Tracing the formation history of intermediate-age star clusters in the Small Magellanic Cloud

Abstract

Colour–magnitude diagrams (CMDs) are presented for the first time for 10 star clusters projected on to the Small Magellanic Cloud (SMC). The photometry was carried out in the Washington system C and T1 filters allowing the determination of ages by means of the magnitude difference between the red giant clump and the main‐sequence turnoff (MSTO), and metallicities from the red giant branch (RGB) locus. The clusters all have ages in the range 1.5–4 Gyr and metallicities between −1.3 < [Fe/H] < −0.6, with respective errors of ∼0.5 Gyr and 0.3 dex. This increases substantially the sample of intermediate‐age clusters in the SMC with well‐derived parameters. We combine our results with those for other clusters in the literature to derive as large and homogeneous a data base as possible (totalling 26 clusters) in order to study global effects. We find evidence for two peaks in the age distribution of SMC clusters, at ∼6.5 and 2.5 Gyr, in good agreement with previous hints involving smaller samples. The most recent peak occurs at a time that corresponds to a very close encounter between the Large Magellanic Cloud (LMC) and the SMC according to the recent dynamical models of Bekki et al. that they used to explain the enhancement of LMC clusters with this age. It appears cluster formation may have been similarly stimulated in the SMC by this encounter as well. We also find very good agreement between cluster ages and metallicities and the prediction from a bursting model from Pagel and Tautvaišienė with a burst that occurred 3 Gyr ago. These two lines of evidence together favour a bursting cluster formation history as opposed to a continuous one for the SMC.

Integrated spectral analysis of 18 concentrated star clusters in the Small Magellanic Cloud

“…L 5 Piatti et al (2005) have derived t = 4.3 Gyr and [Fe/H] = −1.2 for this cluster. Much like K 5, the age of L 5 has been estimated to be 0.8 Gyr according to both methods employed in the present work.…”

“…19. Positions of the eighteen studied cluster fields (crossed boxes) and of the nineteen additional clusters taken from Piatti et al (2002b) and Piatti et al (2005) (triangles) with relation to the SMC bar (straight line) and optical centre (cross). Two ellipses of semi-major axes of 2 • and 4 • (b/a = 1/2), aligned along the SMC Bar, are also drawn.…”

“…Much like K 5, the age of L 5 has been estimated to be 0.8 Gyr according to both methods employed in the present work. A correction to the metallicity provided by Piatti et al (2005) revised it to [Fe/H] = −1.1 for its significantly younger age. Figure 1 shows the best template combination for L 5, i.e., the average of Ia and Yh templates with a reddening of E(B − V) = 0.03.…”

“…Figure 19 shows the positions of the studied clusters (crossed boxes) relative to the SMC optical centre (cross), assumed to be placed (J2000) at: 00 h 52 m 45 s , -72 • 49 43 (Crowl et al 2001). For the sake of completeness, we included 19 additional clusters (triangles) taken from Table 4 of Piatti et al (2002b) and studied by Piatti et al (2005), which have ages and metallicities put onto a homogeneous scale. The collection of these 37 objects constitutes at the present time the largest sample of SMC clusters used to address the issue of the galaxy chemical evolution.…”

The metallicity gradient as a tracer of history and structure: the Magellanic Clouds and M33 galaxies

“…At that time, the SMC had a close encounter with the LMC (Piatti et al 2005, Bekki et al 2004. Was this episode responsible for the formation of the Magellanic Bridge?…”