The formation and evolutionary processes of galaxy bulges are still unclear, and the presence of young stars in the bulge of the Milky Way is largely debated. We recently demonstrated that Terzan 5, in the Galactic bulge, is a complex stellar system hosting stars with very different ages and a striking chemical similarity to the field population. This indicates that its progenitor was probably one of the giant structures that are thought to generate bulges through coalescence. Here we show that another globular cluster-like system in the bulge (Liller 1) hosts two distinct stellar populations with remarkably different ages: only 1–3 Gyr for the youngest, and 12 Gyr for the oldest, which is impressively similar to the old component of Terzan 5. This discovery classifies Liller 1 and Terzan 5 as sites of recent star formation in the Galactic bulge and provides clear observational proof that the hierarchical assembly of primordial massive structures contributed to the formation of the Milky Way spheroid.
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The photometric data that support the plots and other findings of this study are available from the corresponding author upon reasonable request. The catalogues are also publicly downloadable from the website of the Cosmic-Lab project (http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). All the HST images are publicly available from the Mikulski Archive for Space Telescopes (http://archive.stsci.edu/).
Rich, R. M. in Planets, Stars and Stellar Systems Volume 5: Galactic Structure and Stellar Populations (eds Oswalt, T. D. & Gilmore, G.) 271–346 (Springer, 2013).
Scannapieco, C. & Tissera, P. B. The effects of mergers on the formation of disc-bulge systems in hierarchical clustering scenarios. Mon. Not. R. Astron. Soc. 338, 880–890 (2003).
Hopkins, P. F. et al. Mergers and bulge formation in ΛCDM: which mergers matter? Astrophys. J. 715, 202–229 (2010).
Immeli, A., Samland, M., Gerhard, O. & Westera, P. Gas physics, disk fragmentation, and bulge formation in young galaxies. Astron. Astrophys. 413, 547–561 (2004).
Carollo, M., Scarlata, C., Stiavelli, M., Wyse, R. F. G. & Mayer, L. Old and young bulges in late-type disk galaxies. Astrophys. J. 658, 960–979 (2007).
Elmegreen, B. G., Bournaud, F. & Elmegreen, D. M. Bulge formation by the coalescence of giant clumps in primordial disk galaxies. Astrophys. J. 688, 67–77 (2008).
Ferraro, F. R. et al. The cluster Terzan 5 as a remnant of a primordial building block of the Galactic bulge. Nature 462, 483–486 (2009).
Origlia, L. et al. Spectroscopy unveils the complex nature of Terzan 5. Astrophys. J. 726, L20–L24 (2011).
Lanzoni, B. et al. New density profile and structural parameters of the complex stellar system Terzan 5. Astrophys. J. 717, 653–657 (2010).
Origlia, L. et al. The Terzan 5 puzzle: discovery of a third, metal-poor component. Astrophys. J. 779, L5–L8 (2013).
Massari, D. et al. Ceci n’est pas a globular cluster: the metallicity distribution of the stellar system Terzan 5. Astrophys. J. 795, 22–33 (2014).
Ferraro, F. R. et al. The age of the young bulge-like population in the stellar system Terzan 5: linking the Galactic bulge to the high-z universe. Astrophys. J. 828, 75 (2016).
Harris, W. E. A catalog of parameters for globular clusters in the Milky Way. Astron. J. 112, 1487–1488 (1996).
Ortolani, S. et al. HST NICMOS photometry of the reddened bulge globular clusters NGC 6528, Terzan 5, Liller 1, UKS 1 and Terzan 4. Astron. Astrophys. 376, 878–884 (2001).
Valenti, E., Ferraro, F. R. & Origlia, L. Near-infrared properties of 12 globular clusters towards the inner bulge of the Galaxy. Mon. Not. R. Astron. Soc. 402, 1729–1739 (2010).
Saracino, S. et al. GEMINI/GeMS observations unveil the structure of the heavily obscured globular cluster Liller 1. Astrophys. J. 806, 152 (2015).
Pallanca, C. et al. High-resolution extinction map in the direction of the bulge globular cluster NGC 6440. Astrophys. J. 882, 159 (2019).
Ferraro, F. R., Fusi Pecci, F. & Buonanno, R. The galactic globular cluster NGC 5897 and its population of blue stragglers. Mon. Not. R. Astron. Soc. 256, 376–390 (1992).
Girardi, L. et al. Theoretical isochrones in several photometric systems. I. Johnson-Cousins-Glass, HST/WFPC2, HST/NICMOS, Washington, and ESO Imaging Survey filter sets. Astron. Astrophys. 391, 195–212 (2002).
Marigo, P. et al. A new generation of PARSEC-COLIBRI stellar isochrones including the TP-AGB phase. Astrophys. J. 835, 77 (2017).
