The 2dF QSO Redshift Survey – X. Lensing of background QSOs by galaxy groups

We cross-correlate quasi-stellar objects (QSOs) from the 2dF QSO Redshift Survey with groups of galaxies. In the southern region of the 2dF we utilize galaxies from the APM Survey. In the northern strip, galaxies are taken from the recent Sloan Digital Sky Survey Early Data Release. Both galaxy samples are limited to a depth B < 20.5. We use an objective clustering algorithm to detect groups in these galaxy catalogues.

A 3σ anticorrelation is observed between 2dF QSOs and galaxy groups, confirming the effect found by Boyle, Fong & Shanks in an independent data set. This paucity of faint QSOs around groups cannot be readily attributed to a selection effect and is not due to restrictions on the placement of 2dF fibres. By observing the colours of QSOs on the scales of the anticorrelation, we limit the influence intervening dust in galaxy groups can have on background QSO flux, finding a maximum reddening on the scale of the anticorrelation of E(b_j−r) ≤ 0.012 at the 95 per cent level. The small amount of dust inferred from the QSO colours would be insufficient to account for the anticorrelation, supporting the suggestion by Croom & Shanks that the signal is likely to be caused by weak gravitational lensing. The possibility remains that tailored dust models involving grey dust, heavy patches of dust or a combination of dust and lensing, could explain the anticorrelation.

Under the assumption that the signal is caused by lensing rather than dust, we measure the average velocity dispersion of a singular isothermal sphere that would cause the anticorrelation, finding σ∼ 1150 km s⁻¹. Simple simulations reject σ∼ 600 km s⁻¹ at the 5 per cent significance level. We also suppose that the foreground mass distribution consists of dark matter haloes with an Navarro–Frenk–White (NFW) profile and measure the typical mass within 1.5 h⁻¹ Mpc of the halo centre as M_1.5= 1.2 ± 0.9 × 10¹⁵ h⁻¹ M_⊙.

Regardless of whether we utilize a singular isothermal sphere or NFW dark matter profile, our simple lensing model favours more mass in groups of galaxies than is accounted for in a universe with density parameter Ω_m= 0.3. Detailed simulations and galaxy group redshift information will significantly reduce the current systematic uncertainties in these Ω_m estimates. Reducing the remaining statistical uncertainty in this result will require larger QSO and galaxy group surveys.