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New Survey Finds that Single Burst of StarFormation Created Milky Way’s Central Bulge

par Edith Burgey -

Result contradicts previous studies suggesting two or more episodes of star formation. Like most spiral galaxies, the Milky Way has a roughly spherical collection of stars at its center called the bulge. How the bulge formed has been a long-standing mystery, with many studies suggesting that it built up over time through multiple bursts of star formation.New research finds that the majority of stars in our galaxy’s central bulge formed in a single burst of star formation more than 10 billion years ago. To reach this conclusion, astronomers surveyed millions of stars across 200 square degrees of sky—an area equivalent to 1,000 full Moons. The resulting wealth of data is expected to fuel many more scientific inquiries.

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This video zooms in on a view of the center of our galaxy, the Milky Way, starting with a photo that covers 71 degrees of the sky and ending with the new photo obtained by the Dark Energy Camera (DECam) at the Observatory of Cerro-Tololo in Chile as part of this study. The new image covers 0.5 by 0.25 degrees (an area about twice the size of a full moon) and contains over 180,000 stars.

The Full Story

Our Milky Way galaxy is shaped like two fried eggs glued back-to-back. A central bulge of stars sits in the middle of asprawling disk of stars. Though this is a common feature among myriad spiral galaxies, astronomers have spentdecades puzzling out how and when the Milky Way’s central bulge might have formed. Were the stars within thebulge born early in our galaxy’s history, 10 to 12 billion years ago ? Or did the bulge build up over time throughmultiple episodes of star formation ?

Some studies have found evidence for at least two star-forming bursts, leading to stellar populations as old as 10billion years or as young as 3 billion. Now, a comprehensive new survey of millions of stars instead finds that moststars in the central 1,000 light-years of the Milky Way’s hub formed when it was engorged with infalling gas morethan 10 billion years ago. This process might have been triggered by simple accretion of primordial material, orsomething more dramatic like merging with another young galaxy.

“Many other spiral galaxies look like the Milky Way and have similar bulges, so if we can understand how the MilkyWay formed its bulge then we’ll have a good idea for how the other galaxies did too,” said co-principle investigatorChristian Johnson of the Space Telescope Science Institute in Baltimore, Maryland.

“This survey gives us a big picture view of the bulge in a way that many previous surveys have not been able to do,”added co-author Caty Pilachowski of Indiana University in Bloomington, Indiana.

Looking Younger than their Age

To reach their conclusion, the team studied the stars’ chemical compositions. Like many Hollywood stars, stars in thegalactic bulge look like they’ve undergone a cosmic Botox treatment – they appear younger than they are. That’sbecause they contain about the same amount of heavy elements (heavier than hydrogen and helium) as the Sun –what astronomers call metals. That’s surprising because metals take time to accumulate. They must be created byearlier generations of stars, ejected through stellar winds or supernovas, and then incorporated into latergenerations.

Our Sun, at 4.5 billion years old, is a relative newcomer, so it makes sense that it would be replete in metals. Incontrast, most old stars within our galaxy are lacking in heavy elements. And yet bulge stars are metal-enricheddespite their advanced age.

“Something different happened in the bulge. The metals there built up very, very quickly, possibly in the first 500million years of its existence,” said co-principle investigator Michael Rich of the University of California, Los Angeles.

The team used the measured brightness of stars at different wavelengths of light, particularly in the ultraviolet, todetermine their metal content. Stars forming at different times would be expected to have different metallicities onaverage. Instead, they found that stars within 1,000 light-years of the galactic center showed a distribution of metalsclustered around a single average. If stars were students and metallicities were U.S. grades, bulge stars would allhave a solid ‘C’ average, rather than a group of ‘A’ students and a group of ‘D’ students. This suggests that thosestars formed in a brief firestorm of star birth.