Helmi, A. et al. The merger that led to the formation of the Milky Way’s inner stellar halo and thick disk. Nature 563, 85–88 (2018).
Massari, D., Koppelman, H. H. & Helmi, A. Origin of the system of globular clusters in the Milky Way. Astron. Astrophys. 630, L4 (2019).
Mannucci, F. et al. LSD: Lyman-break galaxies Stellar populations and Dynamics—I. Mass, metallicity and gas at z ~ 3.1. Mon. Not. R. Astron. Soc. 398, 1915–1931 (2009).
Valenti, E. et al. Stellar density profile and mass of the Milky Way bulge from VVV data. Astron. Astrophys. 587, L6 (2016).
Hopkins, P. F., Kereš, D., Murray, N., Quataert, E. & Hernquist, L. Stellar feedback and bulge formation in clumpy discs. Mon. Not. R. Astron. Soc. 427, 968–978 (2012).
Portail, M., Gerhard, O., Wegg, C. & Ness, M. Dynamical modelling of the Galactic bulge and bar: the Milky Way’s pattern speed, stellar and dark matter mass distribution. Mon. Not. R. Astron. Soc. 465, 1621–1644 (2017).
VandenBerg, D. A., Stetson, P. B. & Brown, T. M. Color-magnitude diagram constraints on the metallicities, ages, and star formation history of the stellar populations in the Carina dwarf spheroidal galaxy. Astrophys. J. 805, 103 (2015).
Ruiz-Lara, T., Gallart, C., Bernard, E. J. & Cassini, S. The recurrent impact of the Sagittarius dwarf on the star formation history of the Milky Way. Nat. Astron. 4, 965–973 (2020).
Nogueras-Lara, F. et al. Early formation and recent starburst activity in the nuclear disk of the Milky Way. Nat. Astron. 4, 377–381 (2020).
Massari, D. et al. Proper motions in Terzan 5: membership of the multi-iron subpopulations and first constraint on the orbit. Astrophys. J. 810, 69 (2015).
Bensby, T. et al. Chemical evolution of the Galactic bulge as traced by microlensed dwarf and subgiant stars. V. Evidence for a wide age distribution and a complex MDF. Astron. Astrophys. 549, 147–173 (2013).
Dekany, I. et al. The VVV survey reveals classical Cepheids tracing a young and thin stellar disk across the Galaxy’s bulge. Astrophys. J. 812, L29–L36 (2015).
Stetson, P. B. DAOPHOT—a computer program for crowded-field stellar photometry. Publ. Astron. Soc. Pac. 99, 191–222 (1987).
Dalessandro, E. et al. The peculiar radial distribution of multiple populations in the massive globular cluster M80. Astrophys. J. 859, 15 (2018).
Stetson, P. B. The center of the core-cusp globular cluster M15: CFHT and HST observations, ALLFRAME reductions. Publ. Astron. Soc. Pac. 106, 250–280 (1994).
Bohlin, R. C. Perfecting the photometric calibration of the ACS CCD cameras. Astron. J. 152, 60 (2016).
Gaia Collaboration. Gaia Data Release 2. The celestial reference frame (Gaia-CRF2). Astron. Astrophys. 616, A14 (2018).
Minniti, D. et al. VISTA Variables in the Via Lactea (VVV): the public ESO near-IR variability survey of the Milky Way. New Astron. 15, 433–443 (2010).
Miocchi, P. et al. Star count density profiles and structural parameters of 26 Galactic globular clusters. Astrophys. J. 774, 151 (2013).
Lanzoni, B. et al. The surface density profile of NGC6388: a good candidate for harbouring an intermediate-mass black hole. Astrophys. J. 668, L139–L142 (2007).
Lanzoni, B. et al. Star density profiles of six old star clusters in the Large Magellanic Cloud. Astrophys. J. 887, 176 (2019).
Saracino, S. et al. A panchromatic view of the bulge globular cluster NGC 6569. Astrophys. J. 874, 86 (2019).
Ubeda, L. & Kozhurina-Platais, V. ACS/WFC Geometric Distortion: A Time Dependency Study Instrument Science Report ACS 2013-03 (AURA, 2013); https://go.nature.com/2JfTsxx
Dalessandro, E. et al. GeMS/GSAOI photometric and astrometric performance in dense stellar fields. Astrophys. J. 833, 111 (2016).
Nataf, D. M. et al. Reddening and extinction toward the Galactic bulge from OGLE-III: the inner Milky Way’s RV ~ 2.5 extinction curve. Astrophys. J. 769, 88 (2013).
Cardelli, J. A., Clayton, G. C. & Mathis, J. S. The relationship between infrared, optical, and ultraviolet extinction. Astrophys. J. 345, 245–256 (1989).
