The recent literature suggests that an evolutionary dichotomy exists for early-type galaxies (Es and S0s, ETGs) whereby their central photometric structure (cuspy versus core central luminosity profiles), and figure of rotation (fast (FR) vs. slow (S
R) rotators), are determined by whether they formed by wet or dry mergers. We consider whether the mid infrared (MIR) properties of ETGs, with their sensitivity to accretion processes in particular in the last few Gyr (on average z < 0.2), can put further constraints on this picture. We investigate a sample of 49 ETGs for which nuclear MIR properties and detailed photometrical and kinematical classifications are available from the recent literature. In the stellar light cuspy/core ETGs show a dichotomy that is mainly driven by their luminosity. However in the MIR, the brightest core ETGs show evidence that accretions have triggered both AGN and star formation activity in the recent past, challenging a dry merger scenario. In contrast, we do find, in the Virgo and Fornax clusters, that cuspy ETGs, fainter than M$_{K_s}=-24$, are predominantly passively evolving in the same epoch, while, in low density environments, they tend to be more active. A significant and statistically similar fraction of both FR (38$^{+18}_{-11}$%) and SR (50$^{+34}_{-21}$%) shows PAH features in their MIR spectra. Ionized and molecular gas are also frequently detected. Recent star formation episodes are then a common phenomenon in both kinematical classes, even in those dominated by AGN activity, suggesting a similar evolutionary path in the last few Gyr. MIR spectra suggest that the photometric segregation between cuspy and core nuclei and the dynamical segregation between FR and SR must have originated before z~0.2.
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 fr
om 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).
We present optical polarization maps of a sample of four interacting pairs at different phases of encounter, from nearly unperturbed galaxies to on-going mergers. Only the pair RR 24 shows a linear polarization pattern which extends in both galaxies
for several kiloparsecs. The more perturbed member, RR 24b, is lineraly polarized up to the level of ~3%. No polarization is measured in the strongly perturbed late-type pair members of RR 23 and RR 99. Also, in the central part of the double nuclei shell galaxy ESO 2400100 there is no significant polarization. We use the ionized gas velocity field of RR 24 to interpret its linear polarization structure. In RR 24a the quite regular gas kinematics reflect the unperturbed spiral-like polarization structure. In RR 24b a strong velocity gradient in ionized gas could be associated with the polarization structure. We suggest that the large-scale magnetic field of the RR 24 pair members still plays a role in shaping the polarization pattern.
