[Paper Review] The uneven sisters I, NGC 4388 - a strongly constrained ram pressure stripping event

B. Vollmer, C. Pappalardo+2 more|arXiv (Cornell University)|Sep 11, 2018

Galaxies: Formation, Evolution, Phenomena42 references3 citations

TL;DR

This study combines deep multi-wavelength observations of NGC 4388—spanning H I, Hα, FUV, polarized radio continuum, and a deep optical spectrum—with 3D hydrodynamical simulations to constrain the ram pressure stripping event. It finds that the galaxy experienced peak ram pressure ~240 Myr ago at a wind angle of 30° to the disk, and that current asymmetries result from post-stripping gas resettling, not ongoing stripping.

ABSTRACT

Since the Virgo cluster is the closest galaxy cluster in the northern hemisphere, galaxy interactions can be observed in it with a kpc resolution. The spiral galaxy NGC 4388 underwent a ram pressure stripping event ~200 Myr ago caused by its highly eccentric orbit within the Virgo cluster. This galaxy fulfills all diagnostic criteria for having undergone active ram pressure stripping in the recent past: a strongly truncated HI and Halpha disk, an asymmetric ridge of polarized radio continuum emission, extended extraplanar gas toward the opposite side of the ridge of polarized radio continuum emission, and a recent (a few 100 Myr) quenching of the star formation activity in the outer, gas-free galactic disk. We made dynamical simulations of the ram pressure stripping event to investigate the influence of galactic structure on the observed properties of NGC 4388. The combination of a deep optical spectrum of the outer gas-free region of the galactic disk together with deep HI, Halpha, FUV, and polarized radio continuum data permits to constrain numerical simulations to derive the temporal ram pressure profile, the 3D velocity vector of the galaxy, and the time since peak ram pressure with a high level of confidence. From the simulations an angle between the ram pressure wind and the galactic disk of 30 degrees is derived. The galaxy underwent peak ram pressure ~240 Myr ago. The observed asymmetries in the disk of NGC 4388 are not caused by the present action of ram pressure, but by the resettling of gas that has been pushed out of the galactic disk during the ram pressure stripping event. For the detailed reproduction of multi-wavelength observations of a spiral galaxy that undergoes or underwent a ram pressure stripping event, galactic structure, i.e. spiral arms, has to be taken into account.

Motivation & Objective

• To constrain the timing, geometry, and ram pressure profile of a recent ram pressure stripping event in NGC 4388 using multi-wavelength data.
• To investigate how galactic structure—specifically spiral arms—affects the morphology and kinematics of stripped gas during ram pressure stripping.
• To reconcile observational constraints from H I, Hα, FUV, polarized radio emission, and optical spectra with numerical simulations.
• To determine the 3D velocity vector of NGC 4388 and the time since peak ram pressure with high confidence.
• To assess whether the observed asymmetries in NGC 4388 are due to ongoing ram pressure or residual effects from a past event.

Proposed method

• Conduct 3D hydrodynamical simulations of the interstellar medium (ISM) in NGC 4388 under varying ram pressure profiles and wind angles.
• Vary the orientation of spiral arms relative to the galaxy’s motion to assess their influence on gas stripping morphology.
• Incorporate magnetic field evolution and synchrotron emission to model polarized radio continuum emission.
• Compare simulated H I, Hα, FUV, and polarized radio maps with deep archival observations to identify best-fit models.
• Use stellar population synthesis models applied to a deep optical spectrum of the outer gas-free disk to constrain the time since star formation quenching.
• Perform a systematic comparison of ten simulation variants to identify those matching the most observational constraints.

Experimental results

Research questions

• RQ1 What is the time since peak ram pressure in NGC 4388, and how does it align with the quenching time of star formation in the outer disk?
• RQ2 What is the 3D velocity vector of NGC 4388 relative to the intracluster medium, and what is the angle between the ram pressure wind and the galactic disk?
• RQ3 To what extent do spiral arms influence the morphology and kinematics of the stripped H I tail and polarized radio emission?
• RQ4 Are the observed asymmetries in the H I and radio continuum structures caused by ongoing ram pressure or by the relaxation of gas after stripping?
• RQ5 What is the required suppression of thermal conductivity to explain the observed H I tail length and stripping timescale?

Key findings

• The peak ram pressure occurred approximately 240 Myr ago, with a peak pressure of 8.3 × 10⁻¹¹ dyn cm⁻².
• The angle between the ram pressure wind and the galactic disk is constrained to 30°, based on the best-fit simulations.
• The 3D velocity vector of NGC 4388 is (0.11, -0.69, 0.72), corresponding to a total velocity of ~2200 km s⁻¹ relative to the intracluster medium.
• The time since quenching of star formation in the outer, gas-free disk is ~200 Myr, slightly younger than the time since peak ram pressure.
• Only two simulations (models 9 and 10) with an inclination of 75° reproduce the majority of observational constraints, including H I tail extent, H I asymmetry, and polarized radio ridge.
• The presence of a low-column-density H I tail with a stripping timescale of ~200 Myr is consistent with thermal conductivity suppressed by a factor of 25–50.