Exploring the physical properties of local star-forming ULIRGs from the ultraviolet to the infrared

We apply the da Cunha et al. (2008, MNRAS, 388, 1595) model of the spectral energy distribution (SEDs) of galaxies to a small pilot sample of purely star-forming ultra-luminous infrared galaxies (ULIRGs). We interpret the observed SEDs of 16 ULIRGs using this physically-motivated model that accounts for both the emission of stellar populations from the ultraviolet to the near-infrared and for the attenuation by dust in two components: an optically-thick starburst component and the diffuse ISM. The infrared emission is computed by assuming that all the energy absorbed by dust in these components is re-radiated at mid- and far-infrared wavelengths. This model allows us to derive statistically physical properties including star formation rates, stellar masses, the temperatures and masses of different dust components, and plausible star formation histories. We find that, although the ultraviolet-to-near-infrared emission represents only a small fraction of the total power radiated by ULIRGs, observations in this wavelength range are important for understanding the properties of the stellar populations and dust attenuation in the diffuse ISM of these galaxies. Furthermore, our analysis indicates that the use of mid-infrared spectroscopy from the infrared spectrograph on the Spitzer Space Telescope is crucial to obtain realistic estimates of the extinction to the central energy source, mainly via the depth of the 9.7-μm silicate feature, and thus accurately constrain the total energy balance. Our findings are consistent with the notion that, in the local Universe, the physical properties of ULIRGs are fundamentally different from those of galaxies with lower infrared luminosities and that local ULIRGs are the result of merger-induced starbursts. While these are well-established ideas, we demonstrate the usefulness of our SED modelling in deriving relevant physical parameters that provide clues to the star formation mode of galaxies.

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