The Massive Stellar Content in the Starburst NGC 3049: A Test for Hot‐Star Models

Abstract

The objective of this work is twofold. First, we seek evidence for or against the depletion of massive stars in metal-rich starbursts. A second, equally important goal is to perform a consistency test of the latest generation of starburst models in such a high-metallicity environment. We have obtained high spatial resolution ultraviolet and optical STIS spectroscopy and imaging of the metal-rich nuclear starburst in NGC 3049. The stellar continuum and the absorption-line spectrum in the ultraviolet are used to constrain the massive stellar population. The strong, blueshifted stellar lines of C IV and Si IV detected in the UV spectra indicate a metal-rich, compact, massive (~106 M☉) cluster of age 3-4 Myr emitting the UV-optical continuum. We find strong evidence against a depletion of massive stars in this metal-rich cluster. The derived age and the upper mass-limit cutoff of the initial mass function are also consistent with the detection of Wolf-Rayet (W-R) features at optical wavelengths. As a second, independent constraint on the massive stellar content, the nebular emission-line spectrum is modeled with photoionization codes using stellar spectra from evolutionary synthesis models. The morphology of the nuclear starburst of NGC 3049 from the STIS images indicates a simple geometry for the nebular emission-line region. However, the nebular lines are badly reproduced by 3-4 Myr instantaneous bursts, as required by the UV line spectrum, when unblanketed W-R and/or Kurucz stellar atmospheres are used. The corresponding number of photons above 24 and 54 eV in the synthetic models is too high in comparison with values suggested by the observed line ratios. Since the ionizing spectrum in this regime is dominated by emission from W-R stars, this discrepancy between observations and models is most likely the result of incorrect assumptions about the W-R stars. Thus, we conclude that the nebular spectrum of high-metallicity starbursts is poorly reproduced by models for W-R-dominated populations. However, the new model set of Smith et al. with blanketed W-R and O atmospheres and adjusted W-R temperatures predicts a softer far-UV radiation field, providing a better match to the data.