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

Abstract

Aims. 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. Methods. 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 Å) as compared to the forbidden lines of the same ion (17.05 Å and 17.10 Å). The presence and radial dependence of the cooling flow was investigated through spectral modeling. Parallel analysis with deep Chandra Advances CCD Imaging Spectrometer (ACIS) data was conducted to gain additional information about the thermodynamical properties of the IGM.Results. The plasma at the group center to the north shows efficient Fe xvii ion resonant scattering, yielding (I λ17.05 + I λ17.10 )/I λ15.01 line ratios up to 2.9 ± 0.4, corresponding to about twice the predicted line ratio. In contrast, no resonant scattering was detected at the south side. The regions featuring resonant scattering coincide with those embodying large amounts of cool (kT 0.4 keV) gas phases, and the spectral imprints of cooling gas with a total mass deposition rate of ∼0.8 M yr −1 within the examined region of 2.4 × 5.0 . Conclusions. 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.

The Metal Content of the Hot Atmospheres of Galaxy Groups

“…Despite the valuable ability of RGS to measure accurately important ratios (particularly N/Fe and O/Fe, see Figure 9), its limited spectral window-coupled to its sensitivity to the spatial extent of the source and the difficulty to perform spatial spectroscopy (see, however, Reference [201,202])-makes this instrument taken at its best advantage when combined with CCD measurements. Nevertheless, RGS offers the unique advantage to reveal a glimpse of the main transitions populating the Fe-L complex at groups (and clusters) temperature regime(s).…”

Feedback from Active Galactic Nuclei in Galaxy Groups

“…This offers opportunities in a number of important areas, including measurements of bulk flows and turbulence in the hot gas. At present, grating spectra only provide upper limits on the turbulence of the IGrM [240,349], with possible hints of asymmetries that are associated with sloshing motions [350]. Hitomi demonstrated the capabilities of microcalorimeters, but it was only able to make a single turbulence measurement in the Perseus cluster [351].…”

CHEERS: The chemical evolution RGS sample

“…Some examples are the discovery of cool (∼0.2 keV) gas in the CHEERS RGS spectra of elliptical galaxies (Pinto et al 2014) and constraints on turbulent velocities measured with RGS using line broadening (e.g., Pinto et al 2015). The RGS data have also shown for NGC 4636 that spatially resolved resonant scattering analysis is capa-ble of revealing velocity structure in the ICM (Ahoranta et al 2016). This technique will soon be applied to more members of the sample in follow-up papers.…”