P. Focardi — Page 2

arXiv1995

The Kinematics of Emission-Line Galaxies in Clusters from ENACS

By using the ENACS data, we have studied the properties of emission-line galaxies (ELG) in clusters. The fraction of ELG in clusters is 16~%; this fraction is slightly dependent on the cluster velocity dispersion, being larger for ``colder'' systems. ELG tend to occur more frequently than other cluster galaxies in substructures. However, only a minor cluster fraction has an ELG population with a mean velocity significantly different from that of the overall cluster population. The velocity distribution of cluster ELG is broader than that of the other cluster galaxies, which translates in a larger observed velocity dispersion. The velocity distribution for ELG in the inner cluster regions suggests that ELG are on mostly radial orbits.

astro-ph↗

arXiv1994

Star Formation in Dwarf Irregular Galaxies: NGC 6822

Our proposed method to study the star formation histories in nearby irregulars is here applied to NGC 6822. To this purpose we have obtained accurate CCD photometry of three regions in the galaxy, reaching V$\simeq$23.5 with an accuracy of $σ_{BVR}\leq$0.1 mag. The comparison of the observational color-magnitude diagrams and luminosity functions with corresponding synthetic simulations reveals that the star formation activity in the last Gyr has been rather continuous, possibly occurring in long episodes separated by short quiescent periods, and that the initial mass function exponent $α$ is in the vicinity of Salpeter's value. The studied regions of NGC~6822 appear to contain different stellar populations. The preferred star formation regime for Region A is a continuous activity coupled with a moderately flat $α$. For Region C, instead, our best models suggest a discontinuous star formation and a moderately steep $α$. NGC 6822 is the last object of our original sample of five nearby irregulars. We can then draw the scenario resulting from this sample: most of the observed fields appear to have experienced a {\it gasping} regime of star formation, with average exponent $α$=2.35 for the initial mass function. Despite their small sizes, the majority of the sample galaxies contain different stellar populations in different regions.

astro-ph↗