Ram-pressure signatures in the dwarf irregular galaxy Sextans B revealed by deep MeerKAT H I observations

¹ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
² LIRA, Observatoire de Paris, Université PSL, CNRS, 92190 Meudon, France
³ Department of Astronomy, University of Cape Town, Private Bag X3 Rondebosch 7701, South Africa
⁴ Département de physique, Université de Montréal, Complexe des sciences MIL, 1375 Avenue Thérèse-Lavoie-Roux, Montréal, QC H2V 0B3, Canada
⁵ Observatoire d’Astrophysique de l’Université Ouaga I Pr Joseph Ki-Zerbo (ODAUO), BP 7021 Ouaga 03, Burkina Faso
⁶ Max-Planck-Institut für Radioastronomie Radioobservatorium Effelsberg, Max-Planck-Strasse 28, 53902 Bad Münstereife, Germany
⁷ Centre for Radio Astronomy Techniques and Technologies (RATT), Department of Physics and Electronics, Rhodes University, Makhanda 6140, South Africa
⁸ Local Universe and Time-Domain Astronomy Laboratory, Department of Astronomy, China West Normal University, Nanchong 637002, China
⁹ Research Center for Computational Earth and Space Science, Zhejiang Laboratory, Hangzhou 311121, China
¹⁰ The Copperbelt University, Jambo Drive, Riverside, P.o.Box 21692 Kitwe, Zambia

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

Received: 3 April 2026
Accepted: 16 June 2026

Abstract

Context. The impact of extremely low-density environments such as the diffuse intergalactic medium (IGM) on the neutral gas distribution of dwarf galaxies remains poorly explored observationally.

Aims. We present deep MeerKAT H I 21 cm observations of the Local Group dwarf irregular galaxy Sextans B that achieve a spectral resolution of 1.4 km s⁻¹ and reach column-density sensitivities down to N_HI ∼ 3.3 × 10¹⁸ cm⁻², allowing us to trace the extended H I disc and faint outer structures with high sensitivity.

Methods. We analysed the H I distribution and compared it with the stellar component. Three-dimensional kinematic modelling of the H I cube was performed using TiRiFiC. We performed hydrodynamical simulations tailored to Sextans B, which show that the IGM ram-pressure acting on the outer gas disc can produce asymmetric gas distributions, filamentary structures, and kinematic perturbations.

Results. The low-column-density H I distribution is asymmetric and reveals a remarkable filamentary structure arranged in a rosette superposed on the H I disc. A comparison with the stellar distribution shows spatial offsets between the gaseous and stellar components, with the stellar disc remaining relatively symmetric and the H I envelope becoming increasingly disturbed. Three-dimensional kinematic modelling of the H I cube using TiRiFiC reproduces the global velocity gradient but reveals systematic differences between the approaching and receding sides of the rotation curve at large radii, indicating that the outer velocity field departs from simple axisymmetric rotation. While stellar feedback can produce small-scale cavities and turbulence in dwarf galaxies, it cannot generate the H I filamentary structure, the large-scale asymmetric outer H I envelope, or the systematic divergence between the approaching and receding rotation curves observed here. This is consistent with the effects expected from interaction with a diffuse IGM.

Conclusions. The combination of morphological and kinematic signatures therefore suggests that the outer H I disc of Sextans B is affected by a ram-pressure interaction with the diffuse IGM in the outskirts of the Local Group. This is the second strong example in the Local Group, after WLM, showing that even a very low-density IGM can significantly influence the gas distribution and kinematics in the outer parts of dwarf galaxies.