A starburst in the core of a galaxy cluster: The dwarf irregular NGC
  1427A in Fornax
  Gas-rich galaxies in dense environments such as galaxy clusters and massive
groups are affected by a number of possible types of interactions with the
cluster environment, which make their evolution radically different than that
of field galaxies. The dIrr galaxy NGC 1427A, presently infalling towards the
core of the Fornax galaxy cluster, offers a unique opportunity to study those
processes in a level of detail not possible to achieve for galaxies at higher
redshifts. Using HST/ACS and auxiliary VLT/FORS ground-based observations, we
study the properties of the most recent episodes of star formation in this
gas-rich galaxy, the only one of its type near the core of the Fornax cluster.
We study the structural and photometric properties of young star cluster
complexes in NGC 1427A, identifying 12 bright such complexes with exceptionally
blue colors. The comparison of our broadband near-UV/optical photometry with
simple stellar population models yields ages below ~4x10^6 yr and stellar
masses from a few thousand up to ~3x10^4 Msun, slightly dependent on the
assumption of cluster metallicity and IMF. Their grouping is consistent with
hierarchical and fractal star cluster formation. We use deep Ha imaging data to
determine the current Star Formation Rate (SFR) in NGC 1427A and estimate the
ratio, Gamma, of star formation occurring in these star cluster complexes to
that in the entire galaxy. We find Gamma to be among the largest such values
available in the literature, consistent with starburst galaxies. Thus a large
fraction of the current star formation in NGC 1427A is occurring in star
clusters, with the peculiar spatial arrangement of such complexes strongly
hinting at the possibility that the starburst is being triggered by the passage
of the galaxy through the cluster environment.
