Efficient star cluster formation 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 redshits. 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 ~4×10^6 yr and stellar masses from a few thousand up to ~3×10^4 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 Halpha 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 exceptionally large, even after conservatively accounting for the possibility of contamination from intra-cluster light and other modelling uncertainties, implying that recent star formation predominantly occurred in star cluster complexes. This is the first time such high star cluster formation efficiency is reported in a dwarf galaxy within the confines of a galaxy cluster, strongly hinting at the possibility that is being triggered by its passage through the cluster environment.