Jansky Very Large Array (VLA): Solving a 30-year-old problem in massive star formation

This false-colour Very Large Array image of the ionized gas in the star forming region Sgr B2 Main was used to detect small but significant changes in brightness of several of the sources. 

The spots and filaments in this image are regions of ionized gas around massive stars. 

The changes in brightness detected support a model that could solve a 30-year-old question in high mass star formation. 

Credit: NRAO /Agnes Scott College

An international group of astrophysicists has found evidence strongly supporting a solution to a long-standing puzzle about the birth of some of the most massive stars in the universe.

Young massive stars, which have more than 10 times the mass of the Sun, shine brightly in the ultraviolet, heating the gas around them, and it has long been a mystery why the hot gas doesn't explode outwards.

Now, observations made by a team of researchers using the Jansky Very Large Array (VLA), a radio astronomy observatory in New Mexico, have confirmed predications that as the gas cloud collapses, it forms dense filamentary structures that absorb the star's ultraviolet radiation when it passes through them. As a result, the surrounding heated nebula flickers like a candle.

The findings were published recently in The Astrophysical Journal Letters.

"Massive stars dominate the lives of their host galaxies through their ionizing radiation and supernova explosions," said Mordecai-Mark Mac Low, a curator in the American Museum of Natural History's Department of Astrophysics and an author on the paper.

"All the elements heavier than iron were formed in the supernova explosions occurring at the ends of their lives, so without them, life on Earth would be very different."

Observations of the massive star forming region Sgr B2 were made with the Karl G. Jansky Very Large Array (VLA) in 1989 and 2012. 

The VLA has been operational since 1980 and received a major upgrade that was completed in 2011. 

Credit: NRAO/AUI

Stars form when huge clouds of gas collapse. Once the density and temperature are high enough, hydrogen fuses into helium, and the star starts shining.

The most massive stars, though, begin to shine while the clouds are still collapsing.

Their ultraviolet light ionizes the surrounding gas, forming a nebula with a temperature of 10,000 degrees Celsius. Simple models suggest that at this stage, the gas around massive stars will quickly expand.

But observations from the VLA radio observatory show something different: a large number of regions of ionized hydrogen (so-called HII regions) that are very small.

"In the old theoretical model, a high-mass star forms and the HII region lights up and begins to expand."

Chris De Pree

"Everything was neat and tidy," said lead author Chris De Pree, a professor of astronomy and director of the Bradley Observatory at Agnes Scott College.

"But the group of theorists I am working with were running numerical models that showed accretion was continuing during star formation, and that material was continuing to fall in toward the star after the HII region had formed."

More information: arxiv.org/abs/1312.7768

Powerful jets discovered blowing material out of galaxy

Radio-Telescope Image of the galaxy 4C12.50, nearly 1.5 billion light-years from Earth. 

Inset shows detail of location at end of superfast jet of particles, where a massive gas cloud (yellow-orange) is being pushed by the jet. 

Credit: Morganti et al., NRAO /AUI /NSF

Astronomers using a worldwide network of radio telescopes have found strong evidence that a powerful jet of material propelled to nearly light speed by a galaxy's central black hole is blowing massive amounts of gas out of the galaxy.

This process, they said, is limiting the growth of the black hole and the rate of star formation in the galaxy, and thus is a key to understanding how galaxies develop.

Astronomers have theorized that many galaxies should be more massive and have more stars than is actually the case.

Scientists proposed two major mechanisms that would slow or halt the process of mass growth and star formation -- violent stellar winds from bursts of star formation and pushback from the jets powered by the galaxy's central, supermassive black hole.

Raffaella Morganti

"With the finely-detailed images provided by an intercontinental combination of radio telescopes, we have been able to see massive clumps of cold gas being pushed away from the galaxy's center by the black-hole-powered jets," said Raffaella Morganti, of the Netherlands Institute for Radio Astronomy and the University of Groningen.

The scientists studied a galaxy called 4C12.50, nearly 1.5 billion light-years from Earth. They chose this galaxy because it is at a stage where the black-hole "engine" that produces the jets is just turning on.

As the black hole, a concentration of mass so dense that not even light can escape, pulls material toward it, the material forms a swirling disk surrounding the black hole.

Processes in the disk tap the tremendous gravitational energy of the black hole to propel material outward from the poles of the disk.

At the ends of both jets, the researchers found clumps of hydrogen gas moving outward from the galaxy at 1,000 kilometers per second.

One of the clouds has much as 16,000 times the mass of the Sun, while the other contains 140,000 times the mass of the Sun. The larger cloud, the scientists said, is roughly 160 by 190 light-years in size.

"This is the most definitive evidence yet for an interaction between the swift-moving jet of such a galaxy and a dense interstellar gas cloud," Morganti said.

"We believe we are seeing in action the process by which an active, central engine can remove gas -- the raw material for star formation -- from a young galaxy," she added.

The scientists also said their observations indicate that the jets from the galaxy's core can stretch and deform clouds of interstellar gas to expand their "pushing" effect beyond the narrow width of the jets themselves.

In addition, they reported that, at 4C12.50's stage of development, the jets may turn on and off and so periodically repeat the process of removing gas from the galaxy.

In July, another team of scientists, using the Atacama Large Millimeter/submillimeter Array (ALMA), announced they had found gas being blown from a more-nearby galaxy, called NGC 253, by an intense burst of star formation.

"Both processes are thought to be at work, often simultaneously, in young galaxies to regulate the growth of their central black holes as well as the rate at which they can form new stars," Morganti said.

Morganti and her team used radio telescopes in Europe and the U.S., combining their signals to make one giant, intercontinental telescope.

Journal Reference:
R. Morganti, J. Fogasy, Z. Paragi, T. Oosterloo, M. Orienti. Radio Jets Clearing the Way Through a Galaxy: Watching Feedback in Action. Science, 2013; 341 (6150): 1082 DOI: 10.1126/science.1240436