Assigned ID: 14 ----- Attempting Submission 1 (Tue Feb 26 15:51:11 GMT 2008) ----- Outflows from starburst galaxies are now viewed as an important aspect of galaxy formation and evolution. Much of what we know about superwinds comes from optical emission and UV/optical absorption line studies, yet the physical structures within winds that are responsible for these tracers - clouds - are not well understood even in the local Universe. Important issues such as typical cloud sizes, how they are accelerated, and whether they are being destroyed in the wind are not known. In 2006 the Hubble Heritage team obtained a deep and wide-field mosiac of ACS imaging of M82, the nearest brightest starburst galaxy with a superwind, in B, V, I and H-alpha filters. This is and will be the best dataset for studying superwind clouds for the foreseeable future. We will numerically identify clouds in the ACS images, and determine the cloud size and flux distributions as a function of position within the wind. We will combine this with existing information on electron density, extinction and wind velocity as a function of distance the along wind to determine e.g, cloud masses, and mass-flow rates and thus the effective cloud lifetimes. We will also study the relationship between the H-alpha clouds and (a) dust clouds visible in these images, and (b) the soft X-ray emission. This study will provide us with data of unprecedented quality on the physical nature of the one of the fundamental constituents of galactic winds.

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