A Spitzer-IRS view of early-type galaxies with cuspy/core nuclei and fast/slow rotation

摘要

The recent literature suggests that an evolutionary dichotomy exists for early-type galaxies (Es and S0s, ETGs) whereby their central photometric structure (cuspy vs. core central luminosity profiles), and figure of rotation (fast vs. slow rotators), are determined by whether they were formed by "wet" or "dry" mergers. Aims. 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. Methods. We investigate a sample of 49 ETGs for which nuclear MIR properties and detailed photometrical and kinematical classifications are available from the recent literature. Results. 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 MKs =-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 fast rotators (FRs) (38-11+18 %) and slow rotators (SRs) (50-21+34%) shows polycyclic aromatic hydrocarbon 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 Gyrs. Conclusions. MIR spectra suggest that the photometric segregation between cuspy and core nuclei and the dynamical segregation between FRs and SRs must have originated before z ~ 0.2. © 2014 ESO.