The galaxy-dark matter bias in the Garching-Bonn Deep Survey

Abstract

We measured the bias and correlation factor of galaxies with respect to the dark matter using the aperture statistics including the aperture mass from weak gravitational lensing. The analysis was performed for three galaxy samples selected by R-band magnitudes; the median redshifts of the samples were {z} =0.34,0.49 and 0.65, respectively. The brightest sample has the strongest peak in redshift and can therefore be most accurately represented by a single redshift. The data used is the GaBoDS, and the COMBO-17 survey for an accurate estimate of the redshift distribution of the galaxies. Assuming the currently favoured δCDM model as cosmology, we obtained values for the linear stochastic galaxy-dark matter bias on angular scales 1′ ≤θ ≤20′. At 10′, the median redshifts of the samples correspond to a typical physical scale of 0.90,1.25,1.56 Mpc/h with h = 0.7, respectively. Over the investigated range of physical scales the bias factor b stays almost constant, possibly rising on the smallest scales. Here the errors are largest, however. Averaging the measurements for the bias over the range 4′ ≤θap ≤18′, weighting with the cosmic variance error, yields b = 0.89(5),0.79(5),0.89(5), respectively (1σ). Galaxies are thus less clustered than the total matter on that particular range of scales (anti-biased). This is what also has been observed by Hoekstra et al. (2002), albeit that their increasing trend towards a larger bias factor on larger scales is not visible in our analysis. As for the correlation factor r we see, as Hoekstra et al., a slight increase to r =1 in the last angular bin from an almost constant value on smaller scales; the weighted average here over the same range as before is r = 0.8(1),0.8(1),0.5(1), respectively. Therefore, on these scales we find a degree of stochasticity or/and nonlinearity in the relation between dark matter and galaxy density. Within the measurement uncertainties and over the redshift range represented by our galaxy samples we do not see an evolution with redshift of the bias. Show more