Tracing ionized gas kinematics in Lyman-break Analogs

Implications for star formation compactness and outflow properties

¹ Departamento de Astronomía, Universidad de La Serena Av. Juan Cisternas 1200 Norte La Serena, Chile
² Instituto de Astrofísica de Andalucía (CSIC) Apartado 3004 18080 Granada, Spain
³ Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Unidad Asociada al CSIC Plaza San Juan 1 E–44001 Teruel, Spain
⁴ INAF – Osservatorio Astronomico di Roma Via di Frascati 33 00078 Monte Porzio Catone, Italy
⁵ Michigan Institute for Data Science, University of Michigan 500 Church Street Ann Arbor MI 48109, USA
⁶ Space Telescope Science Institute 3700 San Martin Drive Baltimore MD 21218, USA

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

Received: 27 June 2025
Accepted: 12 November 2025

Abstract

Context. The ionized gas kinematics of low-mass starburst galaxies is a tracer of galaxy interactions and feedback processes, which are key for understanding massive star formation, chemical enrichment, and galaxy evolution.

Aims. We studied the ionized gas kinematics and outflow properties of a sample of Lyman-break analogs (LBAs) at z ∼ 0.1 − 0.3; these LBAs are characterized by compact morphologies, high UV luminosities, and strong emission lines, which are common at higher redshifts.

Methods. We used high-resolution VLT/X-shooter spectra of 14 compact, UV-luminous LBAs to model the complex [O III]λλ4959,5007 Å and Balmer line profiles with multi-Gaussian fits.

Results. The kinematics of LBAs are complex, with emission lines best reproduced by narrow (σ < 90 km s⁻¹) and broad (σ > 90 km s⁻¹) components in all galaxies. The narrow-line kinematics is highly turbulent, likely driven by massive star-forming clumps. The luminosities and line ratios of the narrow components are typical of giant H II regions. We interpret the broader components as ionized outflows driven by strong winds of massive stars and supernovae. In galaxies with highly complex profiles and disturbed morphologies, ongoing interactions or mergers are found to contribute to the broad components. We find outflow velocities (v_out) in the range ∼200 km s⁻¹ to 500 km s⁻¹. Simple models yield outflow mass rates of 0.20–2.72 M_⊙ yr⁻¹ and mass-loading factors (η) of ∼0.03–0.81. We find that η shows a mild increasing trend at lower stellar masses, in agreement with previous observational studies and predictions from FIRE-2 and Illustris-TNG simulations. Compact starburst morphologies can modulate the η-M_★ relation, which is strongly correlated as Σ_SFR-η, i.e., more compact starbursts drive stronger outflows. We find a good agreement with similar findings in star-forming galaxies at high redshifts (z ∼ 2 − 9), including those from recent JWST observations.

Conclusions. Our results highlight the relevance of detailed studies of the ionized gas kinematics in local UV-compact starbursts to improve our understanding of feedback processes in low-mass, rapidly star-forming galaxies.