The Search for Multiple Populations in Magellanic Cloud Clusters
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LJMU Research Online Niederhofer, F, Bastian, N, Kozhurina-Platais, V, Larsen, S, Hollyhead, K, Lardo, C, Cabrera-Ziri, I, Kacharov, N, Platais, I, Salaris, M, Cordero, M, Dalessandro, E, Geisler, D, Hilker, M, Li, C, Mackey, D and Mucciarelli, A The Search for Multiple Populations in Magellanic Cloud Clusters II: The Detection of Multiple Populations in Three Intermediate-Age SMC Clusters http://researchonline.ljmu.ac.uk/id/eprint/5046/ Article Citation (please note it is advisable to refer to the publisher’s version if you intend to cite from this work) Niederhofer, F, Bastian, N, Kozhurina-Platais, V, Larsen, S, Hollyhead, K, Lardo, C, Cabrera-Ziri, I, Kacharov, N, Platais, I, Salaris, M, Cordero, M, Dalessandro, E, Geisler, D, Hilker, M, Li, C, Mackey, D and Mucciarelli, A (2017) The Search for Multiple Populations in Magellanic Cloud Clusters II: LJMU has developed LJMU Research Online for users to access the research output of the University more effectively. Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Users may download and/or print one copy of any article(s) in LJMU Research Online to facilitate their private study or for non-commercial research. You may not engage in further distribution of the material or use it for any profit-making activities or any commercial gain. The version presented here may differ from the published version or from the version of the record. Please see the repository URL above for details on accessing the published version and note that access may require a subscription. For more information please contact [email protected] http://researchonline.ljmu.ac.uk/ http://researchonline.ljmu.ac.uk/ MNRAS 465, 4159–4165 (2017) doi:10.1093/mnras/stw3084 Advance Access publication 2016 November 29 The search for multiple populations in Magellanic Cloud clusters – II. The detection of multiple populations in three intermediate-age SMC clusters F. Niederhofer,1‹ N. Bastian,2 V. Kozhurina-Platais,1 S. Larsen,3 K. Hollyhead,2 C. Lardo,2 I. Cabrera-Ziri,2,4 N. Kacharov,5 I. Platais,6 M. Salaris,2 M. Cordero,7 E. Dalessandro,8,9 D. Geisler,10 M. Hilker,4 C. Li,11 D. Mackey12 and A. Mucciarelli8 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 2Astrophysics Research Institute, Liverpool John Moores University, 146 Brownlow Hill, Liverpool L3 5RF, UK 3Department of Astrophysics/IMAPP, Radboud University, PO Box 9010, NL-6500 GL Nijmegen, the Netherlands 4European Southern Observatory, Karl-Schwarzschild-Straße 2, D-85748 Garching bei Munchen,¨ Germany 5Max-Planck-Institut fur¨ Astronomie, Konigstuhl¨ 17, D-69117 Heidelberg, Germany 6Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA 7Astronomisches Rechen-Institut, Zentrum fur¨ Astronomie der Universitat¨ Heidelberg, Monchhofstraße¨ 12-14, D-69120 Heidelberg, Germany 8Department of Physics and Astronomy, University of Bologna, Viale Berti Pichat 6/2, I-40127 Bologna, Italy 9INAF – Osservatorio Astronomico di Bologna, via Ranzani 1, I-40127 Bologna, Italy 10Departamento de Astronomia, Casilla 160-C, Universidad de Concepcion,´ Concepcion,´ Chile 11Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia 12Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia Accepted 2016 November 24. Received 2016 November 24; in original form 2016 October 26 ABSTRACT This is the second paper in our series about the search for multiple populations in Magellanic Cloud star clusters using the Hubble Space Telescope. Here we report the detection of multiple stellar populations in the colour–magnitude diagrams of the intermediate-age clusters Lindsay 1, NGC 416 and NGC 339. With ages between 6.0 and 7.5 Gyr, these clusters are the youngest ones in which chemical abundance spreads have been detected so far. This confirms that the appearance of multiple populations is not restricted to only ancient globular clusters, but may also be a common feature in clusters as young as 6 Gyr. Our results are in agreement with a recent spectroscopic study of Lindsay 1. We found that the fraction of enriched stars in NGC 416 is ∼45 per cent whereas it is ∼25 per cent in NGC 339 and ∼36 per cent in Lindsay 1. Similar to NGC 121, these fractions are lower than the average value for globular clusters in the Milky Way. Key words: stars: abundances – Hertzsprung–Russell and colour–magnitude diagrams – galaxies: individual: SMC – galaxies: star clusters: individual: Lindsay 1 – galaxies: star clus- ters: individual: NGC 339 – galaxies: star clusters: individual: NGC 416. Multiple populations seem to be an inherent and universal prop- 1 INTRODUCTION erty of GCs (with the only known possible exceptions being IC 4499 The increasing precision in observing techniques has