XMM-Newton X-ray and Optical Monitor far UV observations of NGC 7070A and ESO 2400100 shell galaxies

TL;DR

This study combines X-ray and UV observations to analyze the star formation history and hot gas content of shell galaxies NGC 7070A, ESO 2400100, and NGC 474, revealing differences in their evolution and recent activity.

Contribution

It provides new multi-wavelength data and models to understand the recent star formation and accretion processes in shell galaxies, highlighting differences in their X-ray and UV properties.

Findings

NGC 7070A hosts a low luminosity nuclear X-ray source.

ESO 2400100 shows a double nucleus and recent star formation.

UV emission extent aligns with optical, indicating ongoing or recent activity.

Abstract

We present XMM-Newton X-ray observations of two shell galaxies, NGC 7070A and ESO 2400100, and far UV observations obtained with the Optical Monitor for these and for an additional shell galaxy, NGC 474, for which we also have near and far UV data from GALEX. We aim at gaining insight on the overall evolution traced by their star formation history and by their hot gas content. The X-ray and the far UV data are used to derive their X-ray spatial and spectral characteristics and their UV luminosity profiles. We use models developed ad hoc to investigate the age of the last episode of star formation from the (UV - optical) colors and line strength indices. The X-ray spatial and spectral analysis show significant differences in the two objects. A low luminosity nuclear source is the dominant component in NGC 7070A log L_X=41.7 erg s^{-1} in the 2-10 keV band. In ESO 2400100, the X-ray emission is due to a low temperature plasma with a contribution from the collective emission of individual sources. In the Optical Monitor image ESO 2400100 shows a double nucleus, one bluer than the other. This probably results from a very recent star formation event in the northern nuclear region. The extension of the UV emission is consistent with the optical extent for all galaxies, at different degrees of significance in different filters. The presence of the double nucleus, corroborated by the (UV - optical) colors and line strength indices analysis, suggests that ESO 2400100 is accreting a faint companion. We explore the evolution of the X-ray luminosity during accretion processes with time. We discuss the link between the presence of gas and age, since gas is detected either before coalescence or several Gyr (>3) after (Abridged).