We present a general review of the current knowledge of IRAS 20551-4250 and its circumnuclear environment. This Ultraluminous Infrared Galaxy is one of the most puzzling sources of its class in the nearby Universe: the near-IR spectrum is typical of a galaxy experiencing a very intense starburst, but a highly obscured active nucleus is identified beyond ~5？μm and possibly dominates the mid-IR energy output of the system. At longer wavelengths star formation is again the main driver of the global spectral shape and features. We interpret all the available IR diagnostics in the framework of simultaneous black hole growth and star formation and discuss the key properties that make this source an ideal laboratory for the forthcoming James Webb Space Telescope. 1. Introduction Two main physical processes characterize the nuclear regions of active galaxies: intense star formation at rates of ~102–103？ ？yr？1 (starburst, SB) and accretion on to a supermassive black hole (active galactic nucleus, AGN). The issue of SB and AGN connection in both local and distant galaxies is critical for a proper understanding of galaxy formation and evolution, of star formation history and metal enrichment of the Universe, and of the origin of the extragalactic background at low and high energies. There is indeed increasing evidence of a strong link between the starburst and AGN mechanisms in active systems. The empirical correlation between the mass of black holes (BHs) located at the centre of nearby galaxies (both active and passive/quiescent) and the mass of their spheroids (see [1] and references therein) suggests that the formation of bulges and the growth of the central BHs are tightly connected. Also the presence of circumnuclear star formation in a substantial fraction of local AGN [2–5] hints at the relation between the two phenomena. The overall conclusion of these studies is that in 30–50% of the cases the accreting supermassive BHs are associated with young (i.e., of age less than a few ×100？Myr) star-forming regions, with clear evidence of an enhanced star formation rate (reaching up to starburst intensities) in most AGN. However, this does not necessarily imply any causal connection between the two physical processes. It could be simply the natural consequence of massive gas fuelling into the nuclear regions, due to either interactions/mergers or secular evolution such as bar-driven inflows. Both star formation and nuclear accretion, in fact, are triggered and subsequently fed by this gas reservoir. In the local Universe, the optimal targets to study the AGN/SB
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