WLM: Dynamics of an isolated dwarf irregular galaxy under ram pressure in the Local Group

¹ LIRA, Observatoire de Paris, Universite PSL, CNRS, Place Jules Janssen, 92195 Meudon, France
² Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
³ Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, 67000 Strasbourg, France
⁴ Instituto de Astrofísica, Facultad de Ciencias Exactas, Universidad Andres Bello, Fernandez Concha 700, Las Condes, Santiago RM, Chile
⁵ Aix Marseille Univ, CNRS, CNES, LAM, Marseille, France
⁶ Department of Astronomy, University of Cape Town, Private Bag X3, Rondebosch 7701, South Africa
⁷ Département de physique, Université de Montréal, Complexe des sciences MIL, 1375 Avenue Thérèse-Lavoie-Roux Montréal, QC H2V 0B3, Canada
⁸ Observatoire d’Astrophysique de l’Université Ouaga I Pr Joseph Ki-Zerbo (ODAUO), BP 7021 Ouaga 03, Burkina Faso

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

Received: 23 September 2025
Accepted: 18 January 2026

Abstract

The Wolf–Lundmark–Melotte (WLM) galaxy is an archetypal dwarf irregular galaxy that has not experienced interactions with major Local Group galaxies within the past 8 Gyr. It has recently been shown that WLM is losing its gas due to ram pressure forces exerted by the surrounding intergalactic medium. In this work, we explored how ram pressure can also affect the WLM gas kinematics, and we show that its dynamics is especially perturbed at its outskirts, which explains the asymmetric rotation between the approaching and receding sides. Moreover, we have been able to decompose WLM into two main components, a compact one with a solid-body rotation that resembles a bar-like structure, and a more extended one with a characteristic double-horn profile suggesting an edge-on disc. The former is relatively unaffected by ram pressure while the dynamics of the latter is considerably affected. This study shows that mass estimates of a dwarf galaxy like WLM should involve a full modelling of its dynamical components, especially its asymmetric rotation curve.