X-ray Spectra of Circumgalactic Medium Around Star-Forming Galaxies: Connecting Simulations to Observations

TL;DR

This paper uses hydrodynamical simulations to analyze the X-ray spectra of the circumgalactic medium around star-forming galaxies, proposing a log-normal model for better spectral fitting and connecting simulation results to future observations.

Contribution

It introduces a physically motivated log-normal model for fitting CGM X-ray spectra, improving upon simple 1-T or 2-T models and linking simulation insights to upcoming X-ray missions.

Findings

1-T and 2-T models fit spectra reasonably well but lack physical meaning.

Log-normal distribution provides a better fit and reflects the gas temperature distribution.

Spectra inside outflows are hotter and more metal-enriched, especially at high star formation rates.

Abstract

The hot component of the circum-galactic medium (CGM) around star forming galaxies is detected as diffuse X-ray emission. The X-ray spectra from the CGM depend on the temperature and metallicity of the emitting plasma, providing important information about the feeding and feedback of the galaxy. The observed spectra are commonly fitted using simple 1-Temperature (1-T) or 2-T models. However, the actual temperature distribution of the gas can be complex because of the interaction between galactic outflows and halo gas. Here we demonstrate this by analysing 3-D hydrodynamical simulations of the CGM with a realistic outflow model. We investigate the physical properties of the simulated hot CGM, which shows a broad distribution in density, temperature, and metallicity. By constructing and fitting the simulated spectra, we show that, while the 1-T and 2-T models are able to fit the synthesized spectra reasonably well, the inferred temperature(s) does not bear much physical meaning. Instead, we propose a log-normal distribution as a more physical model. The log-normal model better fits the simulated spectra while reproducing the gas temperature distribution. We also show that when the star formation rate is high, the spectra inside the bi-conical outflows are distinct from that outside, as outflows are generally hotter and more metal-enriched. Finally, we produce mock spectra for future missions with the eV-level spectral resolution, such as Athena, Lynx, HUBS and XRISM.