Revealing a ring-like cluster complex in a tidal tail of the starburst galaxy NGC 2146

We report the discovery of a ring-like cluster complex in the starburst galaxy NGC2146. The Ruby Ring, so named due to its appearance, shows a clear ring-like distribution of star clusters around a central object. It is located in one of the tidal streams that surround the galaxy. NGC2146 is part of the Snapshot Hubble U-band Cluster Survey (SHUCS). The WFC3/F336W data have added critical information to the available archival Hubble Space Telescope imaging set of NGC2146, allowing us to determine ages, masses and extinctions of the clusters in the Ruby Ring. These properties have then been used to investigate the formation of this extraordinary system. We find evidence of a spatial and temporal correlation between the central cluster and the clusters in the ring. The latter are about 4Myr younger than the central cluster, which has an age of 7Myr. This result is supported by the Hα emission which is strongly coincident with the ring, and weaker at the position of the central cluster. From the derived total Hα luminosity of the system, we constrain the star formation rate density to be quite high (Σ_SFR = 0.47M_⊙yr^-1kpc^-2). The Ruby Ring is the product of an intense and localized burst of star formation, similar to the extended cluster complexes observed in M51 and the Antennae, but more impressive because it is quite isolated. The central cluster contains only 5 per cent of the total stellar mass in the clusters that are determined within the complex. The ring-like morphology, the age spread and the mass ratio support a triggering formation scenario for this complex. We discuss the formation of the Ruby Ring in a 'collect and collapse' framework. The predictions made by this model agree quite well with the estimated bubble radius and expansion velocity produced by the feedback from the central cluster, making the Ruby Ring an interesting case of triggered star formation.