Washington photometry of 14 intermediate-age to old star clusters in the Small Magellanic Cloud

Abstract

We present CCD photometry in the Washington system C, T1 and T2 passbands down to T1∼ 23 in the fields of L3, L28, HW 66, L100, HW 79, IC 1708, L106, L108, L109, NGC 643, L112, HW 84, HW 85 and HW 86, 14 Small Magellanic Cloud (SMC) clusters, most of them poorly studied objects. We measured T1 magnitudes and C−T1 and T1−T2 colours for a total of 213 516 stars spread throughout cluster areas of 14.7 × 14.7 arcmin2 each. We carried out an in‐depth analysis of the field star contamination of the colour–magnitude diagrams (CMDs) and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C−T1) CMDs, as well as from the δ(T1) index and the standard giant branch procedure, we derived ages and metallicities for the cluster sample. With the exception of IC 1708, a relatively metal‐poor Hyades‐age cluster, the remaining 13 objects are between intermediate and old age (from 1.0 to 6.3 Gyr), their [Fe/H] values ranging from −1.4 to −0.7 dex. By combining these results with others available in the literature, we compiled a sample of 43 well‐known SMC clusters older than 1 Gyr, with which we produced a revised age distribution. We found that the present clusters’ age distribution reveals two primary excesses of clusters at t∼ 2 and 5 Gyr, which engraves the SMC with clear signs of enhanced formation episodes at both ages. In addition, we found that from the birth of the SMC cluster system until approximately the first 4 Gyr of its lifetime, the cluster formation resembles that of a constant formation rate scenario.

Self-consistent physical parameters for five intermediate-age SMC stellar clusters from CMD modelling

“…We note that here too Piatti et al (2011) assumed a distance modulus of 18.90 and also a reddening of 0.04 instead of our fitted value of 0.07. The combination of these two values tends to compensate the effects on age and metallicity.…”

“…In addition, based on integrated spectra, the analysis by Dias et al showed that the clusters HW 1 and Lindsay 3 (as well as NGC 152) can be added to the list of intermediate/old clusters. Since then, newly revealed intermediate-age star clusters (IACs) were studied by Piatti (2011aPiatti ( ,b, 2012a, and Piatti et al (2011).…”

“…In the present work, B and V photometry combined with self-consistent statistical tools are employed to determine ages and metallicities for AM 3, HW 1, HW 40, Lindsay 2 and Lindsay 3, in order to provide more precise constraints for the AMR of the SMC. For these clusters, there are the spectroscopic analysis by Dias et al (2010) of HW 1 and Lindsay 3, and the Washington photometry analysis of AM 3, HW 40, and Lindsay 3 (Piatti 2011a;Piatti et al 2011). The target HW 34 is probably only a field fluctuation.…”

SMC west halo: a slice of the galaxy that is being tidally stripped?

“…Glatt et al 2008a,b;Girardi et al 2013), the VLT (Parisi et al 2014), and 4 m class telescopes (Piatti et al 2005b(Piatti et al , 2007b(Piatti et al , 2011Piatti 2011a,b;Dias et al 2014), as well as CaII triplet spectroscopy (Parisi et al 2009(Parisi et al , 2015. These analyses also showed how difficult it is to assign a unique AMR because the metallicity for a given age is highly dispersed.…”

“…Metallicities derived using resolved spectroscopy received weight 5, those from resolved photometry received weight 3, and integrated spectroscopy received 2. This is not a complete catalogue from the literature, but it covers the most relevant works with large homogeneous samples: Dias et al (2010), Parisi et al (2015Parisi et al ( , 2009Parisi et al ( , 2014, Piatti et al (2007aPiatti et al ( , 2008Piatti et al ( , 2011, Piatti (2011aPiatti ( , 2012a, Glatt et al (2008bGlatt et al ( ,a, 2010, Da Costa & Hatzidimitriou (1998), Mighell et al (1998), Rafelski & Zaritsky (2005 Lindsay 14 2.8 ± 0.4 −1.14 ± 0.11 19.24 ± 0.05 70.6 ± 1.6 0.03 ± 0.02 2.6…”