A SUPERGIANT-DOMINATED STARBURST IN THE NUCLEUS OF NGC 4569

Abstract

ABSTRACT The Virgo spiral NGC 4569 has a very compact optical nucleus whose spectrum is highly composite, which strong Balmer absorption indicating that A stars dominate the optical light. Recent HST imaging shows that the nucleus is extremely compact in the mid-ultraviolet, so that either there must be a central AGN or an extraordinarily luminous and compact central star cluster. I present here IUE and ground-based spectrophotometry and high-resolution optical spectra, which are used to decompose the nucleus into possible stellar and AGN contributions. Accounting for the asymmetric contamination by Balmer emission, at a velocity different from the stellar absorption lines, removes the apparent discrepancy between radial velocities for different stellar components. The nuclear spectrum shows contributions from ordinary bulge light and from a young population. This young component has effective spectral type of mid-A, and the Balmer lines are so narrow that A-type supergiants are the most important contributors to the optical light. The young population is therefore much younger than the spectral type would suggest if interpreted as a main-sequence turnoff age, and the asociated OB stars might account for much of the UV flux. In this case, the nucleus contains an extraordinarily bright star cluster about as large as the core of 30 Doradus but more than 6 magnitudes brighter. While some less luminous supergiant-dominated star clusters have been found, current models of starburst evolution do not give a natural explanation for such a bunching in effective temperature. Evidence for an AGN is equivocal; M/L arguments show that only for the most extreme conditions of timing and burst duration could stars alone give a small enough value to satisfy the observational limit on velocity dispersion, and this condition would be eased if some of the UV radiation and the ionization of the nuclear gas can be attributed to an AGN. While, for very favorable conditions the Balmer absorption might be interpreted as coming from an unusually extensive and face-on accretion disk (rather than stars), the small linewidths and dominance of a single photospheric temperature place very stringent geometric and size constraints on this possibility.