O. Ł. Karczewski, M. J. Barlow, M. J. Page, M. Baes, G. J. Bendo, A. Boselli, A. Cooray, D. Cormier, I. De Looze, I. Ferreras, M. Galametz, F. Galliano, N. P. M. Kuin, V. Lebouteiller, S. C. Madden, M. Pohlen, A. Rémy-Ruyer, M. W. L. Smith, L. Spinoglio
We present an integrated photometric spectral energy distribution (SED) of NGC 4449 from the far-UV to the sub-mm, including new observations acquired by Herschel. We include integrated UV photometry from Swift/UVOT using a measurement technique which is appropriate for extended sources with coincidence loss. The available multi-wavelength data is used to infer a range of ages, metallicities and star formation rates (SFR) for the underlying stellar populations, as well as the composition and the total mass of dust in NGC 4449. Our analysis of the global optical spectrum of NGC 4449 fitted using the spectral fitting code STARLIGHT suggests that the majority of stellar mass resides in old (>~ 1 Gyr old) and metal-poor (Z/Zsolar ~ 0.2) populations, with the first onset of star formation activity deduced to have taken place at an early epoch, approximately 12 Gyr ago. A simple chemical evolution model, suitable for a galaxy continuously forming stars, suggests a ratio of carbon to silicate dust mass comparable to that of the LMC over the inferred timescales. We present an iterative scheme, which allows us to build an in-depth and multi-component representation of NGC 4449 `bottom-up', taking advantage of the broad capabilities of the photoionisation and radiative transfer code MOCASSIN. We fit the observed SED, the global ionisation structure and the emission line intensities, and infer a recent SFR of 0.4 Msolar/yr and a total stellar mass of approximately 1e9 Msolar radiating with a bolometric luminosity of 5.7e9 Lsolar. Our fits yield a total dust mass of 2.9e6 Msolar including 2 per cent attributed to PAHs. We deduce a dust to gas mass ratio of 1/190 within the modelled region. While we do not consider possible additional contributions from even colder dust, we note that including the extended HI envelope and the molecular gas is likely to bring the ratio down to as low as ~ 1/800.
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http://arxiv.org/abs/1302.5430
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