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The Galactic Bulge: A Single Burst of Star Formation?

Discussions of the Milky Way’s center have always attracted me. Here we find ancient stars, with all that suggests about the possibility of long-lived civilizations, but occurring in a place where ionizing radiation associated with the galaxy’s supermassive black hole (Sagittarius A*) may push the habitable regions out into the galactic suburbs. And then there are all those supernovae to contend with! Maybe life in these parts is, if present, single-celled, perhaps underwater, with brief land colonization before new extinction events erase it.

Whatever the case, I’m learning that the galaxy’s central bulge is inspiring a new wave of study. A survey of millions of stars there is producing insights into how the bulge originally formed. We can look at spiral galaxies near and far and find commonality in their central bulge of stars surrounded by the familiar disk, but if the bulge stars formed in a single burst of activity, we could be seeing a population as old as 10 billion years in our home galaxy. But it has also been argued that waves of star formation have occurred in the bulge, so that some stars there may constitute a population as relatively ‘young’ as three billion years, younger than the Sun.

Image: This photo looking toward the center of the Milky Way galaxy covers 0.5 by 0.25 degrees on the sky and contains over 180,000 stars. The image captures a portion of our galaxy about 220 by 110 light-years across. It was taken with the Dark Energy Camera on the Victor M. Blanco 4-meter Telescope at the Cerro-Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab. Credit: CTIO/NOIRLab/NSF/AURA/STScI, W. Clarkson (UM-Dearborn), C. Johnson (STScI), and M. Rich (UCLA).

The new study begins to untangle the contrasting theories of bulge star formation. It is in the hands of co-principal investigators Christian Johnson (Space Telescope Science Institute) and Michael Rich (UCLA), who used the wide-field Dark Energy Camera (DECam) on the 4-meter Victor M. Blanco instrument at Cerro Tololo in Chile to capture 3 square degrees of sky with each exposure. 250 million stars fall into the field of these observations. All told, more than 450,000 photographs have been produced and the chemical compositions of over 70,000 stars analyzed, as described in two papers on this work. Johnson describes the study:

“Our survey is unique because we were able to scan a continuous section of the bulge at wavelengths of light from ultraviolet to visible to near-infrared. That allows us to get a clear understanding of what the various components of the bulge are and how they fit together.”

What Johnson and Rich find is that most stars in the central 1,000 light years of the galaxy’s hub formed more than 10 billion years ago, perhaps triggered by a merger with another galaxy in formation, or else through accretion of infalling gas and dust. The work proceeded by analyzing the stars’ chemical compositions, which produced interesting results. Stars in this region have roughly the same metallicity (elements higher than hydrogen and helium) as the Sun. In other words, these ancient stars are somehow enriched in metals. Here’s Rich:

“Something different happened in the bulge. The metals there built up very, very quickly, possibly in the first 500 million years of its existence.”

Image: This image shows a wide-field view of the center of the Milky Way with a pull-out image taken by the Dark Energy Camera (DECam) at the Cerro-Tololo Inter-American Observatory in Chile. Credit: Milky Way photo: Akira Fujii; Inset photo: CTIO/NOIRLab/NSF/AURA/STScI, W. Clarkson (UM-Dearborn), C. Johnson (STScI), and M. Rich (UCLA).

Measuring stellar brightness at different wavelengths — near-ultraviolet, optical, and near-infrared — is the key to the researchers’ method, with the ultraviolet data taking precedence. Differences in brightness depending on wavelength define photometric colors, which can be used to reveal the composition of stars when the data are calibrated against spectroscopic measurements. Stars forming at different times, we would expect, would show different levels of metallicity, but in this 1,000 light-year region, the distribution of metals is similar. Theories suggesting multiple phases of star formation over billions of years seem ruled out. From the paper:

…little physical motivation exists regarding why the bulge should be a composite of two or more populations that each have a relatively narrow, normally distributed metallicity distribution function. Systems composed of two or more populations with narrow, normally distributed metallicity distributions do exist in nature, but such objects often show extreme heavy element abundance variations that are indicative of a prolonged period of ‘bursty’ star formation and self-enrichment, particularly from low- and intermediate-mass AGB stars (e.g. ω Cen; Johnson & Pilachowski 2010; Marino et al. 2011b). The Galactic bulge shows no evidence supporting this type of enrichment pattern.

