[Paper Review] Molecular gas filaments and fallback in the ram pressure stripped Coma spiral NGC 4921

W. J. Cramer, Jeffrey D. P. Kenney+7 more|arXiv (Cornell University)|Jul 25, 2021

Galaxies: Formation, Evolution, PhenomenaPhysics and Astronomy86 references31 citations

TL;DR

This study presents the first observational evidence of molecular gas fallback in a ram pressure stripped galaxy, NGC 4921, using high-resolution CO(1-0) and CO(2-1) observations from CARMA and ALMA. It identifies compressed molecular gas on the leading side, kiloparsec-scale filaments kinematically linked to the main gas disk, and three blueshifted molecular clouds behind the disk midplane, suggesting gravitational fallback of stripped gas—challenging standard ablation models and implicating magnetic binding in filament formation.

ABSTRACT

We investigate the effects of ram pressure on the molecular ISM in the disk of the Coma cluster galaxy NGC 4921, via high resolution CO observations. We present 6" resolution CARMA CO(1-0) observations of the full disk, and 0.4" resolution ALMA CO(2-1) observations of the leading quadrant, where ram pressure is strongest. We find evidence for compression of the dense interstellar medium (ISM) on the leading side, spatially correlated with intense star formation activity in this zone. We also detect molecular gas along kiloparsec-scale filaments of dust extending into the otherwise gas stripped zone of the galaxy, seen in HST images. We find the filaments are connected kinematically as well as spatially to the main gas ridge located downstream, consistent with cloud decoupling inhibited by magnetic binding, and inconsistent with a simulated filament formed via simple ablation. Furthermore, we find several clouds of molecular gas $\sim 1-3$ kpc beyond the main ring of CO that have velocities which are blueshifted by up to 50 km s$^{-1}$ with respect to the rotation curve of the galaxy. These are some of the only clouds we detect that do not have any visible dust extinction associated with them, suggesting that they are located behind the galaxy disk midplane and are falling back towards the galaxy. Simulations have long predicted that some gas removed from the galaxy disk will fall back during ram pressure stripping. This may be the first clear observational evidence of gas re-accretion in a ram pressure stripped galaxy.

Motivation & Objective

• To investigate the effects of ram pressure stripping on the molecular interstellar medium (ISM) in the nearly face-on spiral NGC 4921 in the Coma cluster.
• To determine the structure, kinematics, and fate of molecular gas in regions of strong ram pressure, particularly in the leading quadrant.
• To test predictions of gas fallback in simulations by searching for kinematically distinct, blueshifted molecular clouds not associated with dust extinction.
• To assess whether observed filaments result from ablation of decoupled clouds or from magnetically inhibited cloud decoupling.
• To quantify the role of compressed and fallback gas in sustaining star formation and delaying quenching in stripped galaxies.

Proposed method

• Conducted 6.5" resolution CARMA CO(1-0) observations of the full disk of NGC 4921 to map molecular gas distribution and surface brightness.
• Performed 0.45" resolution ALMA CO(2-1) observations of the leading quadrant to resolve kinematic and mass profiles of filaments at 220–170 pc resolution.
• Analyzed spatial and kinematic correlation between molecular gas, dust filaments (from HST), and young stellar clusters.
• Compared observed filament mass and velocity profiles with hydrodynamical simulations of ablated cloud tails to assess formation mechanisms.
• Identified and measured blueshifted molecular clouds with velocities 25–50 km s⁻¹ below the galaxy’s rotation curve, using CO emission-to-dust extinction ratios to infer line-of-sight position.
• Used CASA and custom IDL/Python tools (e.g., maskmoment, mommaps) to reduce and analyze ALMA and CARMA data cubes.

Experimental results

Research questions

• RQ1 Is molecular gas compressed on the leading side of NGC 4921 due to ram pressure, and is this compression spatially correlated with enhanced star formation?
• RQ2 Do kiloparsec-scale dust and molecular gas filaments in the leading stripped zone represent ablated tails from decoupled clouds, or are they kinematically and spatially connected to the main gas disk?
• RQ3 Are there molecular gas clouds with blueshifted velocities that lack dust extinction, indicating they are located behind the disk midplane and falling back?
• RQ4 How do the observed filament properties (mass, surface density, kinematics) compare to predictions from hydrodynamical simulations of ram pressure stripping?
• RQ5 What is the role of magnetic fields in inhibiting cloud decoupling and forming persistent, high-surface-density filaments?

Key findings

• The CO(1-0) peak surface brightness is located on the leading side of NGC 4921, where ram pressure is strongest, indicating compression of molecular gas.
• A 2 kpc-long, 0.5 kpc-wide filament with a mass of 4.3 × 10⁷ M⊙ and average surface density of 44 M⊙ pc⁻² is spatially and kinematically connected to the main CO ring, indicating it is not a decoupled ablated tail.
• The observed filament has a much higher surface density and more uniform mass profile than simulated ablated filaments, suggesting it is bound by magnetic fields rather than formed via simple ablation.
• Three molecular clouds with blueshifted velocities (25–50 km s⁻¹) and surface densities of 25–55 M⊙ pc⁻² show no dust extinction, indicating they are located behind the disk midplane and are falling back toward the galaxy.
• The CO emission-to-dust extinction ratio in these fallback clouds is at least 10× higher than in the main CO ring, supporting their location behind the disk.
• The blueshifted velocity residuals are best explained by vertical fallback rather than slower azimuthal motion, based on simulation comparisons.