The Muenster Redshift Project. III. Observational Constraints on the Deceleration Parameter

The redshift-volume test for determining the deceleration parameter q₀ is applied to 89,125 galaxies with redshifts z <= 0.2 (redshift errors σ_z = 0.031) and magnitudes 14.0 <= r_F <= 18.0 mag, obtained within the Muenster Redshift Project (MRSP). With samples of this size, cosmic curvature effects can be measured even at intermediate redshifts. Comparatively small z-values and red photometric magnitudes assure that biased object selection and galaxy evolution do not affect the measurements in uncontrolled ways. In the first step of our analysis, the redshift-volume test assumes a minimum model of passive galaxy evolution. For the cosmological constant Λ = 0 and for the evolutionary models of Rocca-Volmerange & Guiderdoni, the total sample yields the deceleration parameter q₀ = 0.10 with the 95% confidence limit, q₀ < 0.75. In a second step, we evaluate--within the errors of the first step--whether our q₀-value is over- or underestimated, using those observed evolutionary trends that appear to be nearly q₀ independent. The trends indicate that our result q₀ = 0.10 can be regarded as an upper limit. Effects of incompleteness, errors in the (K + E)-corrections due to extreme galaxy mixtures, as well as different models of population synthesis, large-scale clustering, galactic reddening, and gravitational lensing, are discussed. We conclude that the combination of MRSP redshift data, observed evolutionary trends in the galaxy luminosity functions, and passive galaxy aging suggests an open universe.