(ORDO NEWS) — Most galaxies have bulges at their centers. Stars in classical bulges move a little erratically, resembling elliptical galaxies and appear older than their galaxy, while stars in pseudo bulges move by rotation, as in spiral galaxies, and do not differ in age from their galaxy.
Astronomers use the galaxy’s stellar halo as a “fossil” to study these bulges. A stellar halo can, for example, tell astronomers whether a galaxy has merged with another in the past.
“Astronomers believe that when a galaxy merges with another, it results in the deposition of material in the stellar halo of the galaxy it merged with,” said Katya Gozman, lead author of the study.
“By exploring and studying the stars and the stellar population in the stellar halo, we can study and find information about past galaxy mergers. We can say that we are engaged in extragalactic archeology in the stellar halo around M94.
But Gozman and her co-authors found no evidence of a massive merger in the galaxy’s history. Instead, a smaller merger most likely occurred when a galaxy the size of the Small Magellanic Cloud – a dwarf galaxy about three times smaller than the Milky Way – crashed into M94.
Gozman used observations from the Subaru Hyper Suprime-Cam in Hawaii to view M94’s stellar halo, a diffuse halo of stars surrounding the galaxy.
The stellar halo extends far beyond the galaxy, and astronomers can explore it to find remnants of past mergers. One way to learn about these remnants is to calculate the mass of the galaxy’s halo.
In particular, she looked at a type of star called a red-branch star, or RGB star. These stars glow, which is an advantage for imaging them, Gozman said, and the red or blue color of a star is highly correlated with what heavy metal elements they contain.
“We use the mass of the halo to determine the mass of the galaxy that was the last to crash into the galaxy we are studying,” Gozman says.
“You would think that if a really large galaxy crashed into M94 a long time ago, it could have changed the morphology and components of the galaxy significantly.”
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