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Integratedlight VRI Imaging Photometry of Globular Clusters In Mon. Not. R. Astron. Soc. 369, 697–704 (2006) doi:10.1111/j.1365-2966.2006.10314.x Integrated-light VRI imaging photometry of globular clusters in the Magellanic Clouds Paul Goudfrooij,1 Diane Gilmore,1 Markus Kissler-Patig2† and Claudia Maraston3 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 2European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching bei Munchen,¨ Germany 3University of Oxford, Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH Accepted 2006 March 14. Received 2006 March 13; in original form 2006 February 14 ABSTRACT We present accurate integrated-light photometry in Johnson/Cousins V, R and I for a sample of 28 globular clusters in the Magellanic Clouds. The majority of the clusters in our sample have reliable age and metallicity estimates available in the literature. The sample encompasses ages between 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between −1.5 and 0.0 dex. The sample is dominated by clusters of ages between roughly 0.5 and 2 Gyr, an age range during which the bolometric luminosity of simple stellar populations is dominated by evolved red giant branch stars and thermally pulsing asymptotic giant branch (TP-AGB) stars whose theoretical colours are rather uncertain. The VRI colours presented in this paper have been used to calibrate stellar population synthesis model predictions. Key words: techniques: photometric – galaxies: evolution – Magellanic Clouds – galaxies: star clusters. The globular cluster system of the Magellanic Clouds provides 1 INTRODUCTION a unique opportunity to study the influence of the AGB phase to One of the predictions of stellar evolution theory is that evolved red the spectral energy distribution of SSPs as a function of age and giant stars dominate the bolometric luminosity of a simple stellar chemical composition. Globular clusters in the Magellanic Clouds population (SSP) after a few 100 Myr. According to predictions cover a wide range in age and metallicity, and clusters with ages be- based on canonical stellar models (Renzini & Buzzoni 1986), the tween 0.1 and 2 Gyr are present in significant numbers (e.g. Searle, first (and sudden) appearance of a prominent red stellar sequence is Wilkinson & Bagnuolo 1980; Elson & Fall 1985; Sagan & Pandey that of bright asymptotic giant branch (AGB) stars followed by the 1989; Frogel, Mould & Blanco 1990; Girardi et al. 1995). Maraston development of the red giant branch (RGB), both occurring before (1998) presented SSP models whose calibration matches the ener- an age of 1 Gyr. The onset of the RGB is predicted to occur at an getics and colours of globular clusters in the Magellanic Clouds. age of about 0.6 Gyr (Renzini & Buzzoni 1986) and observational The onset of the TP-AGB phase occurs at ∼0.2 Gyr and lasts efforts have verified this prediction (e.g. Ferraro et al. 1995, 2004). until ∼2 Gyr. During this age range, TP-AGB stars dominate However, theoretical modelling of the AGB phase is rather com- the near-infrared (near-IR) and even the bolometric light of SSPs plicated. For example, strong and poorly constrained mass loss and (Maraston 1998). envelope burning affect stellar evolution through the AGB phase The age range of 0.2–2 Gyr, during which AGB stars are expected (e.g. Iben & Renzini 1983; Girardi & Bertelli 1998; Wagenhuber & to dominate the light, is of strong interest and significance to several Groenewegen 1998; Marigo 2001). These phenomena hamper a popular topics in the present-day astronomy and astrophysics. For straightforward theoretical prediction of lifetimes and effective tem- example, it is similar to the crossing time of galaxy-sized stellar peratures during the thermally pulsing part of the AGB phase (the systems, which is a measure of the formation time-scale of a galaxy. so-called TP-AGB phase). Hence, an empirical calibration of the Modern observational facilities like the