2010MNRAS.402.2520S

We have measured the two-dimensional two-point correlation function, ξ_AG, between low-column-density Lyα absorbers and galaxies at a redshift z ∼ 1. We measured Lyα absorbers between redshifts z = 0.68 and 1.51 over a total redshift path length of Δz = 1.08 from Hubble Space Telescope Space Telescope Imaging Spectrograph E230M absorption spectra towards the quasars HE 1122-1648 (z = 2.4) and PKS 1127-145 (z = 1.187). The column density of the Lyα absorbers ranged from 13.2 ≤ log₁₀[N_HI(cm^–2)] ≤ 17.4, with a median column density of log₁₀[N_HI(cm^–2)] = 14.0. A total of 193 galaxy redshifts within the surrounding 6.8x5.7arcmin² field of view of both quasars were identified in a R-magnitude-limited survey (21.5 ≤ R_Vega ≤ 24.5) using the Focal Reducer and Low Dispersion Spectrograph (FORS2) at the Very Large Telescope, of which 95 were higher than the minimum redshift of z = 0.68 to be used in the correlation function. A 3σ upper limit of ξ_AG= 2.8 was found when 145 Lyα absorber-galaxy pairs were binned in redshift space, in a bin of size Δσ = 1.0,Δπ = 2.0h^–1₇₀ Mpc along the projected separation and line-of-sight distances, respectively.

The upper limit in the cross-correlation was found to be 5.4σ lower than the central peak in the galaxy autocorrelation within the same redshift range, ξ_GG, which was in our data equal to 10.7±1.4. Thus, we have shown for the first time that the clustering between low-column-density absorbers and galaxies at a redshift of 1 is weaker than that between galaxies at the same redshift.