Massive star clusters and clumps in the collisional ring galaxy Arp 147

Abstract

We conduct a photometric study of star clusters (or knots) in the collisional ring galaxy (CRG) Arp 147 to trace the star formation history across its empty ring. Using HST F450W, F606W and F814W images, we find that Arp 147 hosts 211 knots and six kpc-size clumps, nearly 60 per cent of which have ages below 10 Myr, and two thirds have masses above $\rm 10^{5}\,M_{\odot}$. The cluster mass function (CMF) of knots with ages between $10 - 200$ Myr deviates from a power-law and follows a Schechter function with a characteristic truncation mass of ${\rm M}_{c} = 6.2 \times 10^{5} \, {\rm M}_{\odot}$. This shape of the CMF is more prominent for a subsample of knots in the eastern region of the ring. Over the same age interval, we derive a low rate of disruption ($\delta \sim 0.25$) from the cluster age function and a cluster formation efficiency (CFE) of $\sim$ 3 per cent. In contrast, the CFE in the $1 - 10$ Myr age range is nearly 40 per cent. We note the lack of high-resolution UV and H$\alpha$ observations to help break age-extinction degeneracy which affects the derived ages for dusty young clusters and old ones with low reddening. Nevertheless, this study has shown, at least to a first-order approximation, that collision-triggered starburst events happening across the CRG offer an ideal environment for a second generation of young blue knots to form in abundance. It also suggests that the drop-through collision between the two galaxies can fuel at least mild cluster disruption over time.