Constraints on turbulent pressure in the X-ray halos of giant elliptical galaxies from resonant scattering

TL;DR

This study uses high-resolution X-ray spectra to measure turbulent pressure in giant elliptical galaxy halos by analyzing resonant scattering effects on Fe XVII emission lines, providing constraints on turbulence and chemical abundance biases.

Contribution

It presents the first detailed measurement of turbulent velocities and pressure support in galaxy cores through resonant scattering analysis of X-ray spectra.

Findings

Turbulent velocities in galaxy cores are less than 100 km/s.

Turbulent pressure support is less than 5% of thermal pressure.

Neglecting resonant scattering biases chemical abundance estimates by 10-20%.

Abstract

The dense cores of X-ray emitting gaseous halos of large elliptical galaxies with temperatures below about 0.8 keV show two prominent Fe XVII emission features, which provide a sensitive diagnostic tool to measure the effects of resonant scattering. We present here high-resolution spectra of five bright nearby elliptical galaxies, obtained with the Reflection Grating Spectrometers (RGS) on the XMM-Newton satellite. The spectra for the cores of four of the galaxies show the Fe XVII line at 15.01 Angstrom being suppressed by resonant scattering. The data for NGC 4636 in particular allow the effects of resonant scattering to be studied in detail and to prove that the 15.01 Angstrom line is suppressed only in the dense core and not in the surrounding regions. Using deprojected density and temperature profiles for this galaxy obtained with the Chandra satellite, we model the radial intensity profiles of the strongest resonance lines, accounting for the effects of resonant scattering, for different values of the characteristic turbulent velocity. Comparing the model to the data, we find that the isotropic turbulent velocities on spatial scales smaller than about 1 kpc are less than 100 km/s and the turbulent pressure support in the galaxy core is smaller than 5% of the thermal pressure at the 90% confidence level, and less than 20% at 95% confidence. Neglecting the effects of resonant scattering in spectral fitting of the inner 2 kpc core of NGC 4636 will lead to underestimates of the chemical abundances of Fe and O by ~10-20%.