It is by now well established that galaxy evolution is driven by intrinsic and environmental processes, both contributing to shape the observed properties of galaxies. A number of early studies, both observational and theoretical, have shown that the
star formation activity of galaxies depends on their environmental local density and also on galaxy hierarchy, i.e. centrals vs. satellites. In fact, contrary to their central (most massive) galaxy of a group/cluster, satellite galaxies are stripped of their gas and stars, and have their star formation quenched by their environment. Large galaxy surveys like SDSS now permit us to investigate in detail environment-driven transformation processes by comparing centrals and satellites. In this paper I summarize what we have so far learnt about environmental effects by analysing the observed properties of local central and satellite galaxies in SDSS, as a function of their stellar mass and the dark matter mass of their host group/cluster.
We present deep imaging of the star-forming dwarf galaxy IC2574 in the M81 group taken with the Large Binocular Telescope in order to study in detail the recent star-formation history of this galaxy and to constrain the stellar feedback on its HI gas
. We identify the star-forming areas in the galaxy by removing a smooth disk component from the optical images. We construct pixel-by-pixel maps of stellar age and stellar mass surface density in these regions by comparing their observed colors with simple stellar populations synthesized with STARBURST99. We find that an older burst occurred about 100 Myr ago within the inner 4 kpc and that a younger burst happened in the last 10 Myr mostly at galactocentric radii between 4 and 8 kpc. We analyze the stellar populations residing in the known HI holes of IC2574. Our results indicate that, even at the remarkable photometric depth of the LBT data, there is no clear one-to-one association between the observed HI holes and the most recent bursts of star formation in IC2574. The stellar populations formed during the younger burst are usually located at the periphery of the HI holes and are seen to be younger than the holes dynamical age. The kinetic energy of the holes expansion is found to be on average 10% of the total stellar energy released by the stellar winds and supernova explosions of the young stellar populations within the holes. With the help of control apertures distributed across the galaxy we estimate that the kinetic energy stored in the HI gas in the form of its local velocity dispersion is about 35% of the total stellar energy.
We have collected archival data on NGC7673 to constrain the star-formation history that produced the young star clusters and the field stellar population in this galaxy during the last 2 Gyr. We have considered the sample of 50 star clusters detected
by HST/WFPC2 in the UV, V and I bands and estimated their age, intrinsic reddening, and mass via comparison of their colours with STARBURST99 models. We have found two prominent epochs of cluster formation occurred about 20 Myr and 2 Myr ago, with somewhat minor events between 3 Myr and 6 Myr ago. The star clusters are characterised by an intrinsic reddening E(B-V) < 0.4 mag and a mass lower than 2e+06 solar masses. Out of the 50 star clusters, we have selected 31 located within the boundaries of the IUE large slit that was employed to obtain the spectrum of NGC7673 between 1150 Ang. and 3350 Ang. For each cluster, we have built a synthetic spectrum corresponding to the age, mass and intrinsic reddening derived from the cluster colours, properly redshifted to NGC7673. The spectra have then been added together in a final, clusters integrated spectrum. This and the IUE and FUSE spectra of NGC7673 have allowed us to describe the star-formation history of the unresolved stars in the field as either exponentially decaying or multi-burst. In the first case, we have derived an e-folding time of 700 (900) Myr and an initial star-formation rate of 16 (13) solar masses per year when the Fitzpatricks (Calzettis) extinction law is used. In the case of a multi-burst star-formation history, the field population turns out to be composed by a young (< 40 Myr) component 3 (2) times brighter than the star clusters, and a component as old as 850 (450) Myr, about 200 (100) times more massive than the star clusters together.
