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The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge


Globular star clusters are compact and massive stellar systems old enough to have witnessed the entire history of our Galaxy, the Milky Way. Although recent results1,2,3 suggest that their formation may have been more complex than previously thought, they still are the best approximation to a stellar population formed over a relatively short timescale (less than 1 Gyr) and with virtually no dispersion in the iron content. Indeed, only one cluster-like system (ω Centauri) in the Galactic halo is known to have multiple stellar populations with a significant spread in iron abundance and age4,5. Similar findings in the Galactic bulge have been hampered by the obscuration arising from thick and varying layers of interstellar dust. Here we report that Terzan 5, a globular-cluster-like system in the Galactic bulge, has two stellar populations with different iron contents and ages. Terzan 5 could be the surviving remnant of one of the primordial building blocks that are thought to merge and form galaxy bulges.

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Figure 1: MAD image of Terzan 5 in the K band.
Figure 2: The two horizontal branch clumps of Terzan 5.
Figure 3: Radial distribution of the two horizontal branch populations in Terzan 5.
Figure 4: Iron abundance and ages of the two populations.


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We thank the MAD team at ESO and in particular P. D’Amico for performing the observations with MAD. This research was supported by “Progetti Strategici di Ateneo 2006” (University of Bologna), “Progetti di Ricerca di Interesse Nazionale 2007 and 2008” (Istituto Nazionale di Astrofisica), Agenzia Spaziale Italiana, and the Ministero dell'Istruzione, dell'Universitá e della Ricerca. We also acknowledge support from the ESTEC Faculty Visiting Scientist Programme. R.M.R. is supported by the NSF and STScI; and R.T.R. is partially supported by STScI. This research has made use of the ESO/ST-ECF Science Archive facility, which is a joint collaboration of the ESO and the Space Telescope – European Coordinating Facility. Part of the data presented here were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and NASA. The Observatory was made possible by the financial support of the W. M. Keck Foundation.

Author Contributions F.R.F. designed the study and coordinated the activity. E.D., A.M., G.B., E.V. and G.C. analysed the photometric dataset. R.M.R. and L.O. secured and analysed the Keck spectra. A.M. designed the reddening correction routine. M.B. performed radial distributions tests. E.D. and M.B performed the proper motion analysis. F.R.F., B.L. and L.O. wrote the paper. S.M.R., R.M.R., R.T.R. and M.B. critically contributed to the paper presentation. All authors contributed to discussion of the results and commented on the manuscript.

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Correspondence to F. R. Ferraro.

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Ferraro, F., Dalessandro, E., Mucciarelli, A. et al. The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge. Nature 462, 483–486 (2009).

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