Multiwavelength Study of Young Massive Star Clusters in the Interacting Galaxy Arp 24

Abstract

We made a multiwavelength study of young massive star clusters (YSCs) in the interacting galaxy Arp 24 using optical and ultraviolet images from the Hubble Space Telescope (HST), the Sloan Digital Sky Survey, and the Galaxy Evolution Explorer, mid-infrared images from the Spitzer Space Telescope, and narrowband Hα images and optical spectra from the National Astronomical Observatories of the Chinese Academy of Sciences 2.16 m telescope. Based on the HST images, we found that the brightest infrared knot in Arp 24 is associated with a complex of five YSCs within a region of ~0.95'' radius (127 pc) in size. The ages and masses of the star clusters in this complex and other regions were estimated using HST broadband photometries and the Starburst99 synthesis models. The star clusters in this complex are very young (within ages of ~3-5 Myr) and massive (masses of ~105 M⊙). The ionization parameter and metallicity of the complex were estimated using the emission-line ratios, and the star formation rates were calculated using monochromatic 24 μm, far-ultraviolet, and Hα-line luminosities. We speculate that Arp 24 may have formed in a retrograde flyby encounter indicated by its one-armed appearance and fanlike structure, and the formation of the YSCs in this galaxy was triggered by the interaction. The clusters in the YSC complex may have formed in a single giant molecular cloud simultaneously. From the ultraviolet to mid-infrared spectral energy distributions, we found that the region of the YSC complex is relatively bluer in the optical and has higher 24 μm dust emission relative to the starlight and 8 μm emission. This warm infrared color may due to a strong UV radiation field or other mechanisms (e.g., shocks) within this region that may destroy the polycyclic aromatic hydrocarbons and enhance the small-grain emission at 24 μm.