Measuring the Cosmic Equation of State with Counts of Galaxies
Abstract
The classical dN/dz test allows the determination of fundamental cosmological parameters from the evolution of the cosmic comoving volume element. It requires the measurement of the redshift distribution of some tracer the evolution of whose number density is known. In the past, this test has been applied using ordinary galaxies as such a tracer; however, in the absence of a complete theory of galaxy formation, that method is fraught with difficulties. In this paper, we propose studying instead the evolution of the apparent numbers of dark matter halos as a function of their circular velocity, observable via the linewidths or rotation speeds of visible galaxies. Upcoming redshift surveys will allow the linewidth distribution of galaxies to be studied at both z~1 and the present day. In the course of studying this test, we have devised a rapid, improved semi-analytic method for calculating the distribution of dark halos in circular velocity based upon the analytic mass function of Sheth et al. (1999) and the formation time distribution of Lacey & Cole (1993). We find that if selection effects are well-controlled and Omega_m and Omega_Q are known, the upcoming DEEP Redshift Survey could allow the determination of the quintessence parameter w to ~1% and provide a constraint on any evolution of w such as that predicted by ``tracker'' models.Keywords
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