Normal galaxies in the XMM-Newton fields. X-rays as a star formation indicator

TL;DR

This study compiles a sample of normal X-ray galaxies using XMM-Newton data to investigate the correlation between X-ray emission and star formation, confirming X-rays as an effective star-formation indicator.

Contribution

It expands the database of X-ray selected normal galaxies at intermediate redshifts and establishes a strong correlation between X-ray and Halpha luminosities.

Findings

X-ray luminosity correlates with Halpha luminosity.

Most X-ray selected galaxies are star-forming or absorption line galaxies.

X-rays are confirmed as a reliable tracer of star formation.

Abstract

Context: We use the first XMM serendipitous source catalogue (1XMM) to compile a sample of normal X-ray galaxies Aims: We seek to expand the database of X-ray selected normal galaxies at intermediate redshifts and examine the relation between X-ray emission and star formation for late-type systems Methods: The candidates are selected based on their X-ray (soft spectra), X-ray to optical [log(fx/fo)<-2] and optical (extended sources) properties. 44 candidates are found and 35 are spectroscopically observed with the Australian National University's 2.3m telescope to examine their nature. Results: Of the 35 sources observed, 2 are AGN, 11 emission line galaxies, 12 absorption line galaxies, 6 have featureless spectra while 4 are associated with Galactic stars. We combine our emission line sample with earlier works forming the most comprehensive X-ray selected galaxy sample for the study of the X-ray luminosity to the Halpha luminosity - a well-calibrated star-formation indicator - relation. Conclusions: We find that the X-ray luminosity strongly correlates with the Halpha luminosity, suggesting that the X-rays efficiently trace the star-formation.