The Stellar Population of Lyα-emitting Galaxies at z ~ 5.7

We present a study of three Lyα-emitting galaxies (LAEs), selected via a narrowband survey in the GOODS northern field and spectroscopically confirmed to have redshifts of z ~ 5.65. Using HST ACS and Spitzer IRAC data, we constrain the rest-frame UV-to-optical spectral energy distributions (SEDs) of the galaxies. Fitting stellar population synthesis models to the observed SEDs, we find best-fit stellar populations with masses between ~10⁹ and 10¹⁰ M_☉, and ages between ~5 and 100 Myr, assuming a simple starburst star formation history. However, stellar populations as old as 700 Myr are admissible if a constant star formation rate model is considered. Very deep near-IR observations may help to narrow the range of allowed models by providing extra constraints on the rest-frame UV spectral slope. Our narrowband-selected objects and other IRAC-detected z ~ 6 i'-dropout galaxies have similar 3.6 μm magnitudes and z'-[3.6] colors, suggesting that they possess stellar populations of similar masses and ages. This similarity may be the result of a selection bias, since the IRAC-detected LAEs and i'-dropouts probably only sample the bright end of the luminosity function. On the other hand, our LAEs have blue i'-z' colors compared to the i'-dropouts and would have been missed by the i'-dropout selection criterion. A better understanding of the overlap between the LAE and the i'-dropout populations is necessary in order to constrain the properties of the overall high-redshift galaxy population, such as the total stellar mass density at z ~ 6.