Soft X-ray to Far Infrared luminosities ratio in star-forming galaxies

TL;DR

This study investigates the correlation between soft X-ray and far infrared luminosities in star-forming galaxies, validating the use of soft X-ray emission as a star formation rate indicator through evolutionary models and observational data.

Contribution

It provides a model-based calibration for soft X-ray luminosity as a star formation rate estimator, supported by observational data and analysis of galaxy samples.

Findings

Soft X-ray and FIR luminosity ratios align with model predictions.

Diffuse soft X-ray emission correlates with star formation episodes.

Models support empirical calibrations for star formation rate estimation.

Abstract

A good correlation has been found in star-forming galaxies between the soft X-ray and the far infrared or radio luminosities. In this work we analyze the relation between the soft X-ray and far infrared luminosities as predicted by evolutionary population synthesis models, aiming first to test the validity of the soft X-ray luminosity as a star formation rate estimator, using the already known calibration of the FIR luminosity as a proxy, and second proposing a calibration based on the predictions of evolutionary synthesis models. We have computed the soft X-ray and far infrared luminosities expected for a massive starburst as a function of evolutionary state, efficiency of the conversion of mechanical energy into soft X-ray luminosity, star formation regime (instantaneous or extended) and dust abundance, and have compared the predictions with observational values for a sample of 62 star-forming galaxies taken from the literature. The observational Lsoftx/Lfir ratios are consistent with the predictions by the models under realistic assumptions (young starbursts, efficiency in the re-processing of mechanical energy of few percent), confirming the direct relation of the diffuse soft X-ray emission with the star formation episode. The soft X-ray emission of the diffuse, extended gas surrounding massive star forming regions can be used as a Star Formation Rate estimator. The empirical calibrations presented in the literature are supported by the predictions of evolutionary synthesis models and by the analysis of a larger sample of star-forming galaxies, but it is biased towards galaxies dominated by relatively unevolved starbursts.