revolutionized and Ruprecht 106 – Walker et al. 2011; Villanova et al. 2013)and our view of star clusters in the last few decades. It is now well their appearance seems not to depend on the environment of the clus- established that old globular clusters (GCs) are not, as previously ter and the type of the host galaxy. Besides the extensively studied thought, simple stellar populations but are composed of multiple GCs in the Milky Way (see e.g. Gratton, Carretta & Bragaglia 2012, populations. The stars of the various populations show different for a review), GCs in nearby dwarf galaxies also host multiple pop- (anti-)correlated abundances of light elements, the most prominent ulations, for example in the Fornax dwarf spheroidal galaxy (Larsen being the Na–O and C–N anticorrelations (e.g. Cannon et al. 1998; et al. 2014), the Sagittarius dwarf galaxy (Carretta et al. 2010, 2014) Carretta et al. 2009a). Moreover, some clusters additionally show a and the Large Magellanic Cloud (LMC; Mucciarelli et al. 2009). Mg–Al anticorrelation (e.g. Carretta et al. 2009b). Recently, Dalessandro et al. (2016) and Niederhofer et al. (2017, hereafter Paper I) also detected multiple populations in NGC 121, the only ‘classical’ GC in the Small Magellanic Cloud (SMC). This E-mail: [email protected] cluster has an age of about 10.5 Gyr (Glatt et al. 2008a). C 2016 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 4160 F. Niederhofer et al. Table 1. Properties of the three clusters analysed in this work. Cluster name RA Dec. Age Ref. Mass Ref. Metallicity Z Ref. (Gyr) (105 M) Lindsay 1 00h03m54s.0 −73◦2818 7.5 (1) ∼2.0 (2) 0.001a (1) NGC 339 00h57m48s.90 −74◦2800. 2 6.0 (1) 0.8 (3) 0.001a (1) NGC 416 01h07m54s.98 −72◦2050. 6 6.0 (1) 1.6 (3) 0.002a (1) Notes. (1) Glatt et al. (2008b); (2) Glatt et al. (2011); (3) McLaughlin & van der Marel (2005). aFrom fitting Padova isochrones (Girardi et al. 2000, 2008) to CMDs in optical filters. It is, however, still unclear if the appearance of multiple popula- tions depends on when or how long ago the cluster formed. To date, there have been no clusters younger than 3 Gyr found with chemical abundance variations (see e.g. Mucciarelli et al. 2008, 2011, 2014; Davies et al. 2009; Cabrera-Ziri et al. 2016). Clusters that fill the age gap between 3 and 10 Gyr have to date been almost com- pletely neglected by studies searching for multiple populations. So far, only Hollyhead et al. (2017) have studied a cluster that falls in this range of ages. They spectroscopically analysed a sample of red giant branch (RGB) stars in the ∼7.5 Gyr old (Glatt et al. 2008b) cluster Lindsay 1 and found a significant variation in N amongst their sample of stars. This result provides the first evidence that multiple populations are likely to be present in clusters younger than about 10 Gyr. Recently, we started a photometric survey of star clusters in the Magellanic Clouds covering a large range of ages and masses to search for anomalies in their colour–magnitude diagrams (CMDs) due to possible spreads in chemical abundances, mainly in N and Figure 1. CMD of NGC 416 before (left) and after (right) the differential C(seePaper I for details of the survey). Using this sample of reddening correction. The direction of the reddening vector in the mF438W targets, our goal is to establish the lower age limit down to which versus mF336W − mF438W colour–magnitude space is indicated in the left- star clusters show evidence for multiple populations. As a first hand panel as a red arrow. result of this study, we detected a bifurcation in the RGB of the ∼10.5 Gyr old SMC cluster NGC 121 (see Paper I), consistent with the independent study by Dalessandro et al. (2016). Here, we the standard HST pipeline for bias, dark, low-frequency flats present the analysis of three more SMC clusters, Lindsay 1, NGC and new improved UVIS zero-points (Ryan et al. 2016). We 339 and NGC 416, which have ages between ∼6.0 and 7.5 Gyr derived the stellar photometry using the spatially variable ‘ef- (Glatt et al. 2008b). Table 1 lists the basic parameters of the three fective point spread function’ (ePSF) method (Anderson, private clusters. The structural parameters of the clusters can be found in communication). The instrumental magnitudes were then trans- Glatt et al. (2009). formed into the VEGAMAG system using the newly derived im- This paper is structured as follows. We briefly describe the ob- proved UVIS VEGAMAG zero-points from the WFC3 instru- servations and data reduction procedures in Section 2. The analysis ment web site. Finally, the derived stellar positions were corrected and the results are shown in Section 3. In Section 4, we present the for the WFC3/UVIS geometric distortion (Bellini, Anderson & discussion of our results and draw final conclusions.