Hubble Survey Finds Nearby Dwarf Galaxies Are Missing Much of Their Oxygen
A NASA Hubble Space Telescope study of 37 nearby dwarf galaxies has found far less oxygen than simulations predict, indicating that supernova-driven winds expel heavy elements from small galaxies more efficiently than current models account for. The findings were published Monday in The Astrophysical Journal as part of the Galaxies Losing Oxygen via Winds project.
NASA
New observations from NASA's Hubble Space Telescope have revealed an apparent shortage of oxygen in the universe's smallest galaxies. In a paper published Monday in The Astrophysical Journal, researchers studying 37 nearby dwarf galaxies, systems containing anywhere from a few thousand to hundreds of millions of stars, report that much of the oxygen they expected to find is simply not there.
Simulations predict that significantly more oxygen, which serves as a tracer for a broader set of heavy elements, should reside within these galaxies. Co-investigator Grace Telford, an assistant professor in the Department of Physics and Astronomy at the University of Utah, said the missing element must have left the system at some point. "Oxygen is produced by stars, which are inside of galaxies. We know that the oxygen had to be inside of the galaxies at some point in the past, so the oxygen must have been ejected," Telford said. Driving that expulsion, she explained, requires energy from massive stars and supernova explosions to push material out of the host galaxies and into the space between them.
According to the study, galactic winds unleashed by supernovae appear to be more effective at stripping oxygen from low-mass galaxies than current models predict. The results indicate that dwarf galaxies can retain only a small fraction of the oxygen forged by their stars.
The work was carried out under the Galaxies Losing Oxygen via Winds, or GLOW, project, which is funded by NASA and the National Science Foundation. The project traces oxygen to understand how it is distributed within stars, in the gas between stars, and in the space between galaxies. Oxygen was chosen because it is the most abundant element after hydrogen and helium, it is relatively easy to detect and measure, and its production is well understood, making it a reliable indicator of how chemical elements are spread through a galaxy.
"So, it's really for practical reasons that we've chosen oxygen," said Kristen Wingfield McQuinn, GLOW principal investigator and mission head of the Science Operations Center for NASA's newly launched Nancy Grace Roman Space Telescope at the Space Telescope Science Institute in Baltimore, Maryland. McQuinn, also an associate professor at Rutgers University in New Brunswick, New Jersey, added that tracing how much oxygen stars produce, and how much ends up inside and outside galaxies, helps scientists understand galaxy evolution and learn where and how galaxies enrich the intergalactic medium.
The team drew on archival optical imaging from Hubble to resolve stellar populations and determine star-formation histories and compositions, supplemented by data from NASA's retired Spitzer Space Telescope and two ground-based radio facilities, the Very Large Array and the Australian Telescope Compact Array. The 37 dwarf galaxies were selected because their masses fall in the range where simulations and theory predict an increasing share of oxygen will be expelled through supernova explosions. They are also close enough for Hubble to resolve their individual stars, a condition met by very few known dwarf galaxies, most of which lie too far away for this kind of analysis.
Astronomers had already suspected that galaxies expel large fractions of oxygen and other heavy elements, but where those metals end up has remained an open question. A 2014 study addressed the issue for Milky Way-like, star-forming galaxies and found they lose a substantial portion of their oxygen, some of which resides in the circumgalactic medium, the diffuse halos of gas surrounding galaxies beyond their disks. The GLOW team extended that work to low-mass galaxies across a wide range of masses and environments. While previous teams have measured oxygen in the circumgalactic medium of dwarf galaxies, it is still unclear whether most of the missing oxygen sits there or has been ejected entirely beyond a galaxy's surroundings.
Future research is already planned. McQuinn has an approved program to use the Roman Space Telescope to search for even lower-mass galaxies across a wide swath of the nearby sky, with follow-up observations from Hubble and NASA's James Webb Space Telescope intended to reconstruct how those faint systems formed stars across cosmic time. Telford, meanwhile, will use Hubble to zoom in on the individual massive stars that produce and expel oxygen, characterizing how such stars differ in small galaxies using several nearby GLOW targets.
The Hubble Space Telescope has operated for more than three decades and is a project of international cooperation between NASA and the European Space Agency. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope and mission operations, with support from Lockheed Martin Space, based in Denver. Hubble science operations are conducted for NASA by the Space Telescope Science Institute in Baltimore, which is operated by the Association of Universities for Research in Astronomy.