Hubble Space Telescope colours of SSPs during this age range is important (see also Renzini (HST), the Spitzer Space Telescope and 10-m-class ground-based 1992; Maraston 1998). telescopes have been able to identify various populations of faint galaxies at high redshift (z 1), for which the accuracy of age determination is relevant to testing the predictions of various galaxy E-mail: [email protected] formation scenarios (see also Maraston 2004). This age range is also †Visiting Astronomer, Cerro Tololo Inter-American Observatory (CTIO). very relevant to the identification and study of post-starburst galaxies The CTIO is operated by the Association of Universities for Research and merger remnants (e.g. Whitmore et al. 1993; Schweizer et al. in Astronomy (AURA), Inc., under contract to the US National Science 1996; Maraston et al. 2001; Goudfrooij et al. 2001a,b, and references Foundation. therein). C 2006 The Authors. Journal compilation C 2006 RAS 698 P. Goudfrooij et al. Table 1. General properties of globular clusters in our sample. Cluster RA Dec. SWB Log (age) Age [Fe/H] [Fe/H] AV σ (AV ) ID J2000 J2000 type (yr) reference (dex) reference (mag) (mag) (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) SMC clusters − +0.09 − ± ∗ Kron 3 00 24 44 72 47 20 VI–VII 9.78−0.11 1 1.16 0.09 1 0.18 0.02 − +0.06 − ± NGC 152 00 32 56 73 06 57 IV 9.15−0.07 2 0.94 0.15 2 0.19 0.02 NGC 265 00 47 12 −73 28 38 III 8.30 3 ..... 0.20 0.02 − +0.08 − ± NGC 339 00 57 49 74 28 00 VII 9.80−0.10 4 1.50 0.14 4 0.18 0.02 − +0.06 − ± NGC 411 01 07 56 71 46 05 V–VI 9.15−0.07 2, 4 0.68 0.07 2, 4 0.17 0.02 − +0.15 − ± NGC 419 01 08 29 72 53 12 V 9.08−0.23 5 0.7 0.3 5 0.32 0.02 LMC clusters NGC 1644 04 37 39 −66 11 58 V 8.94 6 ..... 0.28∗ 0.02 − +0.08 − ± NGC 1651 04 37 32 70 35 06 V 9.30−0.10 7 0.37 0.20 12 0.35 0.05 NGC 1751 04 54 12 −69 48.24 VI 9.18 6 −0.18 ± 0.20 12 0.51 0.02 NGC 1755 04 55 14 −68 12 17 II 7.98 6 ..... 0.55 0.02 NGC 1783 04 59 08 −65 59 20 V 9.11 6 −0.45 ± 0.03 13 0.31 0.02 NGC 1806 05 02 11 −67 59 20 V 8.70 8 −0.23 ± 0.20 12 0.25 0.04 − +0.30 + ± ∗ NGC 1831 05 06 16 64 55 06 IV A 8.50−0.30 9 0.01 0.20 12 0.39 0.02 NGC 1846 05 07 35 −67 27 39 VI 9.08 6 −0.70 ± 0.20 12 0.45 0.03 − +0.30 − ± NGC 1866 05 13 39 65 27 54 III 8.12−0.30 9 0.50 0.10 14 0.26 0.03 − +0.30 − ± ∗ NGC 1868 05 14 36 63 57 18 IV A 8.74−0.30 9 0.50 0.20 12 0.39 0.02 − +0.05 − ± NGC 1978 05 28 45 66 14 12 VI 9.30−0.05 7 0.42 0.20 12 0.55 0.03 NGC 1987 05 27 17 −70 44 06 IV B 8.79 6 ..... 0.33 0.03 NGC 2058 05 36 54 −70 09 44 III 8.06 6 ..... 0.24 0.02 NGC 2134 05 51 56 −71 05 51 III 8.28 6 −1.0 15 0.49 0.02 − +0.10 − ± NGC 2136 05 53 17 69 31 42 III 8.00−0.10 8 0.55 0.23 8 0.46 0.02 NGC 2154 05 57 38 −67 15 43 V 9.01 6 −0.56 ± 0.20 12 0.34 0.02 − +0.06 − ± NGC 2155 05 58 33 65 28 36 VI 9.51−0.07 10 0.55 0.20 12 0.35 0.03 − +0.12 − ± NGC 2162 06 00 31 63 43 18 V 9.11−0.16 7 0.23 0.20 12 0.39 0.02 − +0.20 − ± NGC 2164 05 58 54 68 31 06 II 7.70−0.20 11 0.60 0.20 16 0.41 0.07 − +0.07 − ± ∗ NGC 2173 05 57 58 72 58 42 VI 9.33−0.09 7 0.24 0.20 12 0.39 0.02 − +0.10 − ± NGC 2213 06 10 42 71 31 42 V 9.20−0.12 7 0.01 0.20 12 0.44 0.02 − +0.10 − ± NGC 2231 06 20 44 67 31 06 V 9.18−0.13 7 0.67 0.20 12 0.39 0.02 1. Column (1): cluster ID. Columns (2) and (3): right ascension (RA) (given as hours, minutes and seconds) and declination (Dec.) (given as degrees, arcminutes, and arcseconds)) in J2000.0 equinox, both taken from Welch (1991) for the SMC clusters and from Bica et al. (1999) for the LMC clusters. Column (4): SWB type from Frogel et al. (1990) or Bica et al. (1996). Column (5): logarithm of age from literature. Column (6): age reference code (see below). Column (7): [Fe/H] from literature. Column (8): [Fe/H] reference code (see below). Columns (9) and (10): assumed foreground extinction in V band and its uncertainty, derived from data in Zaritsky et al. (2002, 2004), made available through http://ngala.as.arizona.edu/dennis/lmcdata.html.
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