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

• Published in: Monthly notices of the Royal Astronomical Society Vol. 508; no. 1; pp. 19 - 43
• Main Authors: Chen, Yuguang, Steidel, Charles C, Erb, Dawn K, Law, David R, Trainor, Ryan F, Reddy, Naveen A, Shapley, Alice E, Pahl, Anthony J, Strom, Allison L, Lamb, Noah R, Li, Zhihui, Rudie, Gwen C
• Format: Journal Article
• Language: English
• Published: Oxford University Press 01.11.2021
• Subjects: galaxies: evolution; galaxies: ISM; galaxies: high-redshift
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stab2383

ABSTRACT We present the first statistical analysis of kinematically resolved, spatially extended $\rm Ly\alpha$ 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 $\rm Ly\alpha$ 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 $\rm Ly\alpha$ 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 $\rm Ly\alpha$ 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 $\rm Ly\alpha$ emission along the galaxy principal axes is statistically indistinguishable, with residual asymmetry of ≤2 per cent (∼2σ) of the integrated $\rm Ly\alpha$ emission. The symmetry implies that the $\rm Ly\alpha$ scattering medium is dominated by outflows in all directions within 30 kpc. Meanwhile, we find that the blueshifted component of $\rm Ly\alpha$ emission is marginally stronger along galaxy minor axes for galaxies with relatively weak $\rm Ly\alpha$ emission. We speculate that this weak directional dependence of $\rm Ly\alpha$ emission becomes discernible only when the $\rm Ly\alpha$ escape fraction is low. These discoveries highlight the need for similar analyses in simulations with $\rm Ly\alpha$ radiative transfer modelling.