Over the course of more than a decade, astronomers used the Hubble Space Telescope to gather up 2.5 billion pixels’ worth of imagery focusing on the Andromeda galaxy — and the results could provide clues to the evolutionary history of our galaxy’s celestial neighbor.
The panoramic mosaic of the Andromeda galaxy was unveiled last week in Maryland at the winter meeting of the American Astronomical Society, and in an accompanying research paper published in The Astrophysical Journal.
It’s not just a pretty picture. Hubble was able to resolve more than 200 million of the galaxy’s stars. “This detailed look at the resolved stars will help us piece together the galaxy’s past merger and interaction history,” University of Washington astronomer Benjamin Williams, principal investigator for the project, said in a news release.
The Andromeda galaxy is 2.5 million light-years away, which makes it the nearest large-scale galaxy outside our own Milky Way. Although both galaxies have a spiral structure, there are significant differences. The Milky Way is thought to have somewhere between 100 billion and 400 billion stars, while astronomers estimate that the Andromeda galaxy has a trillion stars — including hundreds of billions of stars that are too small to be resolved by the Hubble Space Telescope.
Researchers say Andromeda seems to be more highly populated with younger stars and unusual features such as coherent streams of stars. Such characteristics suggest that the Andromeda galaxy has been more active than the Milky Way when it comes to recent star formation and interactions with other galaxies.
The newly released mosaic was assembled from about 600 pictures that were collected in near-ultraviolet, visible and near-infrared wavelengths. The first phase of image collection was part of the Panchromatic Hubble Andromeda Treasury program. That phase focused on the northern region of the Andromeda galaxy. The second phase, known as the Panchromatic Hubble Andromeda Southern Treasury, focused on the galaxy’s southern half.
Andromeda’s southern region has a structure that’s different from that of the northern region. Astronomers say it appears to be more disturbed by galactic mergers — including a major merger event that occurred 2 billion to 4 billion years ago.
“The asymmetry between the two halves — now visually evident in this image — is incredibly intriguing,” said study lead author Zhuo Chen, a postdoctoral researcher at the University of Washington. “It’s fascinating to see the detailed structures of an external spiral galaxy mapped over such a large, contiguous area.”
Chen said the newly released image of Andromeda “sets a new benchmark for precision studies of large spiral galaxies.” She and her colleagues are already using the data they’ve collected to get a better sense of the galaxy’s star formation history.
In addition to Chen and Williams, the authors of “PHAST. The Panchromatic Hubble Andromeda Southern Treasury. I. Ultraviolet and Optical Photometry of Over 90 Million Stars in M31” include Dustin Lang, Andrew Dolphin, Meredith Durbin, Julianne Dalcanton, Adam Smercina, Leo Girardi, Claire Murray and Eric Bell.
