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

¹ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436, Macul, Santiago, Chile
² Departamento de Astronomía, Universidad de Chile, Casilla 36-D, Santiago, Chile
³ Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias, Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
⁴ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago, Chile
⁵ Instituto de Física, Pontificia Universidad Católica de Valparaíso, Casilla 4059, Valparaíso, Chile
⁶ Dipartimento di Fisica e Astronomia, Universitá di Firenze, Via G. Sansone 1, 50019, Sesto Fiorentino, Firenze, Italy
⁷ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, I-50125, Italy
⁸ Aix-Marseille Univ., CNRS, CNES, LAM, Marseille, France
⁹ Institut d’Astrophysique de Paris, CNRS-SU, UMR 7095, 98bis bd Arago, 75014, Paris, France
¹⁰ Department of Physics and Astronomy, Camosun College, 3100 Foul Bay Rd, Victoria, B.C., V8P 5J2, Canada

^★★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 September 2025
Accepted: 19 February 2026

Abstract

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.