Galaxies consist of star-forming regions sparkling as fireworks

May 23, 2019

Star formation within interstellar gas clouds proceeds very rapidly, yet highly inefficiently. Most of the gas is quickly dispersed by stellar radiation, leading to a violent cycling in which star-forming regions are flickering as sparkles in fireworks. A team of scientists including researchers at the Max Planck Institute for Extraterrestrial Physics has come to these conclusions based on new observations of the spiral galaxy NGC300.

It is one of the main unsolved problems in astrophysics to determine the physical mechanisms driving star formation. A team of scientists led by Diederik Kruijssen at Heidelberg University and Andreas Schruba at the Max Planck Institute for Extraterrestrial Physics has now managed for the first time to reconstruct the time-evolution of interstellar clouds of gas and dust, so called molecular clouds, and the star formation process within them. Using new observations of the spiral galaxy NGC300, the team was able to demonstrate that star formation proceeds very rapidly, yet is highly “inefficient”. Most of the interstellar gas is not converted into stars, but is instead dispersed by stellar radiation. Molecular clouds are short-lived structures undergoing rapid lifecycles, driven by the intense radiation from the new-born stars within them. This finding shows that galaxies consist of building blocks that flicker as sparkles in fireworks constantly change their appearance.

Star formation in molecular clouds can be explained in two ways. Molecular clouds may be long-lived, and eventually convert all of their gas into stars. In this case, young stars should be found within the molecular clouds from which they have formed. Alternatively, stars may form rapidly and their intense radiation may quickly disperse their birth cloud, causing only a small fraction of the gas to be converted into stars. In this case, young stars and molecular clouds should be generally displaced and coincide only during the short period of star formation.

The image on the left shows that the positions of molecular clouds (blue) and emission from young stars (pink) do not coincide on small spatial scales. The two branches on the right quantify this displacement by showing that molecular clouds and young stars are only correlated when averaging over a large part of the galaxy (1,000 pc, corresponding to 3,000 lightyears).

To answer which of these models for the molecular cloud lifecycle is correct, the scientists combined two different sets of observations of the galaxy NGC300, which is at a distance of about six million lightyears from the Milky Way. The first is a map of light emitted by carbon monoxide, which shows where molecular clouds reside. The second is a map of hot, ionised hydrogen, which marks the positions of massive stars that just formed. These maps were obtained with the Atacama Large Millimeter Array (ALMA, observations carried out by Andreas Schruba) of the European Southern Observatory (ESO) and the 2.2 meter telescope of the Max Planck Society and ESO.

The scientists then analysed the data using a new statistical method, which determines how molecular gas and star formation in galaxies are related on different spatial scales. For the first time, this method enables to precisely quantify the positions of molecular clouds and young stars relative to one another. The results left no doubt: molecular clouds and young, massive stars rarely coincide. This effect becomes stronger on smaller scales. The researchers conclude that stars form and destroy their birth clouds very rapidly, such that gas and young stars represent distinct, subsequent phases in the lifecycle of molecular clouds.

“Our results demonstrate that star formation proceeds very rapidly and highly inefficiently”, explains Diederik Kruijssen from the Heidelberg University, who led the analysis. “Molecular clouds in NGC300 live for about ten million years, and take only about 1.5 million years to be destroyed, well before the most massive stars have reached the end of their lives and explode as supernovae.” Andreas Schruba from the Max Planck Institute for Extraterrestrial Physics, who led the observational program, adds: “ALMA is providing a stunning and detailed picture of nearby galaxies. We can now directly observe that galaxies are highly dynamic systems, consisting of building blocks that constantly change their appearance.”

The team of researchers now wants to apply their new statistical method to observations of large numbers of nearby and distant galaxies, to infer how star formation in molecular clouds proceeds across the history of the Universe.

The movie demonstrates that molecular clouds (top right) and young stars (top left) are anti-correlated in the nearby galaxy NGC300. As the video plays, the spatial resolution increases and the ratio between molecular clouds and emission from young stars (bottom left) changes from white (strong correlation) on large scales to bright red and blue (strong anti-correlation) on small scales. The graph in the bottom-right panel quantifies this behaviour.

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