Active galactic nuclei at z ~ 1.5: I. Spectral energy distribution and
  accretion discs
  The physics of active super massive black holes (BHs) is governed by their
mass (M_BH), spin (a*) and accretion rate ($\dot{M}$). This work is the first
in a series of papers with the aim of testing how these parameters determine
the observable attributes of active galactic nuclei (AGN). We have selected a
sample in a narrow redshift range, centered on z~1.55, that covers a wide range
in M_BH and $\dot{M}$, and are observing them with X-shooter, covering rest
wavelengths ~1200-9800 \AA. The current work covers 30 such objects and focuses
on the origin of the AGN spectral energy distribution (SED). After estimating
M_BH and $\dot{M}$ based on each observed SED, we use thin AD models and a
Bayesian analysis to fit the observed SEDs in our sample. We are able to fit
22/30 of the SEDs. Out of the remaining 8 SEDs, 3 can be fit by the thin AD
model by correcting the observed SED for reddening within the host galaxy and 4
can be fit by adding a disc wind to the model. In four of these 8 sources,
Milky Way-type extinction, with the strong 2175\AA\ feature, provides the best
reddening correction. The distribution in spin parameter covers the entire
range, from -1 to 0.998, and the most massive BHs have spin parameters greater
than 0.7. This is consistent with the "spin-up" model of BH evolution.
Altogether, these results indicate that thin ADs are indeed the main power
houses of AGN, and earlier claims to the contrary are likely affected by
variability and a limited observed wavelength range.
