The association between gas and galaxies - III. The cross-correlation of galaxies and Lyα absorbers† at z ~ 1

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 6.8 × 5.7arcmin² 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^-1₇₀Mpc 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.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile. European Southern Observatory (ESO) programme number 076.A-0312.

Based on observations made with the NASA/ESA Hubble Space Telescope, and obtained from the Hubble Legacy Archive, which is a collaboration between the Space Telescope Science Institute (STScI/NASA), the Space Telescope European Coordinating Facility (ST-ECF/ESA) and the Canadian Astronomy Data Centre (CADC/NRC/CSA). Additional data based on observations from the Ultraviolet and Visual Echelle Spectrograph (UVES) were collected at the European Organisation for Astronomical Research in the Southern hemisphere, Chile. ESO programme numbers 067.A-0567(A), 068.A-0570(A) and 069.A-0371(A). ‡

E-mail: allen.shone@googlemail.com