We present optical measurements of the faint end of the luminosity function in the core of the Coma cluster. Dwarf galaxies are detected down to a limiting magnitude of R approx. 25.75 in images taken with the Hubble Space Telescope. This represents the faintest determination of the Coma luminosity function to date. With the assumption that errors due to cosmic variance are small, evidence is found for a steep faint end slope with alpha < -2. Such a value is expected in theories in which reionization and other feedback processes are dependent on density.
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