Multiwavelength Study of the Starburst Galaxy NGC 7714. II. The Balance between Young, Intermediate‐Age, and Old Stars

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Abstract
We combine existing multiwavelength data (including an HST/GHRS UV spectrum and a ground-based optical spectrum) with unpublished HST/WFPC2 images, near-IR photometry, and K-band spectroscopy. We use these data to constrain the young, the intermediate-age, and the old stellar populations in the central regions of the starburst galaxy NGC 7714. In a previous paper the stellar features in the HST/GHRS ultraviolet (UV) spectrum and the optical emission lines were used to identify an ~5 Myr old, very little reddened stellar population as the main source of UV light in the central ~330 pc. The optical data indicated the existence of an older population. The nature of the latter is investigated here. Stellar absorption features in the optical and the near-IR are used to break partly the strong degeneracy between the effects of aging and those of the inhomogeneous dust distribution on the UV-optical-IR colors. Consistency with far-IR, X-ray, and radio data is also addressed. The successful models have essential features in common. We find that the young burst responsible for the UV light represents only a small part of an extended episode of enhanced star formation, initiated a few times 108 yr ago. The star formation rate is likely to have varied on this timescale, averaging about 1 M☉ yr-1. The mass of young and intermediate-age stars thus formed equals at least 10% of the mass locked in preexisting stars of the underlying spiral galaxy nucleus, and fractions around 25% are favored. The spectrophotometric star formation timescale is long compared to the ~110 Myr elapsed since closest contact with the neighboring NGC 7715, according to the 1992 dynamical models of Smith & Wallin. The initial trigger of the starburst thus remains elusive. NGC 7714 owes its brightness in the UV to a few low extinction lines of sight toward young stars. Our results based on the integrated spectrophotometry of the central ~330 pc are supported by high-resolution images of this area. The different extinction values obtained when different spectral indicators are used result naturally from the coexistence of populations with various ages and obscurations. The near-IR continuum image looks smoothest, as a consequence of lower sensitivity to extinction and of a larger contribution of old stars. We compare the nuclear properties of NGC 7714 with results from studies in larger apertures. We emphasize that the global properties of starburst galaxies are the result of the averaging over many lines of sight with very diverse properties in terms of obscuration and stellar ages. The overall picture is strongly reminiscent of the other nearby "prototypical" starburst, M82.
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