Ram pressure stripping of the multiphase ISM in the Virgo cluster spiral galaxy NGC 4438

Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Halpha emission are closely linked. The kinematics of the northern extraplanar Halpha emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Halpha measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 mum emission are due to this difference in stripping efficiency.

💡 Research Summary

This paper investigates how the multiphase interstellar medium (ISM) of the perturbed Virgo‑cluster spiral galaxy NGC 4438 responds to ram‑pressure stripping (RPS). NGC 4438 experienced a strong tidal encounter roughly 100 Myr ago and is now moving through the dense intracluster medium (ICM) of Virgo, which exerts a substantial dynamic pressure on its gas. The authors present deep Very Large Array (VLA) radio continuum observations at 6 cm and 20 cm, complemented by archival H α, CO(1‑0), and 24 µm data, to trace the magnetic field, warm ionized gas, molecular gas, and dust‑heated emission, respectively.

The radio maps reveal a prominent extraplanar (extra‑disk) emission feature extending west of the galaxy’s nucleus. This feature is roughly twice as extended as the extraplanar structures seen in H α, CO, or 24 µm emission, indicating that relativistic electrons and magnetic fields have been displaced farther than the bulk neutral and ionized gas. The spectral index (α) of the extraplanar radio emission remains roughly constant with distance from the disk, showing no systematic steepening that would be expected from simple synchrotron aging. The authors interpret this as evidence for in‑situ re‑acceleration of cosmic‑ray electrons, likely driven by compression and turbulence of the magnetic field as the ICM wind interacts with the galaxy’s ISM.

A detailed kinematic comparison shows that the northern extraplanar H α component, which is associated with ongoing star formation, shares the same line‑of‑sight velocities as the extraplanar CO emission. This suggests that, in this region, the molecular and ionized phases are still tightly coupled and have not been strongly decoupled by RPS. In contrast, in the western and southern extraplanar zones the H α velocities exceed those of CO by several tens of km s⁻¹. The authors argue that the ionized gas in these regions is being accelerated more efficiently by ram pressure than the denser molecular gas. This differential acceleration naturally produces the observed spatial and velocity offsets between the ionized and neutral components.

The paper also addresses the recently reported “radio‑deficient” zones where the 24 µm dust emission is relatively strong but the radio continuum is weak. The authors propose that these deficits arise because the diffuse ionized gas, which contributes significantly to the synchrotron emission, is stripped more readily than the neutral gas that dominates the 24 µm emission. Consequently, the radio‑to‑infrared ratio drops in regions where the ionized component has been removed or strongly displaced.

Overall, the study provides a coherent picture of how a multiphase ISM reacts to ram‑pressure stripping in a cluster environment. The key insights are: (1) magnetic fields and relativistic electrons can be pushed far beyond the neutral gas, with ongoing particle re‑acceleration maintaining a flat spectral index; (2) the warm ionized medium is more susceptible to ram‑pressure acceleration than the molecular component, leading to measurable velocity offsets; (3) the observed radio‑deficient regions are a natural consequence of the differing stripping efficiencies of the ionized versus neutral phases. These results underscore the importance of considering each ISM phase separately when modelling galaxy evolution in dense environments, and they highlight the role of magnetic field dynamics and cosmic‑ray physics in shaping the observable signatures of ram‑pressure stripping.