Young stars in an old bulge: a natural outcome of internal evolution in the Milky Way

M. Ness, Victor P. Debattista, Thomas Bensby, Sofia Feltzing, R. Roskar, D. R. Cole, J. A. Johnson, K. Freeman

Research output: Contribution to journalArticlepeer-review


The center of our disk galaxy, the Milky Way, is dominated by a boxy/peanut-shaped bulge. Numerous studies of the bulge based on stellar photometry have concluded that the bulge stars are exclusively old. The perceived lack of young stars in the bulge strongly constrains its likely formation scenarios, providing evidence that the bulge is a unique population that formed early and separately from the disk. However, recent studies of individual bulge stars using the microlensing technique have reported that they span a range of ages, emphasizing that the bulge may not be a monolithic structure. In this Letter we demonstrate that the presence of young stars that are located predominantly nearer to the plane is expected for a bulge that has formed from the disk via dynamical instabilities. Using an N-body + smoothed particle hydrodynamics simulation of a disk galaxy forming out of gas cooling inside a dark matter halo and forming stars, we find a qualitative agreement between our model and the observations of younger metal-rich stars in the bulge. We are also able to partially resolve the apparent contradiction in the literature between results that argue for a purely old bulge population and those that show a population comprised of a range in ages; the key is where to look.
Original languageEnglish
Article numberL19
JournalAstrophysical Journal Letters
Issue number2
Publication statusPublished - 2014

Subject classification (UKÄ)

  • Astronomy, Astrophysics and Cosmology

Free keywords

  • Galaxy: bulge
  • Galaxy: evolution
  • Galaxy: formation
  • Galaxy: stellar
  • content


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