A z   ∼  1 galactic-scale outflow transversally mapped to ∼ 50 kpc through gravitational-arc tomography

We report spatially resolved measurements of cool gas traced by Mg II and Fe II absorption in the circumgalactic medium (CGM) of a star-forming galaxy at z ∼ 1 (G1). The fortuitous alignment of a background gravitational arc at z ∼ 2.4 provides seven closely spaced (∼6 kpc) transverse sightlines along the minor axis of G1, probing its CGM out to ∼50 kpc. This geometry allowed us to detect a galactic-scale outflow simultaneously in the down-the-barrel and transverse directions, where we detected blueshifted Mg II absorption lines along both types of sightlines, revealing a large-scale collimated wind. We measured blueshifted line-of-sight velocities of v los ∼ 62 − 239 km s −1 and line-of-sight velocity dispersions of σ los ∼ 53 − 133 km s −1 , suggesting a structure dominated by bulk motion. De-projection of v los along the minor axis indicates that the outflow material barely approaches the escape velocity and is likely to be gravitationally bound to G1. We constrained an outflow opening angle θ c ∼ 18° −25°, and a mass outflow rate of Ṁ out ≳ 0.06 M ⊙ yr −1 , corresponding to a mass loading factor η ≳ 0.004, estimated within ∼10 − 50 kpc (∼0.05 − 0.3 R vir ) of the galaxy centre. Our measurements, combined with previous arc tomography data along the major axis, indicate that normalizing impact parameters by galaxy B-band luminosity substantially reduces scatter in the established anti-correlation between Mg II equivalent width and impact parameter, while also diminishing possible excess of Mg II equivalent width towards the minor axis.