Estimating turbulent velocities in the elliptical galaxies NGC 5044 and NGC 5813

Abstract

Context. The interstellar and intra-cluster medium (ICM) in giant elliptical galaxies and clusters of galaxies is often assumed to be in hydrostatic equilibrium. Numerical simulations, however, show that about 5-30% of the pressure in a cluster is provided by turbulence induced by, for example, the central active galactic nucleus (AGN) and merger activity. Aims. We aim to put constraints on the turbulent velocities and the turbulent pressure in the ICM of the giant elliptical galaxies NGC 5044 and NGC 5813 using XMM-Newton reflection grating spectrometer (RGS) observations. Methods. The magnitude of the turbulence is estimated using the Fe xvii lines at 15.01 Å, 17.05 Å, and 17.10 Å in the RGS spectra. At low turbulent velocities, the gas becomes optically thick in the 15.01 Å line due to resonant scattering, while the 17 Å lines remain optically thin. By comparing the (I 17.05 + I 17.10 )/I 15.01 line ratio from RGS with simulated line ratios for different Mach numbers, the level of turbulence is constrained. The measurement is, however, limited by the systematic uncertainty in the line ratio for an optically thin plasma, which is about 20-30%. Results. We find that the (I 17.05 + I 17.10 )/I 15.01 line ratio in NGC 5813 is significantly higher than in NGC 5044. This difference can be explained by a higher level of turbulence in NGC 5044. The best estimates for the turbulent velocities using resonant scattering and upper limits from the line widths, are 320 < V turb < 720 km s −1 for NGC 5044 and 140 < V turb < 540 km s −1 for NGC 5813 at the 90% confidence limit. Conclusions. The high turbulent velocities and the fraction of the turbulent pressure support of >40% in NGC 5044, assuming isotropic turbulence, confirm that it is a highly disturbed system, probably due to an off-axis merger. The turbulent pressure support in NGC 5813 is more modest at 15-45%. The (I 17.05 + I 17.10 )/I 15.01 line ratio in an optically thin plasma, calculated using AtomDB v2.0.1, is 2σ above the ratio measured in NGC 5044, which cannot be explained by resonant scattering. This shows that the discrepancies between theoretical, laboratory, and astrophysical data on Fe xvii lines need to be reduced to improve the accuracy of the determination of turbulent velocities using resonant scattering.

DEEPCHANDRAOBSERVATIONS OF A2199: THE INTERPLAY BETWEEN MERGER-INDUCED GAS MOTIONS AND NUCLEAR OUTBURSTS IN A COOL CORE CLUSTER

“…As shown in the Appendix (Equation (A5)), turbulence with rms speed σ t and coherence length causes rms radial displacements in a shock front at radius r of roughly √ r (σ t /v s ), where v s is the shock speed. Simulations find typical turbulent velocities in the range 100-300 km s −1 (e.g., Lau et al 2009), roughly consistent with observations (e.g., de Plaa et al 2012;Sanders & Fabian 2013), with plausible values of the coherence length ∼ 0.1r (Rebusco et al 2005) and scaling with σ t (Equation (A7)). For = 0.1r and σ t = 100v 2 km s −1 , this gives displacements at r = 100 of ∼2 v 3/2 2 , so that moderate levels of turbulence can have a marked smearing effect.…”

The relation between gas density and velocity power spectra in galaxy clusters: High-resolution hydrodynamic simulations and the role of conduction

“…The amplitudes of the driven acceleration are evolved in Fourier space and then directly converted to physical space. Since observations (Schuecker et al 2004;Churazov et al 2008;de Plaa et al 2012;Sanders & Fabian 2013) and simulations (Norman & Bryan 1999;Lau et al 2009;Vazza et al 2009Vazza et al , 2011Gaspari et al 2012b;Schmidt et al 2014;Shi & Komatsu 2014) show that the ICM turbulent energies are ∼3-30 percent of the thermal energy, we test subsonic Mach numbers in the range of M ≡ σ v /c s ∼ 0.25-0.75, where σ v is the 3D 1 velocity dispersion (average sound speed of Coma is c s 1500 km s −1 ). The source of turbulence can be various, which includes cosmological flows/mergers, galaxy motions, and feedback processes.…”

Mass Distribution in Galaxy Cluster Cores

“…Nevertheless, deep grating observations suggest that modest nonthermal support (<20%) appears common in relaxed clusters (Sanders & Fabian 2011). Low turbulence (25%) is also seen in at least some isolated ellipticals (e.g., Werner et al 2009;Churazov et al 2010), although the turbulent contribution could be higher in more disturbed galaxies (de Plaa et al 2012).…”