Starburst galaxies are characterized by their prominent multiphase outflows known as galactic winds. These galactic winds have essential effects on their host galaxies, where they alter the disk’s metal content and can enrich the surrounding circumgalactic medium (CGM) and intergalactic medium (IGM).
A new study using NASA’s Chandra X-ray Observatory shows the effects of powerful winds launched from the center of a nearby galaxy, NGC 253. Located nearly 11.4 million light-years from Earth, NGC 253 is a nearby, edge-to-edge galaxy. on starburst. It has a multi-phase outflow that has been well studied across the electromagnetic spectrum.
The wind of the galaxy consists of gas with temperatures of millions of degrees that glows in X-rays. An amount of hot gas equal to about two million Earth masses blows away from the center of the galaxy each year.
The star formation rate of NGC 253 is two to three times faster than that of the Milky Way. Some of these young stars are massive and generate wind by blasting ferocious gas from their surfaces. Even more powerful winds are unleashed when these stars – later in life – explode as supernovae.
Thanks to NGC 253, astronomers have a glimpse into this crucial phase of the life cycle of stars. Materials generated in the young stars enrich the material they project hundreds of light-years into intergalactic space. These elements, many of which are essential to life as we know it on Earth, are incorporated into the emerging stars and planets.
Chandra data (pink and white) is included in a new composite image of NGC 253 in the inset, demonstrating how these winds blow in two opposite directions from the center of the galaxy, to the upper right and lower left. Also included in this image are infrared data from NASA’s Spitzer Space Telescope, visible light data from a 0.9-meter telescope at Kitt Peak Observatory, and hydrogen emission data (red). The wider view in the image, which shows an optical view from the European Southern Observatory’s La Silla Observatory in Chile, shows how NGC 253 looks almost sideways from Earth’s tip.
Using deep Chandra observations, a team led by Sebastian Lopez of Ohio State University in Columbus, Ohio, studied wind properties. They found that the densities and temperatures of the gas in the wind are highest in regions less than about 800 light-years from the galaxy’s center — and then decrease as the distance is further away.
These findings contradict an early theory that suggested that so-called galaxies such as NGC 253 have globular winds. Recent theoretical studies suggest that a ring of “superstar clusters” close to the center of NGC 253 should produce a more directional wind instead. Superstar clusters contain many young, massive stars.
The directed nature of the wind thus supports the assumption that the superstar clusters are a major wind source the team saw. However, the lack of agreement between theory and data raises the possibility that theory falls short in some physics concepts.
Scientists noted, “A hint as to what is missing comes from the observation that the wind cools rapidly as it moves away from the center of the galaxy. This suggests that the wind is plowing cooler gas, cooling and slowing the wind. Such a ‘wind plow’ effect could be the extra physics needed to produce better agreement between theory and observations.’
The team also studied the composition of the wind. In particular, they studied how elements such as oxygen, neon, magnesium, silicon, sulfur and iron are distributed throughout the structure. They found that further away from the center of the galaxy, these elements become much more diluted. Astronomers did not see such a rapid decrease in the amounts of these elements in the winds of another well-studied galaxy undergoing a burst of star formation, M82.
Astronomers will need future observations of other windy galaxies to understand whether this difference is related to the general properties of the galaxies, such as the total mass of the stars they contain.
Magazine reference:
- Sebastian Lopez, Laura Lopez, et al. X-ray properties of the starburst-driven outflow from NGC 253. The Astrophysical Journal. DOI: 10.3847/1538-4357/aca65e
