A study of the motions of different stellar populations in the Andromeda’s disk has revealed a violent history of mergers with dwarf galaxies in the galaxy’s recent past.
Hot stars burn brightly in this image from NASA’s Galaxy Evolution Explorer, showing the UV side of a familiar face. At 2.5 million light-years away, the Andromeda galaxy is our Milky Way’s largest galactic neighbor. The galaxy spans 260,000 light-years across. The bands of blue-white making up the galaxy’s striking rings are neighborhoods that harbor hot, young, massive stars. Dark blue-grey lanes of cooler dust show up starkly against these bright rings, tracing the regions where star formation is currently taking place in dense cloudy cocoons. Eventually, these dusty lanes will be blown away by strong stellar winds, as the forming stars ignite nuclear fusion in their cores. Meanwhile, the central orange-white ball reveals a congregation of cooler, old stars that formed long ago. When observed in visible light, the galaxy’s rings look more like spiral arms. The UV image shows that these arms more closely resemble the ring-like structure previously observed in infrared wavelengths with NASA’s Spitzer Space Telescope; astronomers interpreted these rings as evidence that the galaxy was involved in a direct collision with its neighbor, Messier 32, more than 200 million years ago. Image credit: NASA / JPL-Caltech.
The study combined data from two large surveys of stars in the Andromeda galaxy (also known as Messier 31 or M31), one conducted at the W. M. Keck Observatory in Hawaii and the other using the Hubble Space Telescope.
“In the Andromeda galaxy we have the unique combination of a global yet detailed view of a galaxy similar to our own. We have lots of detail in our own Milky Way, but not the global, external perspective,” said study co-author Prof Puragra Guhathakurta of the University of California, Santa Cruz.
“The high resolution of the Hubble images allows us to separate stars from one another in the crowded disk of Andromeda, and the wide wavelength coverage allows us to subdivide the stars into sub-groups according to their age,” added study lead author Claire Dorman, a graduate student at the University of California, Santa Cruz.
The study presents the velocity dispersion of young, intermediate-age, and old stars in the disk of Andromeda, the first such measurement in another galaxy. It revealed a clear trend related to stellar age, with the youngest stars showing relatively ordered rotational motion around the center of the galaxy, while older stars displayed much more disordered motion.
Stars in a well-ordered population are all moving coherently, with nearly the same velocity, whereas stars in a disordered population have a wider range of velocities, implying a greater spatial dispersion.
“If you could look at the disk edge on, the stars in the well-ordered, coherent population would lie in a very thin plane, whereas the stars in the disordered population would form a much puffier layer,” Dorman said.
The team considered different scenarios of galactic disk formation and evolution that could account for their observations.
One scenario involves the gradual disturbance of a well-ordered disk of stars as a result of mergers with small satellite galaxies.
“Previous studies have found evidence of such mergers in tidal streams of stars in the extended halo of Andromeda, which appear to be remnants of cannibalized dwarf galaxies. Stars from those galaxies can also accrete onto the disk, but accretion alone cannot account for the observed increase in velocity dispersion with stellar age,” Dorman explained.
An alternate scenario involves the formation of the stellar disk from an initially thick, clumpy disk of gas that gradually settled. The oldest stars would then have formed while the gas disk was still in a puffed up and disordered configuration. Over time, the gas disk would have settled into a thinner configuration with more ordered motion, and the youngest stars would then have formed with the disk in a more ordered configuration.
“A combination of these mechanisms could account for the observations. Our findings should motivate theorists to carry out more detailed computer simulations of these scenarios,” Dorman said.
“The comparison to the Milky Way revealed substantial differences suggesting that Andromeda has had a more violent accretion history in the recent past. Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way’s disk.”
The results were presented January 8 at the 225th Meeting of the American Astronomical Society in Seattle, Washington.
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