The KBSS–KCWI survey: The connection between extended Ly α haloes and galaxy azimuthal angle at z ∼ 2–3

Abstract

We present the first statistical analysis of kinematically resolved, spatially extended Lyα emission around z = 2–3 galaxies in the Keck Baryonic Structure Survey (KBSS) using the Keck Cosmic Web Imager (KCWI). Our sample of 59 star-forming galaxies (zmed = 2.29) comprises the subset with typical KCWI integration times of ∼5 h and with existing imaging data from the Hubble Space Telescope and/or adaptive optics-assisted integral field spectroscopy. The high-resolution images were used to evaluate the azimuthal dependence of the diffuse Lyα emission with respect to the stellar continuum within projected galactocentric distances of ≲30 proper kpc. We introduce cylindrically projected 2D spectra (CP2D) that map the averaged Lyα spectral profile over a specified range of azimuthal angle, as a function of impact parameter around galaxies. The averaged CP2D spectrum of all galaxies shows clear signatures of Lyα resonant scattering by outflowing gas. We stacked the CP2D spectra of individual galaxies over ranges of azimuthal angle with respect to their major axes. The extended Lyα emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated Lyα emission. The symmetry implies that the Lyα scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of Lyα emission is marginally stronger along galaxy minor axes for galaxies with relatively weak Lyα emission. We speculate that this weak directional dependence of Lyα emission becomes discernible only when the Lyα escape fraction is low. These discoveries highlight the need for similar analyses in simulations with Lyα radiative transfer modelling.