Observations of asymmetric velocity fields and gas cooling in the NGC 4636 galaxy group X-ray halo

TL;DR

This paper investigates the asymmetric velocity fields and gas cooling in the NGC 4636 galaxy group's X-ray halo, revealing differences in turbulence and cooling processes on opposite sides of the core through detailed spectral analysis.

Contribution

It provides new evidence of asymmetric turbulence and cooling in the NGC 4636 IGM, linking resonant scattering and cool gas phases to gas dynamics and AGN activity.

Findings

Resonant scattering detected in the north, absent in the south.

Cool gas phases are associated with resonant scattering regions.

Asymmetry in turbulence and cooling suggests directional dependence or sloshing effects.

Abstract

This study aims to probe the thermodynamic properties of the hot intragroup medium (IGM) plasma in the core regions of the NGC 4636 galaxy group by detailed measurements of several emission lines and their relative intensities. We analyzed deep XMM-Newton Reflection Grating Spectrometer (RGS) data in five adjacent spectral regions in the central parts of the NGC 4636 galaxy group. We examined the suppression of the Fe xvii resonance line (15.01 {\AA}) as compared to the forbidden lines of the same ion (17.05 {\AA} and 17.10 {\AA}). The presence and radial dependence of the cooling flow was investigated through spectral modeling. In addition, a parallel analysis with deep Chandra Advances CCD Imaging Spectrometer (ACIS) data was conducted to gain additional information about the thermodynamical properties of the IGM. We find that the plasma at the group center to the north shows efficient Fe xvii ion resonant scattering, wheras no resonant scattering was detected at the south side. The regions featuring resonant scattering coincide with those embodying large amounts of cool ($kT\lesssim0.4$ keV) gas phases, and the spectral imprints of cooling gas with a total mass deposition rate of $\sim0.8$ M$_{\odot}$ yr$^{-1}$ within the examined region of $2.4^{\prime}\times 5.0^{\prime}$. We interpret the results as possible evidence of asymmetric turbulence distribution in the NGC 4636 IGM: Turbulence dominates the gas dynamics to the south, while collective gas motions characterize the dynamics to the north. X-ray images show imprints of energetic AGN at both sides, yet we find evidence of turbulence heating at the south and gas cooling at the north of the core. We infer that the observed asymmetry may be the result of the specific observation angle to the source, or arise from the turbulence driven by core sloshing at south side.