RESOLVING THE STELLAR POPULATIONS IN THE CIRCUMNUCLEAR RING OF NGC 7469

摘要

We investigate the stellar populations in the star-forming ring of the luminous infrared galaxy NGC 7469. We use HST multiwavelength (UV through NIR) imaging complemented with new K-band ground-based long-slit spectroscopy, and mid-IR and radio maps from the literature. SEDs and evolutionary synthesis models have been used to characterize the star formation at different scales, from those of individual star clusters (tens of pc) to that of the entire star-forming ring (kpc scale). At the smallest scales two different populations of massive (1-10 x 10~6 solar mass) clusters are identified. About 25% of the clusters are young (1-3 Myr) and extincted (A_V ≈ 3 mag), whereas the vast majority are of intermediate age (～ 9-20 Myr) and less obscured (A_V ≈ 1 mag). At larger (hundreds of pc) scale, an analysis of the integrated SED and spectroscopic data of the ring indicates the presence of two stellar populations. The young (5-6 Myr) and obscured stellar population accounts for the Brγ emission and most of the IR luminosity, and for about one-third of the stellar mass of the ring. The much less obscured intermediate-age population has properties similar to those of the majority of the (older) 1.1 μm—selected star clusters. The distribution of these two populations is clearly different and even spatially anticorrelated. The UV-optical-NIR continuum (including the majority of the clusters) of the ring traces mostly the mildly obscured intermediate-age population, while the MIR and radio peaks mark the location of the youngest and obscured star-forming regions. Moreover, the two brightest MIR and radio peaks are spatially coincident with the ends of the nuclear molecular gas bar. This study emphasizes the need for multiwavelength, high angular resolution observations to characterize the star formation in the dust-obscured regions commonly present in LIRGs.