## Abstract

We suggest to use the observationally measured and theoretically justified correlation between size and rotational velocity of galactic discs as a viable method to select a set of high redshift standard rods which may be used to explore the dark energy content of the universe via the classical angular-diameter test. Here we explore a new strategy for an optimal implementation of this test.We propose to use the rotation speed of high redshift galaxies as a standard size indicator and show how high resolution multi-object spectroscopy and ACS/HST high quality spatial images, may be combined to measure the amplitude of the dark energy density parameter ΩQ, or to constrain the cosmic equation of state parameter for a smooth dark energy component (w = p/ρ, −1 ≤ w < −1/3). Nearly 1300 standard rods with high velocity rotation in the bin V = 200 ± 20 km s−1 are expected in a field of 1 sq. degree and over the

redshift baseline 0 < z < 1.4. This sample is sufficient to constrain the cosmic equation of state parameter w at a level of 20% (without priors in the [Ωm,ΩQ] plane) even when the [OII]λ3727 Å linewidth-diameter relationship is calibrated with a scatter of ∼40%. We evaluate how systematics may affect the proposed tests, and find that a linear standard rod evolution, causing galaxy dimensions

to be up to 30% smaller at z = 1.5, can be uniquely diagnosed, and will minimally bias the confidence level contours in the [ΩQ, w] plane. Finally, we show how to derive, without a priori knowing the specific functional form of disc evolution, a cosmologyevolution diagram with which it is possible to establish a mapping between different cosmological models and the amount of galaxy

disc/luminosity evolution expected at a given redshift.