The Ages and Masses of Lyman Alpha Galaxies at Redshift $z\sim 4.5$

We examine the stellar populations of a sample of 98 redshift 4.5 Lyman alpha emitting galaxies using their broadband colors derived from deep photometry at the 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'). 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 equivalent width 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 stars are causing low EWs. We found masses ranging from 2e7 solar masses for the youngest objects in the sample to 2e9 solar masses for the oldest. It is possible that dust effects could produce large equivalent widths even in older populations by allowing the Lyman alpha photons to escape, even while the continuum is extinguished, and we present models for this scenario also.