Shock-Enhanced C+ Emission and the Detection of H2O from Stephan's Quintet's Group-Wide Shock using Herschel

TL;DR

This study reports Herschel observations of shock-induced [CII], [OI], and water emissions in Stephan's Quintet, revealing turbulence-driven heating processes that influence galaxy collision environments and high-redshift galaxy interpretations.

Contribution

First detection of water and fine-structure lines in Stephan's Quintet's shock filament, highlighting turbulence and shocks as key heating mechanisms beyond traditional models.

Findings

Detection of broad [CII] emission profiles indicating turbulent motions.

Water emission associated with dense, shock-heated gas.

Enhanced [CII] emission driven by turbulence, not star formation or X-ray heating.

Abstract

We present the first Herschel spectroscopic detections of the [OI]63 and [CII]158 micron fine-structure transitions, and a single para-H2O line from the 35 x 15 kpc^2 shocked intergalactic filament in Stephan's Quintet. The filament is believed to have been formed when a high-speed intruder to the group collided with clumpy intergroup gas. Observations with the PACS spectrometer provide evidence for broad (> 1000 km s^-1) luminous [CII] line profiles, as well as fainter [OI]63micron emission. SPIRE FTS observations reveal water emission from the p-H2O (111-000) transition at several positions in the filament, but no other molecular lines. The H2O line is narrow, and may be associated with denser intermediate-velocity gas experiencing the strongest shock-heating. The [CII]/PAH{tot) and [CII]/FIR ratios are too large to be explained by normal photo-electric heating in PDRs. HII region excitation or X-ray/Cosmic Ray heating can also be ruled out. The observations lead to the conclusion that a large fraction the molecular gas is diffuse and warm. We propose that the [CII], [OI] and warm H2 line emission is powered by a turbulent cascade in which kinetic energy from the galaxy collision with the IGM is dissipated to small scales and low-velocities, via shocks and turbulent eddies. Low-velocity magnetic shocks can help explain both the [CII]/[OI] ratio, and the relatively high [CII]/H2 ratios observed. The discovery that [CII] emission can be enhanced, in large-scale turbulent regions in collisional environments has implications for the interpretation of [CII] emission in high-z galaxies.