Stellar populations and Ly α emission in two lensed z & 6 galaxies

We present an analysis of two strongly lensed galaxies at z = 6.56 and z ∼ 7 for which multi-band photometric and spectroscopic observations are available. For one source the data include recent HST and Spitzer observations. Using an SED fitting technique considering a number of parameters (various libraries of empirical and theoretical template spectra, variable extinction and extinction laws) we attempt to constrain the properties of their stellar populations (age, star formation history, mass) and their intrinsic Lyα emission. The following main results are obtained for the individual galaxies: • Triple arc in Abell 2218, probable z ∼ 7 galaxy: The most likely redshift of this source is z ∼ 6.0–7.2 taking into account both our photometric determination and lensing considerations. SED fits indicate generally a low extinction (E(B−V ) . 0.05) but do not strongly constrain the star formation (SF) history. Best fits have typical ages of ∼ 3 to 400 Myr. A reasonable maximum age of (250–650) Myr (1 σ interval) can be estimated . However, the apparent 4000 Å break observed recently from combination of IRAC/Spitzer and HST observations, can also equally well be reproduced with the template of a young (∼ 3–5 Myr) burst where strong restframe optical emission lines enhance the 3.6 and 4.5 μm fluxes. The estimated SFR is typically ∼ 1 M⊙ yr −1 for a Salpeter IMF from 1-100 M⊙, in agreement with previous estimates. The unknowns on the age and star formation history could easily explain the apparent absence of Lyα in this galaxy. • Abell 370 HCM 6A: The available Lyα and continuum observations indicate basically two possible solutions: 1) a young burst or ongoing constant SF with nonnegligible extinction or 2) a composite young + “old” stellar population. In the first case one obtains a best fit E(B − V ) ∼ 0.25, or AV ∼ 0.5–1.8 at a 1 σ level. In consequence we obtain SFR ∼ 11–41 M⊙ yr , higher than earlier estimates, and we estimate a fairly high total luminosity (L ∼ (1 − 4) × 1011L⊙) for this galaxy, in the range of luminous infrared galaxies. A Lyα transmission of ∼ 23– 90 % is estimated from our best fit models. Other properties (age, SF history) remain largely unconstrained. In case of composite stellar populations the SFR, mass, and luminosity estimate is lower. The two scenarios may be distinguishable with IRAC/Spitzer observations at 3.6 and 4.5 μm.