Thanks to painstaking observations and computer modeling, astronomers have discovered that the Milky Way Galaxy is littered with the debris of stellar cannibalism. Small galaxies unlucky enough to be captured by our powerful gravity get torn to shreds and consumed.
It’s sometimes hard to sort out our galaxy’s complex history because we are buried in the midst of it — an astronomical “can’t see the forest through the trees” dilemma. So astronomers have turned to our closest neighbor in the Local Group of galaxies, Andromeda (M31), for a different perspective.
Recently I was updating a feature story about galactic cannibalism by Astronomy Contributing Editor Ray Jayawardhana, “How the Milky Way devours its neighbors.” It appears in our new collector’s edition, The Milky Way Inside and Out on newsstands June 16.
Go inside The Milky Way Inside and Out.
In the course of making sure the research in the article was as current as possible, I had the good fortune of corresponding with Puragra (Raja) Guhathakurta of the University of California, Santa Cruz. He generously brought me up to speed on research in the past decade on Andromeda’s violent history.
Studies suggest that Andromeda has consumed a dwarf galaxy within the past 1 to 2 billion years, possibly a disk-shaped galaxy. Astronomers have found a trail of stars, called the Giant Southern Stream (GSS), falling toward Andromeda’s massive center from behind the galaxy.
In a series of published studies since 2006, Guhathakurta and his students and collaborators have connected the GSS to several other features, called “shelves,” within Andromeda’s disk of stars and gas. They believe the GSS and the shelves mark a continuous trail left behind by the doomed dwarf. Read onward for more details:
PENDICK: When and how did you get interested in the history of the Andromeda Galaxy?
GUHATHAKURTA: I first got interested in the Andromeda Galaxy in the early 1990s. My longtime friend and fellow astronomer Somak Raychaudhury got me hooked. At the time, he was finishing up his Ph.D. on the Local Group at the Institute of Astronomy (IAS) in Cambridge, and I was a postdoc at the Institute for Advanced Studies (IAS) in Princeton, fresh out of graduate school at Princeton University.
Our initial interest in Andromeda involved using it as a tool to pin down the distance scale in the universe and thereby measure the Hubble constant, which in turn relates to the expansion rate of the universe.
Andy Gould, my fellow postdoc at the IAS, got me interested in using Andromeda as a test bed to study the formation and evolution of large galaxies like our own. I continued to get more and more interested in this latter aspect of Andromeda — the history of its formation and evolution — after I moved to UC-Santa Cruz as an assistant professor. Two of my early students, David Reitzel and Phil Choi, carried out their Ph.D. thesis research on this general topic.
PENDICK: Now let's talk a little about the Giant Southern Stream (GSS). Rodrigo Ibata and his collaborators discovered this apparent flow of stars into the Andromeda Galaxy. At the time no one knew for sure what it was, right? What possibilities were you and other researchers thinking about at that time?
GUHATHAKURTA: When Ibata’s group published their first star-count map of Andromeda in 2001, their survey had covered only a relatively small region on the southeastern side of the galaxy. The most prominent feature of their map was a concentration of stars forming a linear streak protruding from the inner parts of the galaxy — the Giant Southern Stream (GSS).
The authors naturally concluded that this stream consisted of the remains of a dwarf galaxy, destroyed by the gravitational (tidal) forces of the Andromeda Galaxy. It was unclear whether the progenitor dwarf galaxy had been completely disrupted or if the central core of the progenitor was still intact.
Around that time, Phil Choi, my Ph.D. student at UCSC, was finishing up a paper on the structure of the nearby Andromeda satellites M32 and NGC 205 showing clear signs of tidal distortion in their outer parts. We added to speculation by Ibata and his colleagues that the GSS may consist of stars ripped out of M32 or NGC 205. There was no hard evidence to support such an association — and such evidence has never been found.
PENDICK: OK, so now we are at the point (in 2001) where people had discovered the Giant Southern Stream but weren’t sure what it is. What have scientists discovered since then?
GUHATHAKURTA: When Ibata and his collaborators reported the discovery of the GSS, they realized what it was. It was pretty obvious that the GSS represented material from a tidally disrupted dwarf satellite galaxy. What wasn't clear yet was which dwarf satellite galaxy. That still isn't clear today.
The following year (2002), our group and Ibata’s started obtaining spectra of individual stars in the GSS to determine their motions. Both groups reported that the stars in the GSS were moving toward us at a higher speed than the speed at which the center of Andromeda was moving toward us. Around that time, Alan McConnachie reported that the GSS was farther away from us than the center of Andromeda. That is, the GSS was behind Andromeda.
PENDICK: What did that mean?
GUHATHAKURTA: The implication of the speed measurements was that the GSS was falling into Andromeda from the back/far side.
PENDICK: And people started to find other features — other debris from the dwarf captured and disrupted by Andromeda.
GUHATHAKURTA: In 2002, Annette Ferguson, Ibata, and other collaborators published a paper showing other concentrations of stars within Andromeda, including the Northeast Shelf. A couple of years later, our group pointed out the existence of the Western Shelf in the map Ferguson and the others made. In the same paper, we presented a computer model in which the GSS was connected to the Northeast and Western shelves as a continuous stream.
The model also predicted that there should be a Southeast shelf present as well. A few months later, we found a definite pattern in our set of stellar velocities indicating the presence of the Southeast shelf. The pattern we saw in our data set was exactly the pattern predicted by our computer model.
PENDICK: What’s left to do to understand these features like the GSS? What are you and your students up to right now?
GUHATHAKURTA: For the GSS itself, it is important to carry out a detailed, statistically robust, forensic reconstruction of this cosmic collision. Such an exercise can tell us what the dwarf galaxy progenitor of the GSS looked like BEFORE it got tidally shredded by Andromeda — i.e. a glimpse into its history. For example, did the dwarf galaxy progenitor consist of a rotating disk of stars? Did it contain gas in the form of neutral hydrogen, and could that explain some of the gas astronomers have recently seen in the vicinity of the GSS?
More generally, it is important to understand how unique such forensic reconstructions are. How precisely can one pin down the orbit and internal properties of the progenitor? This question can be applied to the GSS but should also be applied to debris from other merger events in Andromeda. Astronomers have long known about similar features in other galaxies, and these are starting to get studied in detail.
My students and I are actively studying the many different stages of the hierarchical process through which a large galaxy like Andromeda or the Milky Way is built up from smaller systems of stars (dwarf galaxies). In the context of galactic cannibalism, we are studying the digested material that forms the large belly of Andromeda the cannibal (its smooth stellar halo), the relation of this belly to the cannibal's other internal organs (disk, bulge/inner spheroid), partially digested entrails (GSS and other tidal streams), and the survivors (present day intact dwarf satellite galaxies).
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See more images of the Andromeda Galaxy (M31) in our Online Reader Gallery.