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Astron. Astrophys. 326, 528-536 (1997)

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1. Introduction

NGC 5102 is a blue and gas-rich S0 galaxy that has attracted attention because of a blue color gradient in its center (van den Bergh 1976, Pritchet 1979) and an unusual ultraviolet spectrum (Rocca-Volmerange & Guiderdoni 1987, Burstein et al. 1988). Both characteristics have been interpreted as evidence for recent star formation activity in the central region. Signs of low-level star formation had also been reported in the outer regions (van den Bergh 1976). Van Woerden et al. (1993) have studied the distribution and the motions of atomic hydrogen while summarizing the optical, X-ray and infrared properties. Mc Millan, Ciardullo & Jacoby (1994) recently investigated the distribution of ionized gas and identified a number of planetary nebulae.

This paper reports the first direct detection of individual blue stars in the central region of NGC 5102. These stars are resolved with the Faint Object Camera (FOC) on board the Hubble Space Telescope. In addition to confirming the interpretation of the abnormal blue and UV colors, our motivation was to characterize the star formation phenomenon in an old population and possibly to understand why such a phenomenon has occurred in NGC 5102. Beyond the specific example of NGC 5102, we wished to pursue the broader issues of residual star formation in early-type galaxies, under which conditions it may happen and how, observationally, it may be distinguished from the extreme horizontal branch stars and their evolutionary progeny that are now recognized as the dominant contributors to the UV emission from early-type galaxies (e.g. Greggio & Renzini 1990, Dorman et al. 1993, Dorman et al. 1995, Ferguson et al. 1991, Ferguson & Davidsen 1993, Brown et al. 1995).

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© European Southern Observatory (ESO) 1997

Online publication: October 15, 1997
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