WFPC2 Observations of the Small Magellanic Cloud Intermediate-Age Populous Cluster NGC 416

Abstract

We present our analysis of archival Hubble Space Telescope Wide Field Planetary Camera 2 observations in F555W (∼V ) and F450W (∼B) of the intermediate-age populous star cluster NGC 416 in the Small Magellanic Cloud galaxy. We use published photometry of two other SMC populous star clusters, Lindsay 1 and Lindsay 113, to investigate the age sequence of these three star clusters. We estimate that these clusters have age ratios of age NGC416 /age L1 ≈ 0.73±0.05 and age L113 /age L1 ≈ 0.52±0.09 using an extrapolation of the d (B−V ) method (which uses the color difference between the red horizontal branch and the red giant branch as an age indicator) of Sarajedini, Lee, & Lee [ApJ, 450, 712 (1995)] . These age ratios provide absolute age estimates of 6.6±0.5 Gyr and 4.7±0.8 Gyr for NGC 416 and Lindsay 113, respectively, assuming that Lindsay 1 is 9 Gyr old. Metallicities of [Fe/H] = −1.44±0.12, −1.35±0.08, −1.24±0.11 dex, and reddenings of E(B −V ) = 0.08±0.03, 0.06±0.02, 0.00±0.02 mag for NGC 416, Lindsay 1, and Lindsay 113, respectively, were determined using the simultaneous reddening and metallicity (SRM) method of Sarajedini & Layden [AJ, 113, 264, (1997)]. Accurate (relative) ages for the intermediate-age populous clusters in the Small Magellanic Cloud (e.g. via deep main sequence photometry) would allow the d (B−V ) method to be recalibrated with star clusters that are significantly younger than 7 Gyr. An extended d (B−V ) method could prove to be a very useful age diagnostic for future studies of the intermediate-age metal-poor stellar populations in Local Group galaxies where accurate main-sequence turnoff photometry at M V ≈ +4 mag is currently not possible or practical.

The faint Cepheids of the Small Magellanic Cloud: An evolutionary selection effect?

“…From HST observations Mighell et al (1998b) estimate an absolute age for NGC 416 of 6.6 ± 0.5 Gyr assuming that the Lindsay 1 cluster is 9 Gyr old; Pietrzyński & Udalski (1999b) confirm that NGC 416 is older than 1 Gyr. We underline that neither NGC 330 nor NGC 416 are mentioned in the erratum of Pietrzyński & Udalski (1999a).…”

Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects

“…Points with a connecting line are ages for the same cluster: from measurements of the cluster colors that appear in different fields (x-axis) or from a range in ages given from the CMDs (y-axis). The CMD ages are taken from Hodge & Lee (1984), Geisler et al (1997), Olsen et al (1998), Carney et al (1985), Mighell, Sarajedini, & French (1998a, 1998b. The clusters are identified in the text.…”

“…In the SMC three clusters had CMD ages: NGC 330, 361, and 416(Carney, Janes, & Flower 1985;Mighell, Sarajedini, & French 1998a, 1998b.…”

The SMC star cluster Lindsay 1

“…We decided to apply a technique for the simultaneous determination of metallicity and reddening (SRM) using the (V − I, V) CMD proposed in Sarajedini (1994). The shape of the cluster RGB in our case cannot be represented in a satisfactory way by a second or a third order polynomial so we used a higher-order polynomial and applied the "numerical Mighell et al (1998). A possible explanation is the difference in the polynomial approximations of RGB, or slight difference in the zeropoints of both photometries, or a combination of them.…”