Stanford researchers help to explain how stars are born, cosmic structures evolve

Working with information sent from the Japanese Hitomi satellite, an international team of researchers that include Stanford scientists has obtained the first views of a supermassive black hole stirring hot gas at the heart of a galaxy cluster, like a spoon stirring cream into coffee.

These motions (see video here) could explain why galaxy clusters form far fewer stars than expected – a puzzling property that affects the way cosmic structures evolve.

The data, published today in Nature, were recorded with the X-ray satellite during its first month in space earlier this year, just before it spun out of control and disintegrated due to a chain of technical malfunctions.

“Being able to measure gas motions is a major advance in understanding the dynamic behavior of galaxy clusters and its ties to cosmic evolution,” said study co-author Irina Zhuravleva, a postdoctoral researcher at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC). “Although the Hitomi mission ended tragically after a very short period of time, it’s fair to say that it has opened a new chapter in X-ray astronomy.”

KIPAC is a joint institute of Stanford University and the Department of Energy’s SLAC National Accelerator Laboratory.

Galaxy clusters, which consist of hundreds to thousands of individual galaxies held together by gravity, also contain large amounts of gas. Over time, the gas should cool down and clump together to form stars. Yet there is very little star formation in galaxy clusters, and until now scientists were not sure why.

Norbert Werner, a research associate at KIPAC involved in the data analysis, said, “We already knew that supermassive black holes, which are found at the center of all galaxy clusters and are tens of billions of times more massive than the sun, could play a major role in keeping the gas from cooling by somehow injecting energy into it. Now we understand this mechanism better and see that there is just the right amount of stirring motion to produce enough heat.”

Plasma bubbles stir

About 15 percent of the mass of galaxy clusters is gas that is so hot – tens of millions of degrees Fahrenheit – that it shines in bright X-rays. In their study, the Hitomi researchers looked at the Perseus cluster, one of the most massive astronomical objects and the brightest in the X-ray sky.

Other space missions before Hitomi, including NASA’s Chandra X-ray Observatory, had taken precise X-ray images of the Perseus cluster. These snapshots revealed how giant bubbles of ultra-hot, ionized gas, or plasma, rise from the central supermassive black hole as it catapults streams of particles tens of thousands of light-years into space.

Additional images of visible light from the cluster showed streaks of cold gas that appear to get pulled away from the center of the galaxy. However, until now it has been unclear what effect the plasma bubbles have on this intergalactic gas.

To find out, the researchers pointed one of Hitomi’s instruments – the soft X-ray spectrometer (SXS) – at the center of the Perseus cluster and analyzed its X-ray emissions.

Steve Allen, a co-principal investigator and a professor of physics at Stanford and of particle physics and astrophysics at SLAC, said, “Since the SXS had 30 times better energy resolution than the instruments of previous missions, we were able to resolve details of the X-ray signals that weren’t accessible before. These new details resulted in the very first velocity map of the cluster center, showing the speed and turbulence of the hot gas.”

By superimposing this map onto the other images, the researchers were able to link the observed motions to the plasma bubbles.

Zhuravleva said, “From what we’ve seen in our data, the rising bubbles drag gas from the cluster center, which explains the filaments of stretched gas in the optical images. In this process, turbulence develops. In a way, the bubbles are like spoons that stir milk into a cup of coffee and cause eddies. The turbulence, in turn, heats the gas and suppresses star formation in the cluster.”

Hitomi’s legacy

Astrophysicists can use the new information to fine-tune models that describe how galaxy clusters change over time.

One important factor in these models is the mass of galaxy clusters, which researchers typically calculate from the gas pressure in the cluster. However, motions cause additional pressure, and before this study it was unclear if the calculations need to be corrected for turbulent gas.

“Although the motions heat the gas at the center of the Perseus cluster, their speed is only about 100 miles per second, which is surprisingly slow considering how disturbed the region looks in X-ray images,” said co-principal investigator Roger Blandford, the Luke Blossom Professor of Physics at Stanford and a professor of particle physics and astrophysics at SLAC. “One consequence is that corrections for these motions are only very small and don’t affect our mass calculations much.”

Although the loss of Hitomi cut most of the planned science program short – it was supposed to run for at least three years – the researchers hope their results will convince the international community to plan another X-ray space mission.

Werner said, “The data Hitomi sent back to Earth are just beautiful. They demonstrate what’s possible in the field and give us a taste of all the great science that should have come out of the mission over the years.”

Hitomi is a joint project, with the Japan Aerospace Exploration Agency (JAXA) and NASA as the principal partners. Led by Japan, it is a large-scale international collaboration, boasting the participation of eight countries, including the United States, the Netherlands and Canada, with additional partnership by the European Space Agency (ESA). Other KIPAC researchers involved in the project are Tuneyoshi Kamae, Ashley King, Hirokazu Odaka and co-principal investigator Grzegorz Madejski.