Embargo: 7 pm AEDT, Friday 6 September 2024
(10 am, London time, 05:00 US Eastern Time)
The inside story of a galaxy’s long reach into space
A shroud of gas stretches up to a million light years around every galaxy and is its first interaction with the wider Universe beyond. For the first time scientists have photographed this halo of matter and examined it pixel by pixel.
If this galaxy is typical, then the study, published today in Nature Astronomy, indicates that our galaxy is already interacting with its closest neighbour, Andromeda.
Where does a galaxy end and deep space begin? It seems like a simple question until you look more closely at the gas that surrounds galaxies, known as the circumgalactic medium.
The halo of gas surrounding the stellar disc accounts for about 70% of the mass of the galaxy – excluding dark matter – but until now has remained something of a mystery. In the past we have only been able to observe the gas by measuring the light from a background object, such as a quasar, that is absorbed by the gas.
That limits the picture of the cloud to a pencil-like beam.
A new study, however, has observed the circumgalactic medium of a star-bursting galaxy 270 million light years away, using new deep imaging techniques that were able to detect the cloud of gas glowing outside of the galaxy 100,000 light years into space, as far as they were able to look.
To envisage the vastness of that cloud of gas, consider that the galaxy’s starlight – what we would typically view as the disc – extends just 7,800 light years from its centre.
The current study observed the physical connection of hydrogen and oxygen from the centre of the galaxy far into space and showed that the physical conditions of the gas changed.
“We found it everywhere we looked, which was really exciting and kind of surprising,” says Associate Professor Nikole M. Nielsen, lead author of the paper, and a researcher with Swinburne University, and ASTRO 3D and an Assistant Professor at the University of Oklahoma.
Other authors of the paper came from Swinburne, the University of Texas at Austin, the California Institute of Technology, Pasadena, the University of California, San Diego, and Durham University.
“We’re now seeing where the galaxy’s influence stops, the transition where it becomes part of more of what’s surrounding the galaxy, and, eventually, where it joins the wider cosmic web and other galaxies. These are all usually 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 being heated by something other than typical conditions inside galaxies, this likely includes heating from the diffuse emissions from the collective galaxies in the Universe and possibly some contribution is due to shocks,” says Dr Nielsen.
“It’s this interesting change that is important and provides some answers to the question of where a galaxy ends,” she says.
The discovery has been made possible thanks to the Keck Cosmic Web Imager (KCWI) on the 10-meter Keck telescope in Hawaii, which contains an integral field spectrograph and is one of the most sensitive instruments of its kind in operation.
“These one-of-a-kind observations require the very dark sky that is only available at the Keck Observatory on Mauna Kea,” said one of the paper’s authors, Swinburne’s Associate Professor Deanne Fisher.
ASTRO 3D scientists gained access to KCWI through Swinburne University.
“Swinburne’s Partnership with the W. M. Keck Observatory has allowed our team to really push the boundaries of what is possible,” says another author, Associate Professor Glenn Kacprzak. “KCWI has really changed the game on how we can now measure and quantify the diffuse gas around galaxies.”
Thanks to the instrument, rather than making a single observation providing a single spectrum of the gas in the galaxy, scientists can now obtain thousands of spectra simultaneously with one image from KCWI.
“It is the very first time that we have been able to take a photograph of this halo of matter around a galaxy,” says Professor Emma Ryan-Weber, the Director of ASTRO 3D.
The study adds another 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 huge role in that cycling of that gas,” says Dr Nielsen. “So, being able to understand what the CGM looks like around galaxies of different types – ones that are star-forming, those that are no longer star-forming, and those that are transitioning between the two –we can observe differences in this gas, which might drive the differences within the galaxies themselves, and changes in this reservoir may actually be driving the changes in the galaxy itself.”
The study speaks directly to the ASTRO 3D’s mission. “It helps us understand how galaxies build mass over time,” says Professor Ryan-Weber.
The findings could also hold implications for how different galaxies interact and how they might impact each other. “It’s highly likely that the CGMs of our own Milky Way and Andromeda are already overlapping and interacting,” says Dr Nielsen.
Contacts
Tom Carruthers +61 404 404 026, tom@scienceinpublic.com.au
Niall Byrne +61 417 131 977, niall@scienceinpublic.com.au
Interviews
Dr Nikole (Nikki) Nielsen (ASTRO 3D)
E. nikkimnielsen@gmail.com
Paper at https://www.nature.com/articles/s41550-024-02365-x
Read on for the abstract and author list.
Images
• Visualisation of the gas shroud of starburst galaxy IRAS 08339+6517, credit Cristy Roberts ANU/ASTRO 3D
• Dr Nikole (Nikki) Nielsen visiting Keck in Hawaii
ABOUT ASTRO 3D
The ARC Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D) is a $40m Research Centre of Excellence funded by the Australian Research Council (ARC) and nine collaborating Australian universities – The Australian National University, The University of Sydney, The University of Melbourne, Swinburne University of Technology, The University of Western Australia, Curtin University, Macquarie University, The University of New South Wales, and Monash University.
https://astro3d.org.au/
Authors and abstract
An emission map of the disk-circumgalactic medium transition in starburst IRAS 08339+6517
Nikole M. Nielsen1,2,3*, Deanne B. Fisher1,2, Glenn G. Kacprzak1,2, John Chisholm4, D. Christopher Martin5, Bronwyn Reichardt Chu1,2,6,7, Karin M. Sandstrom8, Ryan J. Rickards Vaught8
- *Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC, 3130, Australia.
- The Australian Research Council Centre of Excellence for All Sky Astrophysics in 3 Dimensions (ASTRO 3D), Australia.
- Homer L. Dodge Department of Physics and Astronomy, The University of Oklahoma, 440 W. Brooks St., Norman, OK 73019, USA.
- Department of Astronomy, University of Texas at Austin, Austin, TX, 78712, USA.
- Cahill Center for Astrophysics, California Institute of Technology, Pasadena, CA, USA.
- Centre for Extragalactic Astronomy, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
- Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK.
- Department of Astronomy & Astrophysics, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA, 92093, USA.
*Corresponding author(s). E-mail(s): nikkimnielsen@gmail.com
Most of a galaxy’s mass is located out to hundreds of kiloparsecs beyond its stellar component. This diffuse reservoir of gas, the circumgalactic medium (CGM), acts as the interface between a galaxy and the cosmic web that connects galaxies. We present kiloparsec-scale resolution integral field spectroscopy of emission lines that trace cool ionized gas from the center of a nearby galaxy to 30 kpc into its CGM. We find a smooth surface brightness profile with a break in slope at twice the 90% stellar radius. The gas also transitions from being photoionized by Hii star-forming regions in the disk to being ionized by shocks or the extragalactic UV background at larger distances. These changes represent the boundary between the interstellar medium (ISM) and the CGM, revealing how the dominant reservoir of baryonic matter directly connects to its galaxy.