O’Donnell, J. E. Rv-dependent optical and near-ultraviolet extinction. Astrophys. J. 422, 158–163 (1994).
Raso, S. et al. Spectral energy distribution of blue stragglers in the core of 47 Tucanae. Astrophys. J. 879, 56 (2019).
Ferraro, F. R. et al. Two distinct sequences of blue straggler stars in the globular cluster M 30. Nature 462, 1028–1031 (2009).
Ferraro, F. R. et al. The Hubble Space Telescope UV Legacy Survey of Galactic globular clusters. XV. The dynamical clock: reading cluster dynamical evolution from the segregation level of blue straggler stars. Astrophys. J. 860, 36 (2018).
Ferraro, F. R. et al. Dynamical age differences among coeval star clusters as revealed by blue stragglers. Nature 492, 393–395 (2012).
Ferraro, F. R. et al. Size diversity of old Large Magellanic Cloud clusters as determined by internal dynamical evolution. Nat. Astron. 3, 1149–1155 (2019).
Ferraro, F. R. et al. The pure noncollisional blue straggler population in the giant stellar system ω Centauri. Astrophys. J. 638, 433–439 (2006).
Origlia, L., Rich, R. M. & Castro, S. High-resolution infrared spectra of bulge globular clusters: Liller 1 and NGC 6553. Astron. J. 123, 1559–1569 (2002).
Horta, D. et al. The chemical compositions of accreted and in situ galactic globular clusters according to SDSS/APOGEE. Mon. Not. R. Astron. Soc. 493, 3363–3378 (2020).
Stephens, A. W. & Frogel, J. A. An infrared spectroscopic study of eight galactic globular clusters. Astron. J. 127, 925–937 (2004).
Dalessandro, E. et al. No evidence of mass segregation in the low-mass galactic globular cluster NGC6101. Astrophys. J. 810, 40 (2015).
Bressan, A. et al. PARSEC: stellar tracks and isochrones with the PAdova and TRieste Stellar Evolution Code. Mon. Not. R. Astron. Soc. 427, 127–145 (2012).
Kroupa, P. On the variation of the initial mass function. Mon. Not. R. Astron. Soc. 322, 231–246 (2001).
This research is part of the project COSMIC-LAB at the Physics and Astronomy Department of the University of Bologna (http://www.cosmic-lab.eu/Cosmic-Lab/Home.html). The research has been funded by project Light-on-Dark, granted by the Italian MIUR through contract PRIN-2017K7REXT. The research is based on data acquired with the NASA/ESA HST under project GO 15231 (principal investigator F.R.F.) at the Space Telescope Science Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555. It is also based on observations obtained at the GEMINI Observatory. S.S. gratefully acknowledges financial support from the European Research Council (ERC-CoG-646928, Multi-Pop). E.V. acknowledges the Excellence Cluster ORIGINS funded by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) under Germany’s Excellence Strategy – EXC-2094 – 390783311. D.G. gratefully acknowledges support from the Chilean Centro de Excelencia en Astrofísica y Tecnologías Afines (CATA) BASAL grant AFB-170002. D.G. also acknowledges financial support from the Dirección de Investigación y Desarrollo de la Universidad de La Serena through the Programa de Incentivo a la Investigación de Académicos (PIA-DIDULS). S.V. gratefully acknowledges the support provided by Fondecyt reg. n. 1170518.
The authors declare no competing interests.
Peer review information Nature Astronomy thanks Annalisa Calamida and the other, anonymous, reviewers for their contribution to the peer review of this work.
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Summary of the photometric dataset used in this work.
a,b, CMDs of the area sampled with the parallel WFC3 observations beyond the tidal radius of Liller1, illustrating the distribution of Galactic field stars in the vicinity of the cluster. c, CMD of a 50"x50" region approximately located at 100" from the centre of Liller1, obtained by combining the NICMOS observations retrieved from the Archive with the optical ACS-HST observations presented in this paper.
The completeness of the sample is shown for three different radial bins from the gravitational centre of Liller 1 (see labels), both in the I-band and in the K-band (panels a,and b, respectively).
The value of the χ2 parameter (see Section ‘Measuring the age of the old stellar population’ in Methods) is plotted as a function of the age of seven isochrones (with t=10, 11, 11.5, 12, 12.5, 13, 14 Gyr) computed for the quoted metallicity of the OP ([M/H]=−0.3). The minimum of the χ2 parameter suggests an age of 12 ± 1 Gyr for the OP of Liller 1.
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Ferraro, F.R., Pallanca, C., Lanzoni, B. et al. A new class of fossil fragments from the hierarchical assembly of the Galactic bulge. Nat Astron 5, 311–318 (2021). https://doi.org/10.1038/s41550-020-01267-y