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Starbursts and Star Clusters in the Ultraviolet 1

We present ultraviolet (UV) images of nine starburst galaxies obtained with the Hubble Space Telescope using the Faint Object Camera. The galaxies range in morphology from blue compact dwarfs to ultra-luminous merging far-infrared galaxies. Our data combined with new and archival UV spectroscopy and far-infrared fluxes allow us to dissect the anatomy of starbursts in terms of the distributions of stars, star clusters and dust. The overall morphology of starbursts is highly irregular, even after excluding compact sources (clusters and resolved stars). The irregularity is seen both in the isophotes and the surface brightness profiles. In most cases the latter can not be characterized by either exponential or R profiles. Most (7/9) starbursts are found to have similar intrinsic effective surface brightnesses, suggesting that a negative feedback mechanism is setting an upper limit to the star formation rate per unit area. Assuming a continuous star formation rate and a Salpeter (1955) IMF slope, this surface brightness corresponds to an areal star formation rate of 0.7 M⊙ Kpc −2 yr in stars in the mass range of 5 – 100 M⊙. All starbursts in our sample contain UV bright star clusters indicating that cluster formation is an important mode of star formation in starbursts. On average about 20% of the UV luminosity comes from these clusters. The clusters with M220 < −14 mag, or super star clusters (SSC) are preferentially found at the very heart of starbursts; over 90% of the SSCs are found where the underlying surface brightness is within 1.5 mag arcsec of its peak value. The size of the SSCs in the nearest host galaxies are consistent with those of Galactic globular clusters. Our size estimates of more distant SSCs are likely to be contaminated by neighboring clusters and the underlying peaked high surface brightness background. The luminosity function of SSCs is well represented by a power law (φ(L) ∝ L) with a slope α ≈ −2. We find a strong correlation between the far infrared excess and the UV spectral slope for our sample and other starbursts with archival data. The correlation is in the sense that as the UV color becomes redder, more far-infrared flux is observed relative to the UV flux. The correlation is well modeled by a geometry where much of their dust is in a foreground screen near to the starburst, but not by a geometry of well mixed stars and dust. Some starbursts have noticeable dust lanes, or completely obscured ionizing sources, indicating that the foreground screen is not uniform but must have some patchiness. Nevertheless, the reddened UV colors observed even in these cases indicates that the foreground screen has a high covering factor and can account for a significant fraction of the far-infrared flux.