Spectral Energy Distribution of Hyper-Luminous Infrared Galaxies
  The relationship between star formation and super-massive black hole growth
is central to our understanding of galaxy formation and evolution.
Hyper-Luminous Infrared Galaxies (HLIRGs) are unique laboratories to
investigate the connection between starburst (SB) and Active Galactic Nuclei
(AGN), since they exhibit extreme star formation rates, and most of them show
evidence of harbouring powerful AGN. Our previous X-ray study of a sample of 14
HLIRGs shows that the X-ray emission of most HLIRGs is dominated by AGN
activity. To improve our estimate of the relative contribution of the AGN and
SB emission to its total bolometric output, we have built broad band spectral
energy distributions (SEDs) for these HLIRGs, and we have fitted empirical AGN
and SB templates to these SEDs. In broad terms, most sources are well fitted
using this method, and we found AGN and SB contributions similar to those
obtained by previous studies of HLIRGs. We have classified the HLIRGs SEDs in
two groups, named class A and class B. Class A HLIRGs show a flat SED from the
optical to the infrared energy range. Three out of seven class A sources can be
modelled with a pure luminosity-dependent QSO template, while the rest of them
require a type 1 AGN template and a SB template. The SB component is dominant
in three out of four class A objects. Class B HLIRGs show SEDs with a prominent
and broad IR bump. These sources can not trivially be modelled with a
combination of pure AGN and pure SB, they require templates of composite
objects, suggesting that >50% of their emission comes from stellar formation
processes. We propose that our sample is actually composed by three different
populations: very luminous QSO, young galaxies going through their maximal star
formation period and the high luminosity tail of ULIRG population distribution.
