Baryonic Ecosystem IN Galaxies (BEINGMgII)

III. Cool gas reservoirs at 0.3 ≤ z ≤ 1.6 in the Dark Energy Survey

¹ Indian Institute of Astrophysics (IIA), Koramangala, Bangalore, 560034, India
² Pondicherry University, R.V. Nagar, Kalapet, Puducherry, 605014, India
³ Università degli Studi di Milano-Bicocca, Dip. di Fisica G. Occhialini, Piazza della Scienza 3, 20126, Milano, Italy
⁴ INAF – Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, I-34143, Trieste, Italy
⁵ Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, VIC, 3122, Australia
⁶ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 21021, Milano, Italy
⁷ European Southern Observatory, Karl-Schwarzschildstrasse 2, D-85748, Garching bei Munchen, Germany
⁸ Aix Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, F-13388, Marseille, France
⁹ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, 100871, PR China
¹⁰ Department of Astronomy, School of Physics, Peking University, Beijing, 100871, PR China

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

Received: 2 September 2025
Accepted: 22 January 2026

Abstract

We investigate the origin of intervening cool Mg II absorption detected in the spectra of background quasars and the nature of associated galaxies across a broad redshift range of 0.3 ≤ z ≤ 1.6, and a largely unexplored galactrocentric distance of ≲20 kpc. Using nebular [O II] λλ3727, 3729 emission lines identified in DESI fibre spectra centred on quasars, we detect 342 galaxies at a typical detection rate of ∼0.45% at z ≲ 1, which increases with the Mg II equivalent width (W₂₇₉₆). A significant fraction (74%) of these galaxies are associated with strong absorbers with W₂₇₉₆ ≥ 2 Å. These absorbers trace galaxies spanning stellar masses of 8.4 ≤ log(M_★/M_⊙)≤11.6 and star formation rates (SFRs) of −1 ≤ log(SFR [M_⊙ yr⁻¹]) ≤ 2.7, located at projected galactocentric distances of 4−24 kpc. We find that the average Mg II absorber strength increases from 2.1 Å to 2.8 Å between redshifts of z ∼ 0.4 and 1.2, indicating evolution in the cool gas content of galaxy halos. The relatively constant absorber strength with galactocentric distance implies a clumpy structure of cool gas in the circumgalactic medium (CGM). Further, we find a positive correlation between W₂₇₉₆ versus stellar mass (M_★), and the SFR, suggesting that the distribution of metal-enriched cool gas in the CGM is closely tied to the properties of the host galaxies. The redshift evolution of gas-phase metallicity suggests that strong Mg II absorbers trace the general population of star-forming galaxies. The velocity dispersion of the cool gas increases with halo mass, and the wide range of the line-of-sight velocity offset (−389 to 364 km s⁻¹) between the galaxy systemic velocity and absorbers highlights the dynamical nature of CGM. However, the majority of this gas remains gravitationally bound to the dark matter halos, consistent with a picture of gas recycling via galactic fountains.