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 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 magnitudes allow a detailed study of
the stellar populations and physical conditions. We compare star formation
rates as measured from the SED fit, the H-alpha and [OII] emission lines, and
the UV+IR bolometric luminosity where 24 micron 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*10^9 M_sun, young ages ~ 100 Myr, little dust content E(B-V)=0.10-0.25, and
star formation rates around 20-100 M_sun/yr. 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 build-up 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.
