Deep imaging techniques reveal galaxies are much larger than previously thought

If this galaxy is typical, then the study, published today in Astronomy of natureindicates that our galaxy is already interacting with its closest neighbor, Andromeda.

Where does a galaxy end and deep space begin? The question seems simple until we take a closer look at the gas surrounding galaxies, known as the circumgalactic medium.

The halo of gas surrounding the stellar disk accounts for about 70 percent of the galaxy’s mass, not including dark matter, but it has remained a mystery until now. In the past, we could only observe the gas by measuring the light from a background object, such as a quasar, that was absorbed by the gas.

This limits the cloud image to a pencil-shaped beam.

A new study, however, has observed the circumgalactic middle of a starburst galaxy 270 million light-years away, using new deep imaging techniques that were able to detect the glowing gas cloud outside the galaxy 100,000 light-years into space, as far as they could look.

To imagine the vastness of this gas cloud, consider that the starlight from the galaxy – what we usually think of as the disk – extends just 7,800 light-years from its center.

The current study observed the physical connection of hydrogen and oxygen from the center of the galaxy to deep into space and showed that the physical conditions of the gas have changed.

“We found it everywhere we looked, which was really exciting and quite surprising,” says Associate Professor Nikole M. Nielsen, lead author of the study, a researcher at Swinburne University and ASTRO 3D and an assistant professor at the University of Oklahoma.

Other authors on the study were from Swinburne, the University of Texas at Austin, the California Institute of Technology in Pasadena, the University of California, San Diego and Durham University.

“We now see where the influence of the galaxy ends, the transition where it becomes part of what surrounds it, and finally where it joins the larger cosmic web and other galaxies. These are all generally fuzzy boundaries,” says Dr. Nielsen.

“But in this case, we seem to have found a fairly clear boundary in this galaxy between its interstellar medium and its circumgalactic medium.”

The study observed stars ionizing gas with their photons within the galaxy.

“In the CGM, the gas is heated by something other than the typical conditions inside galaxies. This probably includes heating from diffuse emissions from the collective galaxies in the universe and perhaps some of the contribution is due to shocks,” says Dr. Nielsen.

“It’s this interesting change that is important and provides answers to the question of where a galaxy ends,” she says.

The discovery was made possible by the Keck Cosmic Web Imager (KCWI) on the Keck 10-meter telescope in Hawaii, which contains an integral field spectrograph and is one of the most sensitive instruments of its kind in operation.

“These unique observations require very dark skies that are only available at Keck Observatory on Mauna Kea,” said study co-author Deanne Fisher, an associate professor at Swinburne.

ASTRO 3D scientists gained access to KCWI through Swinburne University.

“Swinburne’s partnership with the WM Keck Observatory has allowed our team to really push the boundaries of what is possible,” says fellow author Associate Professor Glenn Kacprzak. “KCWI has really changed the game in terms of how we can now measure and quantify diffuse gas around galaxies.”

With this instrument, rather than making a single observation providing a single spectrum of the galaxy’s gas, scientists can now obtain thousands of spectra simultaneously with a single KCWI image.

“This is the first time we have ever managed to take a photograph of this halo of material around a galaxy,” explains Professor Emma Ryan-Weber, director of ASTRO 3D.

This study adds a new piece to the puzzle that is one of the big questions in astronomy and galaxy evolution: How do galaxies evolve? How do they get their gas? How do they process that gas? Where does that gas go?

“The circumgalactic medium plays a critical role in the cycling of this gas,” says Dr. Nielsen. “So by being able to understand what the CGM looks like around galaxies of different types – those that are forming stars, those that are no longer forming stars, and those that are transitioning between the two – we can see differences in this gas that could be driving differences within the galaxies themselves, and changes in this reservoir could actually be driving changes within the galaxy itself.”

The study fits directly into the ASTRO 3D mission. “It helps us understand how galaxies build their mass over time,” explains Professor Ryan-Weber.

These results could also have implications for how different galaxies interact and how they might influence each other.

“It is very likely that the CGMs of our Milky Way and Andromeda are already overlapping and interacting,” says Dr. Nielsen.