A Strategy to Measure the Dark Energy Equation of State using the HII
  galaxy Hubble Relation & X-ray AGN Clustering: Preliminary Results
  We explore the possibility of setting stringent constraints to the Dark
Energy equation of state using alternative cosmic tracers like: (a) the Hubble
relation using HII galaxies, which can be observed at much higher redshifts
(z~3.5) than those currently traced by SNIa samples, and (b) the large-scale
structure using the clustering of X-ray selected AGN,which have a redshift
distribution peaking at z~1.
  We use extensive Monte-Carlo simulations to define the optimal strategy for
the recovery of the dark-energy equation of state using the high redshift (z~2)
Hubble relation, but accounting also for the effects of gravitational lensing,
which for such high redshifts can significantly affect the derived cosmological
constraints. Based on a "Figure of Merit" analysis, we provide estimates for
the number of 2<z<3.5 tracers needed to reduce the cosmological solution space,
presently provided by the Constitution SNIa set, by a desired factor. We find
that it is much more efficient to increase the number of tracers than to reduce
their individual uncertainties.
  Finally, we propose a framework to put constraints on the dark energy
equation of state by using the joint likelihood of the X-ray AGN clustering and
of the Hubble relation cosmological analyses. A preliminary joint analysis
using the X-ray AGN clustering of the 2XMM survey and the Hubble relation of
the Constitution SNIa set provide: Omega_m= 0.31+-0.01 and w=-1.06+-0.05. We
also find that the joint SNIa-2XMM analysis provides significantly more
stringent cosmological constraints, increasing the Figure of Merit by a factor
~2, with respect to that of the joint SNIa-BAO analysis.
