Image Credit: Robert Gendler

More than a millennia ago, Andromeda was described as a small, patchy cloud occupying a section our the night sky. Flash forward to 1887, when English astronomer Isaac Roberts took the first long-exposure image of the "Andromeda Nebula," revealing it is not a nebula at all, but a large, bustling galaxy with billions upon billions of stars. Surely Abd al-Rahman al-Sufi (an Iranian scientists who was likely the first person to know of Andromeda's existence), Charles Messier, and Roberts himself would be astonished that we can now resolve individual stars and see its halo. 

In a new study, after analyzing Andromeda's disk, and looking at the motions' of multiple populations of stars, researchers have learned that there is stark contrast between its disk and the Milky Way. This ultimately tells the tale, that Andromeda's past was more turbulent than we previously believed, with it undergoing exponentially more collisions and mergers than our own galaxy.

By analyzing the structure and internal motions of a galaxy's disk, we can unlock a lot of information about the way in which a spiral galaxy formed and evolved. Understanding Andromeda is paramount to understanding our own history, for not only is Andromeda similar to the Milky Way, but our fates are intrinsically linked.

Puragra Guhathakurta, professor of astronomy and astrophysics at the University of California, Santa Cruz, explains: "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."

The Research: 

For the purpose of this study, the team utilized the datasets from two detailed surveys focused on Andromeda; the 'SPLASH' survey (short for 'Spectroscopic and Photometric Landscape of Andromeda's Stellar Halo') and Hubble's 'PHAT' Survey. The former, which was carried out by Keck's Deep Imaging Multi-Object Spectrograph, looked at the individual radial velocity — or the change in the distance from the Sun to a star — of around 10,000 of Andromeda's stars.

Stars included in the spectroscopic survey have been superimposed on an image of Andromeda. (Dots are color coded according to their velocity relative to the Milky Way) (Credit: Claire Dorman and ESA)

When paired with PHAT (otherwise known as the 'Panchromatic Hubble Andromeda Treasury' survey), we can see around half of these stars, which vary in age, in unprecedented detail, from ultraviolet through the near infrared wavelengths.

"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," remarked Dorman. The team also notes that this is the first time anything of this magnitude has been attempted.

What Did They Find?

When Dorman and the team announced their findings at the January 8th American Astronomical Society meeting, they revealed that they found a bonafide link between the age of stars and rotational motion. The younger stars orbit Andromeda's core in an ordered manner, while the older ones are more spontaneous. "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," according to the team.

"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 explained.

Artist rendering of Andromeda's thick disk structure, a place where stars are much older than those in the thin disk (Credit: Amanda Smith, IoA graphics officer)

In order to explain their observations, the researchers developed two very different scenarios. One of which suggests that, over time, the ordered stars were gradually integrated into the disk from satellite galaxies Andromeda absorbed. The other suggests the groundwork for the configuration of these stars was laid very early on in Andromeda's history, namely when it first formed,. At the time, its disk was thick and rather clumpy. The older stars took shape when the disk was still in its initial disordered state, but as it settled, the disk became thinner and more ordered, with the younger stars popping up then.

Of the two scenarios, the first has more evidence backing it. In several other studies, researchers have pinpointed several tidal streams of material  surrounding Andromeda. Computer simulations and observations both support the notion that these streams of material (mostly gas, dust and stars) come from mergers and other gravitational disturbances. However, Dorman made it clear that gradual accretion can't solely account for the distinct velocity dispersion in young and old stellar populations. As such, Dorman suggests that it might not be as simple as "either or," that a combination of both could work. 

Turbulent Past:

When it boils down to it, by comparing this dataset to that of our galaxy's disk, we see that Andromeda's past (recent and distant) was far more bloody than the Milky Way's. "Even the most well ordered Andromeda stars are not as well ordered as the stars in the Milky Way's disk," Dorman said.

As a final note, "In the currently favored 'Lambda Cold Dark Matter' paradigm of structure formation in the universe, large galaxies such as Andromeda and the Milky Way are thought to have grown by cannibalizing smaller satellite galaxies and accreting their stars and gas. Cosmologists predict that 70 percent of disks the size of Andromeda's and the Milky Way's should have interacted with at least one sizable satellite in the past 8 billion years. The Milky Way's disk is much too orderly for that to have happened, whereas Andromeda's disk fits the prediction much better." [Reference: "Study of Andromeda's stellar disk indicates more violent history than Milky Way," UCSC]

"In this context, the motion of the stars in Andromeda's disk is more normal, and the Milky Way may simply be an outlier with an unusually quiescent accretion history," Guhathakurta concluded. 

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