The low redshift Universe (z<~0.5) is not a dull place. Processes leading to the suppression of star formation and morphological transformation are prevalent: this is particularly evident in the dramatic upturn in the fraction of S0-type galaxies in
clusters. However, until now, the process and environment of formation has remained unidentified. We present a HST-based morphological analysis of galaxies in the redshift-space selected group and field environments at z~0.4. Groups contain a much higher fraction of S0s at fixed luminosity than the lower density field, with >99.999% confidence. Indeed the S0 fraction in groups is at least as high as in z~0.4 clusters and X-ray selected groups, which have more luminous Intra Group Medium (IGM). An 97% confident excess of S0s at >=0.3Mpc from the group centre at fixed luminosity, tells us that formation is not restricted to, and possibly even avoids, the group cores. Interactions with a bright X-ray emitting IGM cannot be important for the formation of the majority of S0s in the Universe. In contrast to S0s, the fraction of elliptical galaxies in groups at fixed luminosity is similar to the field, whilst the brightest ellipticals are strongly enhanced towards the group centres (>99.999% confidence within 0.3Mpc). We conclude that the group and sub-group environments must be dominant for the formation of S0 galaxies, and that minor mergers, galaxy harassment and tidal interactions are the most likely responsible mechanisms. This has implications not only for the inferred pre-processing of cluster galaxies, but also for the global morphological and star formation budget of galaxies: as hierarchical clustering progresses, more galaxies will be subject to these transformations as they enter the group environment.
We have used the VIMOS IFU to map the properties of the Seyfert 1.9 galaxy LEDA 135736. These maps reveal a number of interesting features including: an Extended Narrow Line Region detectable out to 9 kpc, an area of intense star formation located at
a projected distance of 12 kpc from the centre, an elliptical companion galaxy, and kinematic features, aligned along the long-axis of the ENLR, that are consistent with radio jet-driven mass outflow. We propose that the ENLR results from extra-planar gas ionized by the AGN, and that the AGN in turn might be triggered by interaction with the companion galaxy, which can also explain the burst of star formation and morphological features. Only about two percent of the ENLRs kinetic energy is in the mass outflow. We infer from this that the bulk of mechanical energy imparted by the jet is used to heat this gas.
The most massive galaxies in the Universe are also the oldest. To overturn this apparent contradiction with hierarchical growth models, we focus on the group scale haloes which host most of these galaxies. A stellar mass selected M_* >~ 2x10^10M_sol
sample at z~0.4 is constructed within the CNOC2 redshift survey. A sensitive Mid InfraRed (MIR) IRAC colour is used to isolate passive galaxies. It produces a bimodal distribution, in which passive galaxies (highlighted by morphological early-types) define a tight MIR colour sequence (Infrared Passive Sequence, IPS). This is due to stellar atmospheric emission from old stellar populations. Significantly offset from the IPS are galaxies where reemission by dust boosts emission at 8microns (InfraRed-Excess or IRE galaxies). They include all known morphological late-types. Comparison with EW[OII] shows that MIR colour is highly sensitive to low levels of activity, and allows us to separate dusty-active from passive galaxies. The fraction of IRE galaxies, f(IRE) drops with M_*, such that f(IRE)=0.5 at a ``crossover mass of ~1.3x10^11M_sol. Within our optically-defined group sample there is a strong and consistent deficit in f(IRE) at all masses, and most clearly at M_* >~10^11M_sol. Using a mock galaxy catalogue derived from the Millenium Simulation we show that the observed trend of f(IRE) with M_* can be explained if suppression of star formation occurs primarily in the group environment, and particularly for M_*>~10^11M_sol galaxies. In this way, downsizing can be driven solely by structure growth in the Universe.
