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 metal content of the disk 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 almost 11.4 million light-years from Earth, NGC 253 is a nearby, edge-on starburst. It has a multiphase outflow that has been well-studied across the electromagnetic spectrum.
The galaxy’s wind is composed of gas with temperatures of millions of degrees that glows in X-rays. An amount of hot gas equivalent to about two million Earth masses blows away from the galaxy’s center yearly.
The star formation rate of NGC 253 is two to three times faster than Milky Way. Some of these young stars are massive and generate wind by ferociously blowing gas from their surfaces. Even more powerful winds are unleashed when these stars- later in their life- explode as supernovae.
Astronomers have a window into this crucial stage of the star life cycle thanks to NGC 253. Materials generated inside 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 upcoming stars and planets.
Chandra data (pink and white) are included in a new composite image of NGC 253 in the inset, demonstrating how these winds sweep away from the galaxy’s center in two opposed directions, to the upper right and lower left. Furthermore seen 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 broader image in the graphic, which displays an optical image from the European Southern Observatory’s La Silla Observatory in Chile, reveals how NGC 253 appears almost edge-on from Earth’s point.
Using deep Chandra observations, a team led by Sebastian Lopez of The Ohio State University in Columbus, Ohio, studied wind properties. They found that the densities and temperatures of the gas in the wind are the highest in regions less than about 800 light-years from the center of the galaxy — and then decrease with distance farther away.
These findings conflict with an early theory that suggested so-called starburst galaxies like NGC 253 have spherical winds. Recent theoretical studies suggest that a ring of “super star clusters” situated close to the center of NGC 253 should instead produce a more focused wind. Many young, massive stars can be found in superstar clusters.
The focused nature of the wind consequently supports the assumption that the superstar clusters are a significant wind source that the team saw. However, the lack of concordance between theory and data raises the possibility that the theory is deficient in some physics concepts.
Scientists noted, “A hint about what is missing comes from the observation that the wind cools rapidly as it moves away from the galaxy’s center. This suggests that the wind is plowing cooler gas, causing the wind to cool and slow down. Such a ‘wind plow’ effect might be the extra physics required to produce a better agreement between theory and observations.”
The team also studied the composition of the wind. In particular, they studied how elements like oxygen, neon, magnesium, silicon, sulfur, and iron are scattered across the structure. They found that these elements become much more diluted farther away from the galaxy’s center. Astronomers did not see such a rapid decrease in the amounts of these elements in the wind from another well-studied galaxy undergoing a burst of star formation, M82.
Astronomers will need future observations of other galaxies with winds to understand whether this difference is related to the general properties of the galaxies, such as the total mass of the stars they contain.
- Sebastian Lopez, Laura Lopez, et al. X-Ray Properties of NGC 253’s Starburst-driven Outflow. The Astrophysical Journal. DOI: 10.3847/1538-4357/aca65e