forming regions are influenced by their parent galaxies.

The census was conducted with the use of the Atacama Large Millimeter/ submillimeter Array (ALMA) located on the Chajnantor plateau, in the Atacama Desert of northern Chile. Whilst not marking the first time stellar nurseries have been studied with ALMA, this is the first census of its kind to observe globular clusters across more than either one galaxy or a small region of a single galaxy.

“We have carried out the first real ‘census’ of these stellar nurseries, and it provided us with details about their masses, locations, and other properties,” Adam Leroy, Associate Professor of Astronomy at Ohio State University (OSU) tells ZME Science. “Some people thought that all stellar nurseries across every galaxy look more or less the same, and it took having a really big, sensitive, and high-resolution survey of many galaxies with a telescope such as ALMA to see that this is not the case. This survey allows us to see how the stellar nurseries change across different galaxies. “

As a result, this is the first time that astronomers have been granted a look at the ‘big picture’ when it comes to these star-forming regions. Erik Rosolowsky, Associate Professor of Physics at the University of Alberta, and a co-author of the research points out that what ALMA has allowed the team of astronomers to create is essentially a new form of ‘cosmic cartography’ consisting of 90 maps of unparalleled detail detailing the regions of space where the next generation of stars will be born.

“By doing this we will combine what we are learning from ALMA about the clouds that form stars with pictures of newly formed stars from these other telescopes. This promises to give us the best view ever of the full life cycle of these stellar nurseries, and our most complete picture ever of the full cycle of star birth and death.”

“Our survey is the first one to capture the demographics of these stellar nurseries across a large number of the galaxies near the Milky Way,” adds Leroy, the lead author of a paper presenting the PHANGS ALMA survey. “We used these measurements to measure the characteristics of these nurseries, their lifetimes, and the ability of these objects to form new stars.”

How Galactic Neighborhoods Influence Star-Forming Clouds

The variety displayed by the molecular clouds surveyed in the PHANGS project was visible due to ALMA’s ability to take millimeter-wave images with the same sharpness and quality as images taken in the visible spectrum.

“While optical pictures show us light from stars, these ground-breaking new images show us the molecular clouds that form those stars,” says Leroy. “That helped us to see that stellar nurseries actually change from place to place.”

The team compared the changes displayed by molecular clouds from galaxy to galaxy to changes in houses, neighbourhoods and cities from region to region here on Earth.

“How stellar nurseries relate to their parent galaxies has been a big question for a long time. We’re able to answer this because our survey expands the amount of data on stellar nurseries by a factor of almost 100,” says Leroy. “Before this, it was very common to study a few hundred nurseries in one galaxy. So it was kind of like trying to learn about houses in general by looking only at neighbourhoods in Columbus, Ohio.

“You will learn some things about houses, but you miss the big picture and a lot of the variation, complexity, and commonality With this survey we looked at houses in many cities across many countries.”

Adam Leroy, Ohio State University

Leroy continues by explaining that stellar nurseries ‘know’ about their neighbourhood, meaning that molecular clouds are different depending on what galaxy they live in or where in that galaxy they are located. “So the stellar nurseries that we see in the Milky Way won’t be the same as those in a different galaxy, and the stellar nurseries in the outer part of a galaxy–where we live–aren’t the same as those near the galaxy centre.”

The team found clouds in the dense central regions of galaxies tend to be more massive, denser, and more turbulent than those located on the outskirts of a galaxy. In addition to this, the census revealed the lifecycle of clouds also depends on their environment. Annie Hughes, an astronomer at L’Institut de Recherche en Astrophysique et Planétologie (IRAP) explains that this means that both the rate at which a cloud forms stars and the processes that ultimately destroy clouds both seem to depend on where the cloud lives.

How Differences in Globular Clusters Influence the Birth of Stars

Because all stars are formed in molecular clouds, understanding the differences in these clouds of gas and dust and how they are caused by the conditions in which they exist is key to better understanding the processes that are driving the birth of stars like our own Sun.

These molecular clouds are so vast that they can birth anywhere from thousands to hundreds of thousands of stars before being exhausted of raw materials. These new observations have shown astronomers that each cosmic neighbourhood can have an effect on where stars are born and how many stars are spawned.

“Every star in the sky, in fact, every star in every galaxy, including our Sun, was born in one of these stellar nurseries. These are really the engines that build galaxies and make planets, and they’re just an essential part of the story of how we got here.”

Adam Leroy, Ohio State University

The next step for the astronomers will be to combine the data provided by ALMA with surveys conducted by other telescopes including the Hubble space telescope, and the Very Large Telescope (VLT) also located in the Atacama desert, Chile. Leroy hopes that this along with observations made with the James Webb Space Telescope (JWST), will help astronomers answer the question of how the diversity of molecular structures affects the stars which form within them. He explains: “By doing this we will combine what we are learning from ALMA about the clouds that form stars with pictures of newly formed stars from these other telescopes.

This promises to give us the best view ever of the full life cycle of these stellar nurseries, and our most complete picture ever of the full cycle of star birth and death.”

Adam Leroy, Ohio State University

Leroy concludes by pointing out why the study of these star-forming regions is so important. “This is the first time we have gotten a clear view of the population of these stellar nurseries across the whole nearby universe,” the researcher says. “It’s a big step towards understanding where we come from.”