Big Pictures, Big data

The team surveyed a portion of the sky covering more than 200 square degrees – an area approximately equivalentto 1,000 full Moons. They used the Dark Energy Camera (DECam) on the Victor M. Blanco 4-meter Telescope at theCerro Tololo Inter-American Observatory in Chile, a Program of NSF’s NOIRLab (https://noirlab.edu/public/). Thiswide-field camera is capable of capturing 3 square degrees of sky in a single exposure.

The team collected more than 450,000 individual photographs that allowed them to accurately determine chemicalcompositions for millions of stars. A subsample of 70,000 stars were analyzed for this study.

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

The wealth of data collected by this survey will fuel additional scientific inquiries. For example, the researchers arelooking into the possibility of measuring stellar distances to make a more accurate 3D map of the bulge. They alsoplan to seek correlations between their metallicity measurements and stellar orbits. That investigation could locate“flocks” of stars with similar orbits, which could be the remains of disrupted dwarf galaxies, or identify signs ofaccretion like stars orbiting opposite the galaxy’s rotation.

Is the Milky Way’s bulge-formation history unique or common in galaxy evolution ? To answer that question,astronomers will have to look at galaxy assembly in the distant, young universe – a task for which NASA’s JamesWebb Space Telescope (https://webbtelescope.org/) was specifically designed. “With Webb, we’ll have a front-rowseat to watching galaxies like our Milky Way forming,” said Rich.

The Blanco DECam Bulge Survey is named in honor of Victor and Betty Blanco. Victor Blanco was the first Directorof the Cerro-Tololo Inter-American Observatory ; he and Betty Blanco also pioneered study of the galactic bulge andMagellanic Clouds using the observatory’s 4-meter telescope.

These results are being reported in two companion papers accepted for publication in the Monthly Notices of theRoyal Astronomical Society.

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Vue de la région centrale de la Voie lactée obtenue par la Dark Energy Camera (DECam) à l’Observatoire interaméricain de Cerro-Tololo au Chili, qui a servit à cette étude. Crédit : STScI-2020-56
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Zoom sur les régions centrales de la Voie lactée (video). Cette vidéo fait un zoom sur une vue du centre de notre galaxie, la Voie lactée, en commençant par une photo qui couvre 71 degrés du ciel et en se terminant sur une nouvelle photo obtenue par la Dark Energy Camera (DECam) à l’Observatoire interaméricain de Cerro-Tololo au Chili. La nouvelle image couvre 0,5 sur 0,25 degrés (une zone environ deux fois plus large que la pleine lune) et contient plus de 180000 étoiles. Crédit : https://media.stsci.edu/preview/video/1294/news_release/2020-56.

This research was partially supported by the National Science Foundation through grant AST-1413755.
The Space Telescope Science Institute is expanding the frontiers of space astronomy by hosting the scienceoperations center of the Hubble Space Telescope, the science and operations center for the James Webb Space Telescope, and the science operations center for the future Nancy Grace Roman Space Telescope. STScI alsohouses the Mikulski Archive for Space Telescopes (MAST) which is a NASA-funded project to support and provide tothe astronomical community a variety of astronomical data archives, and is the data repository for the Hubble, Webb,Kepler, K2, TESS missions and more.

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Contact :

  • Christine PulliamSpace Telescope Science Institute, Baltimore, Maryland410-338-4366cpulliam chez stsci.edu (mailto:cpulliam chez stsci.edu)

Science Contacts :

  • Christian JohnsonSpace Telescope Science Institute, Baltimore, Marylandchjohnson1 chez stsci.edu (mailto:chjohnson1 chez stsci.edu)
  • Michael RichUniversity of California, Los Angelesrmr chez astro.ucla.edu (mailto:rmr chez astro.ucla.edu)
  • Will ClarksonUniversity of Michigan, Ann Arbor, Michiganwiclarks chez umich.edu (mailto:wiclarks chez umich.edu)
  • Caty PilachowskiIndiana University, Bloomington, Indianacpilacho chez indiana.edu (mailto:cpilacho chez indiana.edu