Soft X-ray emission from the inner disk of M33

TL;DR

This study analyzes the soft X-ray emission from M33's inner disk, revealing its correlation with star formation activity and distinguishing between emission linked to recent star formation and older stellar populations.

Contribution

It provides a detailed spectral and spatial analysis of the residual X-ray emission in M33, identifying distinct components associated with star formation and old stars.

Findings

Soft X-ray emission correlates with FUV emission and star formation activity.

The ratio of X-ray luminosity to SFR is consistent with previous studies.

A significant portion of unresolved emission may originate from old stellar populations.

Abstract

We present a study, based on archival XMM-Newton observations, of the extended X-ray emission associated with the inner disk of M33. After the exclusion of point sources with L_X > 2 x 10^{35} erg/s (0.3-6 keV), we investigate the morphology and spectrum of the residual X-ray emission. This residual emission has a soft X-ray spectrum which can be fitted with a two-temperature thermal model, with kT = 0.2 keV and 0.6 keV. The soft X-ray surface brightness distribution shows a strong correlation with FUV emission, indicative of a close connection between recent star-formation activity and the production of soft X-rays. Within 3.5 kpc of the nucleus of M33, the soft X-ray and FUV surface brightness distributions exhibit similar radial profiles. This implies that the ratio of the soft X-ray luminosity (0.3-2.0 keV) to the star formation rate (SFR) per unit disk area remains fairly constant within this inner disk region. We derive a value for this ratio of 1-1.5 x 10^{39} (erg/s)/(M_sun/yr), consistent with previous studies. In the same region, the ratio of soft X-ray luminosity to stellar mass (derived from K-band photometry) is 4 x 10^{28} erg/s/M_sun, a factor of 5-10 higher than is typical of dwarf elliptical galaxies, suggesting that 10-20% of the unresolved emission seen in M33 may originate in its old stellar population. The remainder of the soft X-ray emission is equally split between two spatial components, one which closely traces the spiral arms of the galaxy and the other more smoothly distributed across the inner disk of M33. The former must represent a highly clumped low-filling factor component linked to sites of recent or ongoing star formation, whereas the distribution of the latter gives few clues as to its exact origin.