Chandra determines what makes a galaxy’s wind blow

This version features an optical image of the spiral galaxy NGC 253 and a separate composite close-up of the galaxy’s bright center. In both images, the galaxy is contrasted against the blackness of space, which is dotted with specks of light. – X-RAY: NASA/CXC/THE OHIO STATE UNIV/S. LOPEZ ET A

A new study with NASA’s Chandra X-ray Observatory shows the effects of powerful winds blown from the center of a nearby galaxy, NGC 253, located 11.4 million light-years away.

This galactic wind is made of gas with temperatures in the millions of degrees that glows in X-rays. About two million Earth masses of hot gas moves away from the center of the galaxy each year. NGC 253 is a spiral galaxy, which makes it similar to our Milky Way. However, stars are forming in NGC 253 two to three times faster than in our home galaxy. Some of these young stars are massive and generate wind by fiercely ejecting gas from their surfaces. Even more powerful winds are unleashed when, later in their relatively short lives, these stars explode as supernovae, spewing waves of material into space.

NGC 253 provides astronomers with a keyhole through which to study this important phase in the stellar life cycle. The material that young stars send out into intergalactic space over hundreds of light-years is enriched with elements forged within. These elements, which include many responsible for life on Earth, are integrated into the next generations of stars and planets.

A new composite image of NGC 253 in the inset includes Chandra data (pink and white) showing that these winds blow in two opposite directions away from the center of the galaxy, toward the upper right and lower left. Also shown in this image are visible light (cyan) and hydrogen emission (orange) data, both from a 0.9-meter telescope at Kitt Peak Observatory, and infrared data from NASA’s Spitzer Space Telescope (red). From Earth’s vantage point, NGC 253 appears almost edge-on, as seen in the larger image of the graphic, which shows an optical image from the European Southern Observatory’s La Silla Observatory in Chile.

A team led by Sebastian Lopez of Ohio State University in Columbus, Ohio used deep Chandra observations, made over four days, to study the properties of the wind. They found that gas densities and temperatures in the wind are highest in regions less than 800 light-years from the center of the galaxy, then decrease with further distance. These results do not agree with an early model in which the winds of so-called starburst galaxies like NGC 253 are spherical. Instead, recent theoretical work predicts that a ring of “super star clusters” located near the center of NGC 253 should form a more concentrated wind. Super star clusters contain large numbers of young and massive stars.

The focused nature of the wind observed by López and his team therefore supports the idea that superstar clusters are an important source of the wind. However, there is not complete agreement between the theory and the observations, suggesting that physics is missing from the theory, NASA reports. A clue as to what is missing comes from the team’s observation that the wind cools rapidly as it moves away from the center of the galaxy. This suggests that the wind is picking up cooler gas, causing it to cool and slow down. Such a ‘wind plowing’ effect could be the additional physics required to produce better agreement between theory and observations.

López and his colleagues also studied the composition of the wind, including how elements such as oxygen, neon, magnesium, silicon, sulfur, and iron disperse through the structure. They found that these elements dilute much further from the center of the galaxy. The astronomers did not see such a rapid decline in the amounts of these elements in the wind of another well-studied galaxy undergoing burst star formation, M82.

Astronomers will need future observations of other windy galaxies to understand whether this difference is related to general properties of the galaxies, such as the total mass of the stars they contain. An article describing these results was published in The Astrophysical Journal