Hα Imaging of the Candidate Protogalaxy MS 1512−[CLC]cB58[/CLC]

We have obtained an image of the candidate protogalaxy MS 1512-cB58 through a narrow passband filter centered on Hα redshifted to z = 2.72. MS 1512-cB58 shows excess emission that is spatially extended, has a rest equivalent width of 56.7 Å, and luminosity of 3 × 10 43 h −2 75 ergs s -1 ( q 0 = 0.1). Assuming that the excess emission arises from nebular Hα associated with H II regions, the observed luminosity implies a star formation rate of 285 M ☉ yr -1 (neglecting any possible magnification by gravitational lensing). By comparison, the luminosity of the stellar continuum at 1500 Å, along with models for the spectral energy distribution, indicate a star formation rate about a factor of 12 greater. We discuss plausible explanations for this discrepancy and possible future observations that may clarify the situation.