The stars analyzed in this region of the galactic bulge are enriched in metals despite their age, with a distribution of metals clustered around a single average. We seem to be looking at a single burst of star formation here. The full dataset for the Blanco DECam Bulge Survey will soon be released, offering a major resource for those investigating stellar populations within the inner disk and the galactic bulge that will be useful when combined with other imaging surveys. Up next for the authors is research into any correlations between metallicity and stellar orbits, which could uncover the remains of dwarf galaxies disrupted in this ancient phase of star formation.

The papers are Johnson et al., “Blanco DECam Bulge Survey (BDBS) II: project performance, data analysis, and early science results,” Monthly Notices of the Royal Astronomical Society 499 (3 September 2020), 2357–2379 (full text); and Rich et al., “The Blanco DECam bulge survey. I. The survey description and early results,” Monthly Notices of the Royal Astronomical Society 499 (3 September 2020), 2340–2356 (full text).


Comments on this entry are closed.

  • Ron S. October 29, 2020, 14:14

    “0.5 by 0.25 degrees (an area about twice as wide as the full Moon)”

    It’s a trivial error but since it shows up in the captions of both images I would like to point out the error. The Moon’s disk is 0.5 degrees so one or more of these numbers are wrong.

    • Alex Tolley October 29, 2020, 17:38

      Now that you have pointed that out, how does one compare a 2-dimensional area to a 1-dimensional line?

      I can see that if the Moon subtended an angle of 0.25 degrees, then the galactic image would be 2x the area of a square with sides of 0.25 degrees.

      As you say, this is wrong unless the 0.25 degrees is the radius of the Moon, not the diameter as usually used.

  • Geoffrey Hillend October 29, 2020, 15:48

    It is correct that the Moon and the Sun appear only half a degree. Interesting paper about the chaotic inner part of our galaxy.

  • Michael Fidler October 30, 2020, 6:55

    Surprisingly mature galaxies in the early universe.

    Galaxies in the infant universe were surprisingly mature.

    Two articles on the same material on the same day on phys.org makes you wonder???

    Galaxies are considered more “mature” than “primordial” when they contain a significant amount of dust and heavy elements. “We didn’t expect to see so much dust and heavy elements in these distant galaxies,” said Faisst. Dust and heavy elements (defined by astronomers as all elements heavier than hydrogen and helium) are considered to be a by-product of dying stars. But galaxies in the early universe have not had much time to build stars yet, so astronomers don’t expect to see much dust or heavy elements there either.

    So where have all the old M6 to M10 dwarfs gone???

    Makes you wonder, but there is a simple answer, Halton Arps many many mini bangs.


  • Andrew Palfreyman October 30, 2020, 10:30

    How very encouraging that these ancient stars show comparable metallicity to Sol. This appears to imply that the most ancient possible civilisations in our galaxy have a 6 billion year head start on us – a not inconsiderable figure, surely.

    • Alex Tolley October 31, 2020, 7:07

      If so, they have not apparently left any obvious visible signs of their technological presence.

      • Michael Fidler October 31, 2020, 22:29

        Because their tech does not leave any obvious visible signs of their tech. Their Tech is equal to nature meaning that have advanced enough not to disturb the natural order of the universe, ie pollution. The movie 2067 may a very good point of this.

        • Alex Tolley November 1, 2020, 11:36

          At the end of the movie, when Whyte looks over the cliff he sees a city. While the future Earth had successfully restored the lost vegetation and O2 to breathe, the city, even if it was partially living, is still a potentially recognizable structure. There were lights that should be detectable on the surface.

          Despite the time travel technology, it seems surprising that technology artifacts would not exist, such as dams and orbiting satellites. Glints from shiny structures, wakes from ships, or regular spaced hot spots from the tree dwellings might be proxy signs of technology.

          I agree that today we could not detect these technologies. However, they are in principle detectable. The world of 2067 hasn’t abandoned manufacturing AFAICS, just ensured that a living planet is sustained as a result of the conditions depicted at the start of the movie.

          We don’t know if they have built large space platforms and habitats that we might be able to detect today, or in teh very near future.

          If you have seen the movie Clara the aliens have built a moon-sized structure at L1. Ignoring the incorrect detection of that structure by the transit method in the plot, such a structure could be detected in principle by other means.

          It seems unlikely that none of the many civilizations postulated as being in the galactic bulge have not developed technologies that are detectable, unless Cixin Liu’s “Dark Forest” hypothesis is correct and all civilizations hide or cloak themselves as revealing their locations results in near-instant destruction.

