First statistical constraints on galactic-scale outflow properties traced by their extended Mg II emission with MUSE

¹ Leibniz-Institut for Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
² Univ of Lyon1, Ens de Lyon, CNRS, Centre de Recherche Astrophysique de Lyon (CRAL) UMR5574, F-69230 Saint- Genis-Laval, France
³ NSF’s National Optical-Infrared Astronomy Research Laboratory, 950 N. Cherry Ave., Tucson, AZ 85719, USA
⁴ Department of General Systems Studies, Graduate School of Arts and Sciences, The University of Tokyo, 3-8-1 Komaba, Meguro-ku, Tokyo 153-8902, Japan

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

Received: 28 October 2025
Accepted: 10 February 2026

Abstract

Galaxies evolve within vast gaseous halos that fuel star formation and carry signatures of feedback-driven outflows. Deep integral field data have enabled the study of Mg IIλλ2796, 2803 halos, which trace galaxy-scale outflows in emission, and while individual detections of Mg II halos have revealed extended circumgalactic structures, their faintness has limited studies to single-object analyses. Here, we present the first statistical study of Mg II-emitting halos using deep MUSE observations of 47 star-forming galaxies at 0.7 < z < 2.0. Building on our previous work, where we developed and applied an outflow modeling framework to a single Mg II halo, we now extend this approach to a larger sample, enabling robust population-level insights into the properties of circumgalactic outflows traced by their extended Mg II emission for the first time. We detected extended Mg II emission out to tens of kiloparsecs and modeled the outflows as an ensemble of radially accelerating shells. Galaxies with Mg II outflows tend to have higher star formation rates (SFRs) and specific star formation rates (sSFRs) and younger stellar populations–consistent with star-formation-driven winds. The observations are consistent with winds that accelerate linearly with radius (v ∝ r) from launching velocities of v ∼ 60 km s⁻¹ up to maximum velocities that correlate with the stellar mass of galaxies, and are of about ∼490 km s⁻¹. The inner regions of the outflows are highly opaque (log τ ∼ 2.6), and we also identify a tentative trend between stellar mass and central optical depth. The opening angle of the outflow shows some dependency on the host-galaxy stellar mass, with less massive galaxies showing primarily wide opening angles (i.e., nearly isotropic outflows), and more massive galaxies showing a broader range of values, with both wide and narrow opening angles. The distribution of the spatial extent of Mg II halos exhibits a clear peak at half-light radius (HLR) of ∼5 kpc, with an extended tail of larger HLR values, up to ∼20 kpc. Compact halo sizes (HLR < 8 kpc) correlate with stellar mass, but extended halos do not, which could suggest a difference in the powering mechanism between compact and extended halos. This work provides new insight into the structure and dynamics of the circumgalactic medium and its role in galaxy evolution by constraining, for the first time, the properties of circumgalactic outflows with a statistically significant sample of galaxies with an extended Mg II emission halo.