The ages and masses of Lyα galaxies at z ∼ 4.5

We examine the stellar populations of a sample of 98 z ∼ 4.5 Lyα-emitting galaxies using their broadband colors derived from deep photometry at the Multiple Mirror Telescope (MMT). These galaxies were selected by narrowband excess from the Large Area Lyman Alpha survey. Twenty-two galaxies are detected in two or more of our MMT filters (g′, r′, i′, and z′), with calculated rest-frame equivalent widths (EWs) from 5 to 800 A. By comparing broad- and narrowband colors of these galaxies to synthetic colors from stellar population models, we determine their ages and stellar masses. The highest EW objects have an average age of 4 Myr, consistent with ongoing star formation. The lowest EW objects show an age of 40-200 Myr, consistent with the expectation that larger numbers of older stars are causing low EWs. We found masses ranging from 2 × 10⁷ M _⊙ for the youngest objects in the sample to 2 × 10 ⁹ M_⊙ for the oldest. It is possible that dust effects could produce large EWs even in older populations by allowing the Lyα photons to escape, even while the continuum is extinguished, and we present models for this scenario also.