The complex evolutionary paths of local infrared bright galaxies: a high
  angular resolution mid-infrared view
  We investigate the evolutionary connection between local IR-bright galaxies
($\log L_{\rm IR}\ge 11.4\,L_\odot$) and quasars. We use high angular
resolution ($\sim$ 0.3-0.4 arcsec $\sim$ few hundred parsecs) $8-13\,\mu$m
ground-based spectroscopy to disentangle the AGN mid-IR properties from those
of star formation. The comparison between the nuclear $11.3\,\mu$m PAH feature
emission and that measured with Spitzer/IRS indicates that the star formation
is extended over a few kpc in the IR-bright galaxies. The AGN contribution to
the total IR luminosity of IR-bright galaxies is lower than in quasars.
Although the dust distribution is predicted to change as IR-bright galaxies
evolve to IR-bright quasars and then to optical quasars, we show that the AGN
mid-IR emission of all the quasars in our sample is not significantly
different. In contrast, the nuclear emission of IR-bright galaxies with low AGN
contributions appears more heavily embedded in dust although there is no clear
trend with the interaction stage or projected nuclear separation. This suggests
that the changes in the distribution of the nuclear obscuring material may be
taking place rapidly and at different interaction stages washing out the
evidence of an evolutionary path. When compared to normal AGN, the nuclear star
formation activity of quasars appears to be dimming whereas it is enhanced in
some IR-bright nuclei, suggesting that the latter are in an earlier
star-formation dominated phase.
