Formation histories of galactic halos via chemical abundance analyses of near-by stellar systems

• Applicant: Dr. Andreas Koch-Hansen, Ph.D.
• Subject Area: Astrophysics and Astronomy
• Term: from 2011 to 2018
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 193251414

• Final Report Year: 2017

Final Report Abstract

Our group set out to study the question of how and on what times scales galaxies like our own Milky Way (MW) formed and evolved and in particular to investigate the interplay between their outermost component, namely the halo, and its building blocks – to wit, fields stars, globular clusters (GCs), and faint dwarf spheroidal (dSph) galaxies. Thus we have vastly increased the amount of information about many constituents of MW halo, and also studied the halos of distant galaxies in the Local Universe. By measuring chemical element abundances in several GCs that have no published studies in the literature, we confirmed that these objects host several stellar generations and in some cases (e.g., M75) we found an even more complex mix of (up to four different) stellar populations than seen in other GCs. While the stellar halo of the Galaxy shows a clear dichotomy in its stellar populations, and it is known that the outer halos display, on average, younger ages than their inner halo counterparts, we do not see any chemical differences between the inner and outer halo GCs. As for the dSphs, we gained further insight into their internal chemical evolutions by detailed chemical abundance measurement and subsequent modeling of chemical evolution. For one of the most massive systems in the Galactic halo, the Fornax dSph, we could show that it formed stars during discrete periods rather than continuously, indicating that small-scale, localized events such as mergers with other stellar systems played an important role in its evolution. Furthermore, we could show that the GCs associated with this dwarf are chemically coupled to the evolution of the surrounding field star population. On the other extreme side, we studied some of the faintest, lowest-mass companions to the MW, the Hercules and Bootes II dSphs. The chemical abundances imprinted in their stars also revealed a number of peculiarities such as a puzzling deficiency in heavy elements, which can only be explained by a very low efficiency in forming stars and a very high degree of spatial inhomogeneities. This again emphasizes that the evolution of these low-mass inhabitants of the MW halo proceeds in a highly complex manner on the smallest scales. The discovery and analysis of distant galaxies and their halos provided another mainstay of our group’s research. Here, the morphology and kinematics of dwarf galaxies that are in the process of being accreted by larger structures provided important constraints to the time scales for such mergers that will eventually add to the growth of the galactic halos. In particular, our discoveries of a disrupting dSph in a massive galaxy cluster and a dwarf-galaxy-dwarf-galaxy merger in the Local Universe posed important chances to investigate how such mergers proceed in various environments (i.e., the dense galaxy cluster vs. the less dense regions of our own MW halo). Overall, the progress of this project emphasized the large power of serendipity – many of the surprising results (a spectroscopic binary in a faint dSph; stars with very peculiar abundance pattern; tidally disrupted dSphs) were discovered within larger surveys of “regular” halo objects. This opens an important window to exploiting present and upcoming surveys, such as the Gaia-ESO survey or 4MOST, both of which will spectroscopically target millions of stars and in which members of my group were participating. Results of our research were not only published in 51 refereed articles and numerous international conference contributions, but also presented in press releases.

Publications

• 2012, ApJL, 755, L13: ‘Threshing in Action – The tidal disruption of a dwarf galaxy by the Hydra I Cluster’
  Koch, A., Burkert, A., Rich, R.M., Collins, M.L.M., Black, C.S., Hilker, M., & Benson, A.
  
• 2013, A&A, 554, A5: ‘Neutron-capture element deficiency of the Hercules dwarf Spheroidal galaxy’
  Koch, A., Feltzing, S., Adén, D., & Matteucci, F.
  
• 2013, A&A, 554, A81: ‘A comprehensive chemical abundance study of the outer halo globular cluster M 75’
  Kacharov, N., Koch, A., & McWilliam, A.
  
• 2013, AN, 334, 197: ‘Velocity and abundance precisions for future high-resolution spectroscopic surveys: a study for 4MOST’
  Caffau, E., Koch, A., Sbordone, L., et al.
  
• 2014, A&A, 567, 69): ‘A study of rotating globular clusters – The case of the old, metal poor globular cluster NGC 4372’
  Kacharov, N., Bianchini, P., Koch, A., Frank, M.J., Martin, N.F., van de Ven, G., Puzia, T.H., McDonald, I., et al.
  
• 2014, ApJ, 780, 91: ‘A spectroscopic binary in the Hercules dwarf Spheroidal galaxy’
  Koch, A., Hansen, T., Feltzing, S., & Wilkinson, M.I.
  
• 2014, ApJ, 785, 102: ‘The metal-poor knee in the Fornax dwarf Spheroidal galaxy’
  Hendricks, B., Koch, A., Lanfranchi, G.A., Boeche, C., Walker, M., Johnson, C.I., Peñarrubia, J., & Gilmore, G.F.
  
• 2015, A&A, 581, 72: ‘Strömgen ubvy photometry of the peculiar globular cluster NGC 2419’
  Frank, M.J., Koch, A., Feltzing, S., Kacharov, N., Wilkinson, M.I., & Irwin, M.
  
• 2015, ApJ, 807, 153: ‘A detection of gas associated with the M31 Stellar Stream’
  Koch, A., Danforth, C.W., Rich, R.M., Ibata, R.A., & Keeney, B.A.
  
• 2017, A&A, 599, 97: ‘Insights into the chemical composition of the metal-poor Milky Way halo globular cluster NGC 6426’
  Hanke, M., Koch, A., Hansen, C.J., & McWilliam, A.