Andromeda 'Puffed Up' with Old Stars


Gorgeous, spiral structures seem to dominate the universe of galaxies around us at the present time. However, as we well know around here, galaxies also collide and interact in beautiful and shape-warping ways. So how do galaxies keep such neat disks for long periods of time? The answer may lie in the fact that the disks are not so neat after all.

When you look at the Andromeda Galaxy above, the bright disk of stars with a spiral pattern is immediately apparent. You may also notice that there is a spherical “bulge” of stars at the center of that disk.

ANALYSIS: Star Birth Watered-Down in Galaxies with Bars

Less apparent, but well known to astronomers, is the large, spherical halo around the disk that contains the oldest stars. Astronomers have to look very carefully to spot yet another component: the “thick disk.”

When gas falls into a galaxy, it tends to spin faster and faster, creating a thin disk of material from which stars form. However, if the stars are perturbed by interacting with each other, or interactions with other galaxies, over billions of years, they may stray from that thin layer, “puffing out” to form a thicker disk component.

Finding such a sparsely populated component is not easy however, and the existence of a thick disk on our Milky Way Galaxy is hotly debated. So, a group led by Michelle Collins, Ph.D. student at the Institute of Astronomy in Cambridge, looked to our nearest spiral neighbor.


The Andromeda Galaxy is often called the twin to our galaxy and allows us a close-up, but still outsider’s view, of the whole structure. Since the galaxy is not seen edge-on from our vantage point, the group studied the motions of the stars in a region of Andromeda and carefully teased out a sample that had statistically different motions.

This population of stars is consistent with a thick disk model and can help shed some light into thick disk formation and prevalence in spiral galaxies.

For example, these stars had on average less of the elements heavier than hydrogen and helium, dating their birth back to an earlier time than the average for the thin disk. It’s not yet clear from this first data set how these stars got into a thick disk after all, but observations are already being planned for more sections of Andromeda to search for more evidence of the thick disk.

No need to rush, of course. It’ll be another 5 billion years or so before the Milky Way crashes into Andromeda, making a whole mess of the stellar populations for quite some time to come!

Images: Top – The Andromeda Galaxy. Credit – Robert Gendler. Center – Diagram showing the components of a galaxy such as Andromeda. Credit – Amanda Smith, IoA graphics officer.

This work has been published in the Monthly Notices of the Royal Astronomical Society, and a preprint is available on astro-ph. More information is available on Collins’s website.

Thanks to my colleage and office-neighbor Gail Zasowski for reminding me of all we learned about thick disks back in classes!

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