Color-magnitude diagrams for clusters 2 and 4, and the field in the Fornax dwarf spheroidal galaxy
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We present new results on the cluster system and the field of Fornax based on imaging and spectroscopic observations conducted at CTIO and CFHT. The color-magnitude diagram of Cluster 2 (C2), which reaches well below the horizontal branch confirms that it is a "bona fide" old globular cluster similar to the metal-poor globulars of the galaxy with well-developed blue horizontal branches. From the magnitude of its HB, V=21.4±0.2, and assuming E(B-V)=0.03±0.03, and MV(HB)=+0.6±0.2 we derive a corrected distance modulus of (V-MV)0=20.7±0.3 corresponding to a distance of 138+20-18 kpc. We have partially resolved Cluster 4 (C4) using HRCam at the CFH telescope and found its HB at V=21.3 in excellent agreement with the results for C2. The giant branch of C4, which in our diagram consists of a small number of stars, indicates a low metallicity comparable to that of C2. However, integrated-light spectroscopy of the near IR Ca II triplet confirms the higher value of the metallicity found by other workers also from the study of the integrated light. The measurements of reliable parameters from the CMD of this very compact and highly contaminated cluster requires resolution closer to its center than we have been able to obtain. Our V, B, and I CMDs of the field of Fornax reach well below the level of the HB. They reveal a young main sequence, indicating that star formation has continued until about 2 Gyr ago. There is little evidence for the presence of a blue HB, and the giant branch contains a well-developed clump similar to those observed in intermediate-age populations in the Magellanic Clouds, but of much lower mean metallicity. The metallicity of the field is well represented by [Fe/H]=-1.34±0.40 (full range). These abundance values are in good agreement with the results of previous investigations. The metallicity spread of 0.8 dex found in the field is similar to that found in the cluster system since [Fe/H]Cluster4-[Fe/H]Cluster3≃0.7. Our CMD of C4 suggests that it is an old cluster not grossly different in age from the more metal-poor Fornax clusters. This in turn suggests that the metal enrichment of Fornax was rapid in comparison to its long history of star formation.
author list (cited authors)
Beauchamp, D., Hardy, E., Suntzeff, N. B., & Zinn, R
complete list of authors
Beauchamp, Dominique||Hardy, Eduardo||Suntzeff, Nicolas B||Zinn, Robert