Ocular Shock Front in the Colliding Galaxy IC 2163

TL;DR

This study uses ALMA CO observations to analyze gas dynamics and density enhancements in the ocular structure of the colliding galaxy IC 2163, revealing gas inflow, compression, and turbulence caused by galactic interaction.

Contribution

It provides detailed kinematic and density analysis of the ocular feature in IC 2163, highlighting the gas flow and turbulence induced by galaxy collision, which was not previously characterized at this resolution.

Findings

Gas piles up in narrow eyelids with large velocity gradients.

Radial inflow of gas slows and causes density increase in eyelids.

High turbulence and shear observed in the eyelids.

Abstract

ALMA observations in the CO 1 - 0 line of the interacting galaxies IC 2163 and NGC 2207 at 2" x 1.5" resolution reveal how the encounter drives gas to pile up in narrow, ~ 1 kpc wide, "eyelids" in IC 2163. IC 2163 and NGC 2207 are involved in a grazing encounter, which has led to development in IC 2163 of an eye-shaped (ocular) structure at mid-radius and two tidal arms. The CO data show that there are large velocity gradients across the width of each eyelid, with a mixture of radial and azimuthal streaming of gas at the outer edge of the eyelid relative to its inner edge. The sense of the radial streaming in the eyelids is consistent with the idea that gas from the outer part of IC 2163 flows inward until its radial streaming slows down abruptly and the gas piles up in the eyelids. The radial compression at the eyelids causes an increase in the gas column density by direct radial impact and also leads to a high rate of shear. We find a strong correlation between the molecular column densities and the magnitude of dv/dR across the width of the eyelid at fixed values of azimuth. Substantial portions of the eyelids have high velocity dispersion in CO, indicative of elevated turbulence there.