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Enter ‘Galactic Archaeology’

I’ve used the term ‘interstellar archaeology’ enough for readers to know that I’m talking about new forms of SETI that look for technological civilizations through their artifacts, as perhaps discoverable in astronomical data. But there is another kind of star-based archaeology that is specifically invoked by the scientists behind GALAH, as becomes visible when you unpack the acronym — Galactic Archaeology with HERMES. A new $13 million instrument on the Anglo-Australian Telescope at Siding Spring Observatory, HERMES is a high resolution spectrograph that is about to be put to work.


Image: I can’t resist running this beautiful 1899 photograph of M31, then known as the Great Andromeda Nebula, when talking about our evolving conception of how galaxies form. Credit: Isaac Roberts (d. 1904), A Selection of Photographs of Stars, Star-clusters and Nebulae, Volume II, The Universal Press, London, 1899. Via Wikimedia Commons.

And what an instrument HERMES is, capable of providing spectra in four passbands for 392 stars simultaneously over a two degree field of view. What the project’s leaders intend is to survey one million stars by way of exploring how the Milky Way formed and evolved. The idea is to uncover stellar histories through the study of their chemistry, as Joss Bland-Hawthorn (University of Sydney) explains:

“Stars formed very early in our galaxy only have a small amount of heavy elements such as iron, titanium and nickel. Stars formed more recently have a greater proportion because they have recycled elements from other stars. We reach back to capture this chemical state – by analysing the mixture of gases from which the star formed. You could think of it as its chemical fingerprint – or a type of stellar DNA from which we can unravel the construction of the Milky Way and other galaxies.”

Determining the histories of these stars with reference to 29 chemical signatures as well as stellar temperatures, mass and velocity should help the researchers create a map of their movements over time. This should be a fascinating process, for views of galaxy formation have changed fundamentally since the days when Allan Sandage and colleagues proposed (in 1962) that a protogalactic gas cloud that settled into a disk could explain galaxies like the Milky Way.

That concept suggested that the oldest stars in the galaxy were formed from gas that was being drawn toward the galactic center, collapsing from the halo to the plane, and in Sandage’s view, this collapse was relatively rapid (on the order of 100 million years), with the initial contraction beginning roughly ten billion years ago. Later we begin to see a different model developing, one in which the galaxy formed through the agglomeration of smaller elements like satellite galaxies. Both these processes are now believed to play a role, with infalling satellite systems affecting not just the galactic halo but also the disk and bulge.


Image: Structure of the Milky Way, showing the inner and outer halo. Credit: NASA, ESA, and A. Feild (STScI).

Galactic archaeology is all about detecting the debris of these components, making it possible to reconstruct a plausible view of the proto-galaxy. How the galactic disk and bulge were built up is the focus, determined by using what the researchers call ‘the stellar relics of ancient in situ star formation and accretion events…’ The authors explain the challenges they face:

… unraveling the history of disc formation is likely to be challenging as much of the dynamical information such as the integrals of motion is lost due to heating. We need to examine the detailed chemical abundance patterns in the disc components to reconstruct [the] substructure of the protogalactic disc. Pioneering studies on the chemodynamical evolution of the Galactic disc by Edvardsson et al. (1993) followed by many other such works (e.g. Reddy et al. 2003; Bensby, Feltzing & Oey 2014), show how trends in various chemical elements can be used to resolve disc structure and obtain information on the formation and evolution of the Galactic disc, e.g. the abundances of thick disc stars relative to the thin disc. The effort to detect relics of ancient star formation and the progenitors of accretion events will require gathering kinematic and chemical composition information for vast numbers of Galactic field stars.

Two days ago we looked briefly at globular clusters and speculated on what the view from a planetary surface deep inside one of these clusters might look like. The globular clusters, part of the galaxy’s halo, contain some of its oldest stars, and the entire halo is poor in metals. Going into the GALAH survey, the researchers believe that a large fraction of the halo stars are remnants of early satellite galaxies that evolved independently before being acquired by the Milky Way, a process that seems to be continuing as we discover more dwarf satellites and so-called ‘stellar streams,’ associations of stars that have been disrupted by tidal forces.

