WLM: Dynamics of an isolated Dwarf Irregular Galaxy Under Ram Pressure in the Local Group

TL;DR

This work demonstrates that the isolated dwarf irregular WLM experiences ram-pressure–driven perturbations in its outer HI disk while retaining a resilient central, bar-like component. Using high-resolution MeerKAT data (re-analyzed MeerKAT-16 and a new MeerKAT-64 mosaic) and 3D tilted-ring modeling with TiRiFiC, the authors reveal a two-component HI structure: a compact inner component with solid-body rotation and a more extended outer disk with a double-horn profile, the latter being heavily influenced by ram pressure. The kinematic decomposition confirms distinct, physically meaningful components, and the rotation curves show that ram pressure reshapes gas dynamics beyond ~1 kpc, leading to shifts in the dynamical center and asymmetries between approaching and receding sides. These findings imply that mass estimates for dwarf galaxies must account for multi-component dynamics and environmental effects, and motivate hydrodynamical simulations to constrain the Local Group intergalactic medium density and the galaxy’s total mass.

Abstract

WLM 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 (IGM). In this work, we explore how ram pressure may also affect the WLM gas kinematics, and we show that its dynamics is especially perturbed at its outskirts, explaining the asymmetric rotation between the approaching and receding sides. Moreover, we have been able to decompose WLM in 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 disk. The former is relatively unaffected by ram pressure while the latter has its dynamics considerably affected by ram pressure. This study shows that mass estimates of a dwarf galaxy like WLM should account for a full modeling of its dynamical components, especially accounting for its asymmetric rotation curve.

Figures (17)

• Figure 1: Top and bottom panels show our new masked MeerKAT16 moment-0 and 1 maps of WLM. Both panels reveal the stripped HI tail. The green arrow indicates proper motion and the transparent circle showing the 1$\sigma$ uncertainty as presented in battaglia_gaia_2022 and yang_evidence_2022. The green circle at the bottom left marks the beam size.
• Figure 2: HI high-resolution moment maps of MeerKAT 64 dish observation of WLM. All the figures are masked by our new MeerKAT64 moment-0 map (left panel).The contours of the moment-0 map correspond to the ticks of the colourbar. Central and right panels show respectively the rest frame moment-1 velocities and moment-2 dispersion map corrected from instrumental broadening. The contours of the moment-1 map correspond to a velocity range between -40 to 40 km s$^{-1}$ with a spacing of 5 km s$^{-1}$ and for the moment-2 map they correspond to 6,9 and 12 km s$^{-1}$, respectively The green circle shows the beamsize.
• Figure 3: Global PV diagram of WLM taken along the PA reported in Table 1. The contours correspond to the ticks on the colour bar.
• Figure 4: Top: A reproduction of the two Sersic components of the stellar profile of WLM from higgs_solo_2021. Bottom: Two Sersic components of the HI profile.
• Figure 5: Emission profile of the whole body of WLM from 3 major HI observations. The GBT observations are from the same data cube used in ianjamasimanana_meerkat-16_2020, Little things VLA data is from hunter_little_2012. All observations were masked by SoFIA-2 and a common velocity range was set. Velocities are in the rest-frame .