Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by the Spitzer Space Telescope

We present ground-based optical and Spitzer Space Telescope infrared imaging observations of the interacting galaxy UGC 10214, the Tadpole galaxy (z = 0.0310), focusing on the star formation activity in the nuclear, disk, spiral arms, and tidal tail regions. The ground-based optical data set spans a wavelength range between 0.3 and 0.8 μm, the near-IR data set spans 1–2.2 μm, and the Spitzer IR data set spans 3–70 μm. The major findings of this study are that the Tadpole is actively forming stars in the main disk outside of the nucleus and in the tidal plume, with an estimated mean star formation rate of ∼2–4 M⊙ yr-1. The most prominent sites of mid-IR emission define a "ring" morphology that, combined with the overall morphology of the system, suggests the interaction may belong to the rare class of off-center collisional ring systems that form both shock-induced rings of star formation and tidal plumes. In stark contrast to the disk star formation, the nuclear emission is solely powered by older stars, with little evidence for ongoing star formation at the center of the Tadpole. Extranuclear star formation accounts for >50% of the total star formation in the disk and spiral arms, featuring infrared-bright "hot spots" that exhibit strong polycyclic aromatic hydrocarbon (PAH) emission, the band strength of which is comparable to that of late-type star-forming disk galaxies. The tidal tail, which extends 2′ (∼75 kpc) into the intergalactic medium, is populated by supermassive star clusters, M ∼ 106 M⊙, likely triggered by the galaxy-galaxy interaction that has distorted UGC 10214 into its current "tadpole" shape. The Tadpole is therefore an example of an off-nuclear or tidal-tail starburst, with several large sites of massive star formation in the disk and in the plume, including the most prominent Hubble Space Telescope–revealed cluster, J160616.85+552640.6. The clusters exhibit remarkable IR properties, including exceptionally strong 24 μm emission relative to the underlying starlight, hot dust continuum, and PAH emission, with an estimated current star formation rate of ∼0.1–0.4 M⊙ yr-1, representing >10% of the total star formation in the system. We estimate the mass of the largest cluster to be ∼ × 106 M⊙ based on the g′-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes in combination with an assumed mass-to-light ratio appropriate to young clusters, or large enough to be classified as a nascent dwarf galaxy or globular cluster.