Optical Spectroscopy of Halpha Filaments in Cool Core Clusters: Kinematics, Reddening, and Sources of Ionization

TL;DR

This study uses high-resolution spectroscopy of Halpha filaments in galaxy cluster cool cores to analyze their ionization, kinematics, and reddening, revealing that shocks and star formation primarily ionize the gas and that turbulence influences filament velocities.

Contribution

It provides detailed spectroscopic analysis of filaments in galaxy cluster cores, ruling out several ionization mechanisms and supporting a shock plus star formation model, with insights into gas kinematics and turbulence.

Findings

Small reddening in filaments (E(B-V) < 0.2)

Ionization dominated by shocks and star formation, not cosmic rays or X-ray photoionization

Line widths decrease with radius, indicating turbulence and kinematic complexity

Abstract

We have obtained deep, high spatial and spectral resolution, long-slit spectra of the Halpha nebulae in the cool cores of 9 galaxy clusters. This sample provides a wealth of information on the ionization state, kinematics, and reddening of the warm gas in the cool cores of galaxy clusters. We find evidence for only small amounts of reddening in the extended, line-emitting filaments, with the majority of filaments having E(B-V) < 0.2. The combination of [O III]/Hb, [N II]/Ha, [S II]/Ha, and [O I]/Ha allow us to rule out collisional ionization by cosmic rays, thermal conduction, and photoionization by ICM X-rays and AGN as strong contributors to the ionization of the warm gas in both nuclei and filaments. The data are adequately described by a composite model of slow shocks and star formation. This model is further supported by an observed correlation between the linewidths and low ionization line ratios which becomes stronger in systems with more modest star formation activity based on far ultraviolet observations. We find that the more extended, narrow filaments tend to have shallower velocity gradients and narrower linewidths than the compact filamentary complexes. We confirm that the widths of the emission lines decrease with radius, from FWHM \sim 600 km/s in the nuclei to FWHM ~ 100 km/s in the most extended filaments. We suggest that this radial dependence of the velocity width may in fact be linked to ICM turbulence and, thus, may provide a glimpse into the amount of turbulence in cool cores. In the central regions (r < 10 kpc) of several systems the warm gas shows kinematic signatures consistent with rotation. We find that the kinematics of the most extended filaments in this sample are broadly consistent with both infall and outflow, and recommend further studies linking the warm gas kinematics to both radio and X-ray maps in order to further understand the observed kinematics.