          • Michael Fidler November 2, 2020, 8:44

            Yes, thanks for the movie Clara, I was surprised because I must of overlooked it. The plot was excellent and could we detect an object in a L1 orbit around an exoplanet??? Reports by astronomers in the early 1760’s were seen crossing the Sun and may have been observing Earth,. In the same time period satellite/moons were observed near Venus. L1 is a good place to hide!

            • Michael Fidler November 2, 2020, 22:50

              Interesting, I thought someone was already observing this area.
              Moon patrols could be a future reality for the U.S. military.
              by Sandra Erwin — November 2, 2020
              “It’s a brave new world for the DoD to embark on,” said Capt. David Buehler, manager of the AFRL experiment named CHPS, for Cislunar Highway Patrol System.


          • Mike Serfas November 2, 2020, 11:06

            The berserkers are a reasonable explanation, but there are others; the most cheerful being that the aliens, having understood all aspects of consciousness and thought, have translated them to the densest possible level and the most versatile organization, which is to say, there are alien consciousnesses of unimaginable potency hidden in the patterns of starlight, air, of paint on your wall, the ribosomes in your cells, each random-seeming atom secretly entangled with countless others across the universe in a vast quantum computing network even as we remain blissfully unaware.

  • wdk October 30, 2020, 22:01

    if I am not mistaken, the moon’s angular width ( as that of the sun) is about 0.5 degrees. So the narrow part of the rectangle would be about half.

    While I would not necessarily dispute that the stars could be ten billion years old, haven’t got to the part of the study where the enrichment mechanism is explained. It would seem like the “easy” answer about looking toward the galactic center and finding metallically enriched stars would be to assume that the massive objects in the central region still stir the galactic medium. Perhaps the trouble is the nucleation or condensation process in clouds of gas and dust?

  • Geoffrey Hillend October 31, 2020, 16:02

    There is a large black hole in the center of the galaxy, Sagittarius A. The idea that the gravity is stronger there so there are more heavily elements trapped there seems dubious, but there is also a fast rate star formation or starburst as the result of the stronger gravity around Sagittarius A with supernova’s forming more often there, so it would make sense that there are young stars enriched with heavy elements there. Wikipedia, Galactic Center.

  • wdk November 1, 2020, 12:40

    Delving into the paper this topic is based on, I’d say that it is one of the more difficult ones I’ve seen on Centauri-dreams to follow. My summary thus far would be that a lot of effort was taken to examine 250 million stars within a couple hundred light year box near galactic center and streams of stars had different metallicities. Somewhere in the midst of this emerge some stars with metallicities similar to our sun’s or the sun’s “population”. But the hypothesis that the medium had been long enriched by Sagittarius X -1 proximity or activity does not leap out at me. Taking something of a devil’s advocacy, other possibilities strike me:
    1. Could it be stars of a similar age as the sun enriched by the same medium?
    2. Could it be stars annexed by wandering paths froom the disk?

    And, if Sagittarius X-1 is influencing stellar evolution, how wide a
    net does it cast? Several hundred light years? While some of the recent studies of Sag X have examined its ability to perturb stars with several year or decade elliptic orbits ( e.g., similar to the sun on Mercury), perhaps more attention should be directed to those captured items chemical content. Or since Sag is relatively inactive now, what would be the consequences of a phase similar to M-87 (?) where there are polar jets fueled by accretion disks.

    But most of this speculation on my part is the “odd” idea that observations of Bulge stars suggest arrival at the same point as Disk stars, except with a very different, complicated path. Not exactly
    an Occam’s Razor illustration for students.

  • Michael Fidler November 3, 2020, 11:00

    This may be the reason they are not talking to us:

    Neanderthals And Humans Were at War For Over 100,000 Years, Evidence Shows.


    • ljk November 5, 2020, 15:16

      Did species on other worlds evolve as saints and angels right from the start? Or did they too go through a rough adolescence? I am hard pressed to think of many terrestrial species that have not had to fight and defend their way up the evolutionary ladder.

      There may be highly advanced beings who are the epitome of civilized peace, but did they and the creatures they evolved with on their worlds start out that way? Maybe we will be in for a big surprise, but I have to wonder.

  • ljk January 22, 2021, 17:19

    Secret Ingredient Found to Power Supernovas

    Three-dimensional supernova simulations have solved the mystery of why they explode at all.

    Thomas Lewton
    Contributing Writer

    January 21, 2021