Orientation effects on cool gas absorption from gravitational-arc tomography of a z = 0.77 disc galaxy

We use spatially resolved spectroscopy of a distant giant gravitational arc to test orientation effects on Mg II absorption equivalent width (EW) and covering fraction (κ) in the circumgalactic medium of a foreground star-forming galaxy (G1) at z ∼ 0.77. Forty-two spatially-binned arc positions uniformly sample impact parameters (D) to G1 between 10 and 30 kpc and azimuthal angles α between 30^◦ and 90^◦ (minor axis). We find an EW-D anticorrelation, akin to that observed statistically in quasar absorber studies, and an apparent correlation of both EW and κ with α, revealing a non-isotropic gas distribution. In line with our previous results on Mg II kinematics suggesting the presence of outflows in G1, at minimum a simple 3D static double-cone model (to represent the trace of bipolar outflows) is required to recreate the EW spatial distribution. The D and α values probed by the arc cannot confirm the presence of a disc, but the data highly disfavour a disc alone. Our results support the interpretation that the EW-α correlation observed statistically using other extant probes is partly shaped by bipolar metal-rich winds.