Higher-resolution optical spectra of M_*< 10¹⁰ M_⊙ galaxies reveal outflow signatures unresolved by the SDSS

¹ Institute of Theoretical Astrophysics, University of Oslo, P.O. Box 1029 Blindern 0315, Oslo, Norway
² Dipartimento di Fisica e Astronomia Augusto Righi, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
³ INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 101, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Brera, Via Brera 28, 20121 Milano, Italy
⁵ INAF – Fundación Galileo Galilei, Rambla José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁶ Armagh Observatory and Planetarium, College Hill, Armagh BT61 9DG, UK
⁷ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
⁸ Department of Physics and Astronomy, University College London, London WC1E 6BT, UK
⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy

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

Received: 18 December 2025
Accepted: 1 April 2026

Abstract

Galactic outflows are predicted to be ubiquitous in low-mass galaxies, but observational evidence is lacking. Both a low signal-to-noise ratio and a low spectral resolution can severely hamper the detection of galactic outflows, especially in small galaxies that have intrinsically narrow spectral lines. In an effort to overcome these issues, we obtained new, medium-high resolution (FWHM_inst ∼ 50 − 110 km/s) optical spectra of 52 local star-forming galaxies (0.01 < z < 0.03) with stellar masses 10^8.5 < M_*/[M_⊙]< 10¹⁰, using the TNG/DOLORES and NTT/EFOSC2 instruments. Our parent sample consists of galaxies from the Sloan Digital Sky Survey (SDSS) with available heterodyne single-dish molecular (i.e., CO) line data. The targets of this study are selected among those that, based on the comparison between CO line widths, SDSS spectral resolution, and corresponding SDSS-based Hα line widths, have a high chance of being unresolved by SDSS spectroscopy. Our new, medium-high resolution spectra reveal overall narrower Hα and [OIII]λ5007 lines, with signs of asymmetries and broad wings that are absent in the SDSS spectra of the same galaxies. This confirms our hypothesis that SDSS spectroscopy does not resolve the narrow emission lines of low-M_* galaxies, which hinders the detection of outflows. We identify outflow signatures in ∼30% of our targets based on the Hα line spectra. Assuming a typical bi-conical outflow geometry, this detection rate is consistent with theoretical predictions of ubiquitous outflows in the low-mass regime. The outflow incidence is enhanced (∼60%) for galaxies with above average star-formation rates (SFR) for the sample (SFR > 10^−0.74 M_⊙/yr). We estimate ionized gas mass outflow rates ranging from ∼0.1 − 50 × 10⁻³ M_⊙/yr (mean ∼20 × 10⁻³ M_⊙/yr) and corresponding mass loading factors between 0.03 and 0.14 (mean ∼0.07) for the sample.