Seventy astronomers from seventeen institutions in eight countries are involved in GALAH, which is led by Bland-Hawthorn, Gayandhi De Silva (University of Sydney) and Ken Freeman (Australian National University). Their work should give us much new information not just about the halo and globular clusters but the interactions of stars throughout the disk and central bulge. The paper on the project is De Silva et al., “The GALAH survey: scientific motivation,” Monthly Notices of the Royal Astronomical Society Vol. 449, Issue 3 (2015), pp. 2604-2617 (abstract). A University of Sydney news release is also available.

And in case you’re interested, the classic paper by Sandage et al. is “Evidence from the motions of old stars that the Galaxy collapsed,” Astrophysical Journal, Vol. 136 (1962), p. 748 (abstract).


Comments on this entry are closed.

  • RobFlores April 9, 2015, 13:39

    I think one of the great puzzles about galaxies is their associated globular
    clusters. In particular.

    1) Why didn’t they become incorporated into the main body
    as dwarf galaxies are prone to. They have been around a long time.

    2) It is mentioned they tend orbit close to the galactic core.
    Does this mean that they cross the main body of the galaxy on occasion.
    Because, I don’t see how a gravity bound object could orbit the central bulge of our galaxy above the plane of the galaxy w/o crossing into the bulge

    3) Do galaxy merging tend to destroy them and leave only ‘lucky’
    Clusters intact. Do we have examples suspected Star Streams that be part of
    a former cluster.

    4) Are their orbits also part of the reason we need Dark Matter, which explains the main galaxy’s structure that normal matter cannot account for.
    (Dark Matter is one of the more odious place holders in modern astronomy IMO)

  • Harold Daughety April 9, 2015, 15:28

    RE: RobFlores on dark matter
    Dark matter reminds me of the ancient cartographer’s notation on the unknown fringes of their maps: “Here Be Dragons.”
    There seems to be a problem with the previously Respected Model of the cosmos. So a solution is to create a new concept, a substance, an energy: a fudge factor to make the cosmos fall in line with the Respected Model. Fact is, there never were any dragons, just ignorance, and one might suspect, arrogance.

  • ljk April 9, 2015, 15:29

    Our Sun Came Late to the Milky Way’s Star-Birth Party

    April 9, 2015

    In one of the most comprehensive multi-observatory galaxy surveys yet, astronomers find that galaxies like our Milky Way underwent a stellar “baby boom,” churning out stars at a prodigious rate, about 30 times faster than today.

    Our sun, however, is a late “boomer.” The Milky Way’s star-birthing frenzy peaked 10 billion years ago, but our sun was late for the party, not forming until roughly 5 billion years ago. By that time the star formation rate in our galaxy had plunged to a trickle.

    Missing the party, however, may not have been so bad. The sun’s late appearance may actually have fostered the growth of our solar system’s planets. Elements heavier than hydrogen and helium were more abundant later in the star-forming boom as more massive stars ended their lives early and enriched the galaxy with material that served as the building blocks of planets and even life on Earth.

    Astronomers don’t have baby pictures of our Milky Way’s formative years to trace the history of stellar growth so they studied galaxies similar in mass to our Milky Way, found in deep surveys of the universe. The farther into the universe astronomers look, the further back in time they are seeing, because starlight from long ago is just arriving at Earth now. From those surveys, stretching back in time more than 10 billion years, researchers assembled an album of images containing nearly 2,000 snapshots of Milky Way-like galaxies.

    The new census provides the most complete picture yet of how galaxies like the Milky Way grew over the past 10 billion years into today’s majestic spiral galaxies. The multi-wavelength study spans ultraviolet to far-infrared light, combining observations from NASA’s Hubble and Spitzer space telescopes, the European Space Agency’s Herschel Space Observatory, and ground-based telescopes, including the Magellan Baade Telescope at the Las Campanas Observatory in Chile.

