[ITAL]HUBBLE SPACE TELESCOPE[/ITAL][ITAL]Hubble Space Telescope[/ITAL] Observations of Star Clusters in NGC 1023: Evidence for Three Cluster Populations?

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Abstract
Using Hubble Space Telescope images we have carried out a study of cluster populations in the nearby S0 galaxy NGC 1023. In two WFPC2 pointings we have identified 221 cluster candidates. The small distance (~9 Mpc) combined with deep F555W and F814W images allows us to reach about 2 mag below the expected turnover of the globular cluster luminosity function. NGC 1023 appears to contain at least three identifiable cluster populations: the brighter clusters show a clearly bimodal color distribution with peaks at (V-I)0 = 0.92 and at (V-I)0 = 1.15, and in addition there are a number of fainter, more extended objects with predominantly red colors. Among the brighter clusters, we find that the blue clusters have somewhat larger sizes than the red ones with mean effective radii of Re ~ 2 and Re ~ 1.7 pc, respectively. These clusters have luminosity functions (LFs) and sizes consistent with what is observed for globular clusters in other galaxies. Fitting Gaussians to the LFs of the blue and red compact clusters, we find turnover magnitudes of MTO(blue) = -7.58 and MTO(red) = -7.37 in V and dispersions of σV(blue) = 1.12 and σV(red) = 0.97. The fainter, more extended clusters have effective radii up to Re ~ 10–15 pc, and their LF appears to rise at least down to MV ~ -6, few of them being brighter than MV = -7. We suggest that these fainter objects may have a formation history distinct from that of the brighter GCs.
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