The MUSE Ultra Deep Field (MUDF)

VIII. The cool gas distribution surrounding galaxies at redshifts z ≈ 0.5–2

¹ Dipartimento di Fisica “G. Occhialini”, Università degli Studi di Milano-Bicocca, Piazza della Scienza 3 I-20126 Milano, Italy
² Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Stra β e 1 85748 Garching b. München, Germany
³ IUCAA, Postbag 4 Ganeshkind, Pune 411007, India
⁴ INAF – Osservatorio Astronomico di Trieste, Via G. B. Tiepolo 11, I-34143 Trieste, Italy
⁵ INAF – Osservatorio Astronomico di Brera, Via Brera 28, I-21021 Milano, Italy
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA

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

Received: 18 July 2025
Accepted: 19 February 2026

Abstract

We use deep MUSE data from the MUDF survey to investigate the cool gas around galaxies at redshifts 0.5 ≲ z ≲ 2. We constructed two samples: one sample for a down-the-barrel analysis, probing outflows via Mg II absorption against galaxy continua, and the other sample for projected galaxy pairs to examine the gas around the foreground galaxies in the transverse direction. From down-the-barrel stacked spectra, we detected blueshifted Mg II absorption, indicative of outflows, in which the absorption strength increases with stellar mass and star formation rate. Lower-mass galaxies exhibit weaker absorption, but higher outflow velocities, whereas higher-mass systems retain more cool gas with slower outflows. In the transverse direction, the absorption of Mg II decreases with the impact parameter, following a shallow profile. Comparing observations with radiative transfer models, we found that extrapolating an expanding halo model constrained with down-the-barrel measurements to halo scales overestimates the observed equivalent widths, likely due to the outflow geometry and the absence of the interstellar medium in the model. Our results highlight that mass, outflow geometry, and gas retention shape the cool circumgalactic medium, and that the combination of absorption and emission diagnostics provides powerful constraints on the properties of the cold halo gas.