    “This study allows us to see what the Milky Way may have looked like in the past,” said Casey Papovich of Texas A&M University in College Station, lead author on the paper that describes the study’s results. “It shows that these galaxies underwent a big change in the mass of its stars over the past 10 billion years, bulking up by a factor of 10, which confirms theories about their growth. And most of that stellar-mass growth happened within the first 5 billion years of their birth.”

    Full article here:


    To quote:

    The new analysis reinforces earlier research which showed that Milky Way-like galaxies began as small clumps of stars. The galaxies swallowed large amounts of gas that ignited a firestorm of star birth.

    The study reveals a strong correlation between the galaxies’ star formation and growth in stellar mass. So, when the galaxies slow down making stars, their growth decreases as well. “I think the evidence suggests that we can account for the majority of the buildup of a Milky Way-like galaxy through its star formation,” Papovich said. “When we calculate the star-formation rate of a Milky Way-like galaxy in the past and add up all the stars it would have produced, it is pretty consistent with the mass growth we expected. To me, that means we’re able to understand the growth of the ‘average’ galaxy with the mass of a Milky Way galaxy.”

    The astronomers selected the Milky Way-like progenitors by sifting through more than 24,000 galaxies in the entire catalogs of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), taken with Hubble, and the FourStar Galaxy Evolution Survey (ZFOURGE), made with the Magellan telescope.

    They used the ZFOURGE, CANDELS, and Spitzer near-infrared data to study the galaxy stellar masses. The Hubble images from the CANDELS survey also provided structural information about galaxy sizes and how they evolved. Far-infrared light observations from Spitzer and Herschel helped the astronomers trace the star-formation rate.

    The team’s results will appear in the April 9 issue of The Astrophysical Journal.

    For images and more information about the Hubble Space Telescope, visit:

    http://www.nasa.gov/hubble or http://hubblesite.org/news/2015/11

  • ljk April 9, 2015, 16:14

    Speaking of celestial archaeology:


    I wonder how many times astronomers have witnessed something artificial out there that we just do not recognize as such yet?

  • Ron S April 9, 2015, 16:34

    These anti-DM people amuse me. Astronomers careful collect empirical data over many years and the theorists propose a solution to the data, and all now seek to confirm/disprove the theory. This is called the scientific method. What credible alternative explanation do the doubters propose, including criteria for falsification? Liking or disliking DM is utterly irrelevant.

  • Adam April 9, 2015, 17:06

    For the non-Aussies, a galah is a gregarious, noisy pink-and-white parrot. Their name is also slang for noisy, gregarious humans – as typically found on a Friday night at any Australian pub.

  • Alex Tolley April 9, 2015, 19:36

    @Ron S – there is always the Modified Newtonian dynamics (MOND) idea. It is testable too. Whether it is sufficient to explain newer observations that supposedly demonstrate DM, I don’t know.

  • Ross Turner April 9, 2015, 20:38

    I’m an anti-DM, anti-DE person as well, and I’ll stop being one as soon as they find some of either. So far they have lots of indirect evidence based on data interpretations, but not one particle of DM and not one quanta of dark energy.

    There are actually alternatives to DM and DE that other scientists are proposing, but so far they aren’t being take seriously by the majority. These aren’t crackpots or fringe scientists proposing these alternatives, but published researchers. MOND is a serious field of study, and more than one real scientist think it is a viable alternative, see this link for example:


    Check this alternative DE link out:


    It seems like we are in the dark ages of cosmology. I’m actually hoping they find DM soon, because if they don’t, the spending will continue, perhaps without results. How will someone ever prove it doesn’t exist?

    All of us who oppose or dislike DM aren’t total morons, realize that our disliking the idea is irrelevant, realize it could be true, do know (have heard about) the scientific method. But until they find some of it, some of us actually look for alternatives being proposed by other mainstream scientists, just to satisfy our own egos, nothing more. Show me an actual particle of DM, and I’ll stop disliking the idea of it so intensely.

