XMM-Newton X-ray and optical monitor far UV observations of NGC 7070A and ESO 2400100 shell galaxies^*

¹ INAF-Osservatorio Astronomico di Brera, via Brera 28, 20121 Milano, Italy e-mail: ginevra.trinchieri@brera.inaf.it
² INAF-Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy e-mail: [roberto.rampazzo;antonietta.marino]@oapd.inaf.it
³ Dipartimento di Astronomia Università di Padova, Vicolo dell'Osservatorio 2, 35122 Padova, Italy e-mail: [cesare.chiosi;rosaria.tantalo]@unipd.it
⁴ Institute for Astronomy (IfA), University of Vienna, Türkenschanzstrasse 17, 1180 Vienna, Austria e-mail: gruetzbauch@astro.univie.ac.at 


Received: 22 April 2008
Accepted: 27 June 2008

Abstract

Context. Shell galaxies are considered to be the debris of recent accretion/merging episodes. Their high frequency in low density environments suggests that these episodes could drive the secular evolution of at least some fraction of the early-type galaxy population.

Aims. 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 attempt to gain insight into the overall evolution as traced by their star formation history and hot gas content.

Methods. The X-ray and far UV data are used to derive their X-ray spatial and spectral characteristics and their UV luminosity profiles. We use ad hoc models developed to investigate the age of the most recent episode of star formation from the (UV – optical) colors and line strength indices.

Results. 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 is probably due to a very recent star formation event in the northern nuclear region. The extension of the UV emission is consistent with the optical extent of 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 in 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.