How cosmic winds transform galactic environments

Using observations made by NASA's Chandra X-ray Observatory, a new study details how these energetic winds, once released from the center of a galaxy, directly influence the temperature and metal distribution of the rest of the region.

"Galactic winds are a large part of galaxy evolution in general," said Sebastian Lopez, lead author of the study and a graduate student in astronomy at The Ohio State University. "As they blow from one end of a galaxy to another, they alter the distribution of metals across the disk and enrich the surrounding intergalactic space."

In investigating the nearby spiral galaxy NGC 253, researchers found that while the amount of these elements can vary, the abundances of oxygen, neon, magnesium, silicon, sulfur and iron peaked in the center of the galaxy and decreased with distance from it. This indicates that as hot gas cools the farther away it travels from the center, it leaves behind a lower concentration of these elements.

Learning more about how the celestial detritus that make up these vast galaxies are disseminated across the cosmos could help astronomers more deeply understand how galactic formation works in other areas of the universe. "Our research could reflect that the size of a galaxy, or even its morphology, could impact how gas leaves these systems," Lopez said. The study was published online in The Astrophysical Journal.

Between 1999 and 2018, Chandra observed NGC 253 only seven times, but by analyzing image and spectral data taken from those observations, Lopez and his team were able to use specialized computer software to identify the emission lines left by passing winds. While compiling this data, they found that the research runs counter to previous X-ray studies done on NGC 253, which posit that galactic winds expand spherically, or in a bubble-like shape.

Instead, the models Lopez's team created show how the winds move in opposite directions from the middle of the galaxy and then radiate outwards toward the upper right and lower left regions. Lopez places much of this discrepancy on the data available at the time of the previous studies and the technological strides scientists have made since.

Still, there were a few similarities to previous work that did catch researchers' interest. To determine how galactic emission differences arise and if these differences depend on the galaxy's properties, they compared NGC 253 to the results of studies done on the galaxy M82, a similar starburst system located some 12 million light-years away from Earth. After detecting the same metals and similar distributions within M82 that they did with NGC 253, Lopez said that comparing the two led the team to discern that a process called charge exchange -- the stripping of an electron from a neutral atom by an ion -- plays a large part in X-ray emission.

"In order for scientists to create a realistic galaxy in simulations, we need to know where these heavy elements are going," Lopez said. "Because if you were to model it and not include charge exchange into these models, they wouldn't match up." If such calculations were inherently wrong, he said, scientists would have a hard time using their observations to make educated guesses about what the universe looks like and how it operates.

But Lopez imagines the more accurate models created from this study will help astronomers study the winds of other galaxies, such as calculating their velocities and discovering what makes them so good at creating unique stellar environments. "Next, we want to do this analysis for a larger set of different galaxies and see how things change," Lopez said.

This research was supported by NASA. Co-authors were Laura Lopez, Dustin Nguyen, Todd Thompson, Smita Mathur and Amy Sardone of Ohio State, Alberto Bolatto of the University of Maryland, and Neven Vulic of Eureka Scientific.