  • Larry Kennedy April 9, 2015, 21:41

    Hurray for Harvard and the NSF. The sort of thing that doesn’t make headlines, but a huge return on the money.

  • Ron S April 9, 2015, 23:29

    Alex, there are as many variants of MOND as there are tint selections in a paint store. Yet all of them can explain some of the data but not all. MOND can be curve fit to explain some or many galactic rotation data. When it comes to other data, especially gravitational lensing, they don’t do so well. There is serious work being done on MOND so perhaps a version that does better to explain the data will appear. My own opinion is that this branch of inquiry will fail.

    Ross, have we ever detected a gravitational wave? No, we haven’t. Do you doubt their existence? Indirect detection has been done and matches theory extremely well. That we don’t know what DM is (I’ll pass on the DE discussion for now since I know far less about it) is not a argument against its existence. Its effects are measured.

    I doubt I’ll ever understand why some people have such distaste for DM and are determined to prejudge the research. It explains the data quite well. There is some real opportunity for new and interesting physics if it can be detected.

    If it turns out to be a mirage I will be happy with that, too, since we will have learned something interesting about the universe. I am happy no matter where the data leads. Others, it seems, insist on projecting their prejudices onto the universe. The universe doesn’t care about our feelings.

  • Adam April 10, 2015, 2:20

    I don’t know why there’s so much angst about Dark Matter or Dark Energy. Seems to be symptomatic of an inability to tolerate mystery in the world. But the only decent way to reduce the uncertainty is to actually find some DM and pin down “lambda” if is a new field, space-time’s internal energy, a cosmological constant, or weird systematics.

    My personal bet is that Dark Matter will prove to be a mix of quark matter and neutrinos. “Dark Energy”… might be telling us something so obvious that we’ll smack our foreheads and go “why didn’t we think of that before?”

  • xcalibur April 10, 2015, 15:03

    To my understanding, dark matter and dark energy are placeholders. Basically, the equations for how the universe works don’t balance out, so we assume that there are large amounts of unknown matter and energy present to explain our observations. It seems like a very reasonable hypothesis.

    Considering that dark matter/energy are hypothesized to be the vast majority of matter/energy in the universe, if their existence is disproven, this would bring a major change to our understanding of normal matter/energy.

  • Black Sci-Fi April 13, 2015, 1:31

    Experts have their favorite theories about DM and DE. There is, however, mathematical and observational data that would “infer” a link to some substance or unknown interaction of matter. These physical traits have to be the result of something other than a smudged telescope lens. So, hubris aside, I’m open to any reasonable theory for the issues at hand.

    And, it’ not reasonable to argue with certainty about the unknown to the exclusion of other well reasoned ideas. As always, when we hit a wall we’ll dust off an old theory and use more modern instruments and find an answer. More ideas are always better.

  • ljk April 24, 2015, 10:38

    Look what some astronomers mining celestial data found – 11 compact elliptical galaxies ejected from their primary stellar islands:


    To quote:

    Runaway stars can be ejected from their host galaxies if they are travelling at a greater speed than that galaxy’s “escape velocity.” Like a rocket leaving Earth’s gravitational well, escape velocity can only be achieved if the rocket is supplied with enough energy to exceed 11.2 kilometers per second (25,000 miles per hour). In the case of a star being ejected from our galaxy, it would need to be traveling a speed of 537 km/s (over 1.2 million miles per hour!).

    So you can probably imagine the astronomical speed an entire galaxy would need to travel to leave the gravitational heft of an entire galaxy cluster — a velocity of up to 3,000 km/s (6 million miles per hour), depending on the mass of the cluster.

    The 11 runaway galaxies were found by chance while Chilingarian and co-investigator Ivan Zolotukhin, of the L’Institut de Recherche en Astrophysique et Planetologie and Moscow State University, were scouring publicly-available data (via the Virtual Observatory) from the Sloan Digital Sky Survey and the GALEX satellite for compact elliptical galaxies.

  • Larry Kennedy April 24, 2015, 14:56

    Great stuff. Anyone know of a peer reviewed paper on these runaways?