Stellar populations of highly magnified lensed galaxies: Young starbursts at z ∼ 2

We present a comprehensive analysis of the rest-frame UV to near-IR spectral energy distributions (SEDs) and rest-frame optical spectra of four of the brightest gravitationally lensed galaxies in the literature: RCSGA 032727-132609 at z = 1.70, MS1512-cB58 at z = 2.73, SGAS J152745.1+065219 at z = 2.76, and SGAS J122651.3+215220 at z = 2.92. This includes new Spitzer imaging for RCSGA0327 as well as new spectra, near-IR imaging and Spitzer imaging for SGAS1527 and SGAS1226. Lensing magnifications of 3-4 mag allow a detailed study of the stellar populations and physical conditions. We compare star formation rates (SFRs) as measured from the SED fit, the Hα and [O II] λ3727 emission lines, and the UV+IR bolometric luminosity where 24 μm photometry is available. The SFR estimate from the SED fit is consistently higher than the other indicators, which suggests that the Calzetti dust extinction law used in the SED fitting is too flat for young star-forming galaxies at z ∼ 2. Our analysis finds similar stellar population parameters for all four lensed galaxies: stellar masses (3-7) × 109M⊙, young ages ∼ 100Myr, little dust content E(B - V) = 0.10-0.25, and SFRs around 20-100M⊙yr-1. Compared to typical values for the galaxy population at z ∼ 2, this suggests we are looking at newly formed, starbursting systems that have only recently started the buildup of stellar mass. These results constitute the first detailed, uniform analysis of a sample of the growing number of strongly lensed galaxies known at z ∼ 2. © 2012. The American Astronomical Society. All rights reserved.