Multi-wavelength Study of Young Massive Star Clusters in the Interacting   Galaxy ARP 24
  We made a multi-wavelength study of young massive star clusters (YSCs) in the interacting galaxy ARP 24, using the optical and ultraviolet images from Hubble Space Telescope (HST), Sloan Digital Sky Survey, and Galaxy Evolution Explorer; the mid-infrared images from Spitzer Space Telescope; and the narrow-band Ha image and optical spectra from the NAOC 2.16m telescope. Based on the HST images, we found that the brightest infrared knot in ARP 24 is associated with a complex of five young massive star clusters, within a region of ~ 0.95" radius (127pc) 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 ~ 10^5 Msun). The ionization parameter and metallicity of the complex were estimated using the emission line ratios, and the star formation rates were calculated using monochromatic 24um, FUV, and Ha line luminosities. We speculate that ARP 24 may formed by a retrograde fly-by encounter indicated by its one-armed appearance and fan-like structure, and the formation of the YSCs in this galaxy is triggered by the interaction. The clusters in the YSC complex may 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 optical and has higher 24um dust emission relative to the starlight and 8um emission. This warm infrared color may due to strong UV radiation field or other mechanisms (e.g., shocks) within this region which may destroy the Polycyclic Aromatic Hydrocarbons and enhance the small grain emission at 24um. 
Chen Cao (NAOC; Gucas), Hong Wu (NAOC)