Stellar populations and Lya emission in two lensed z>~6 galaxies

Abstract

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 we attempt to constrain the properties of their stellar populations and their intrinsic Lya emission. The following main results are obtained for the individual galaxies: * Triple arc in Abell 2218: The most likely redshift of this source is z ~ 6.0-7.2. SED fits indicate generally a low extinction but do not strongly constrain the star formation (SF) history. Best fits have typical ages of ~3 to 400 Myr. The apparent 4000Ang 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). The estimated SFR is typically ~1 Msun/yr, in agreement with previous estimates. The unknowns on the age and star formation history could easily explain the apparent absence of Lya in this galaxy. * Abell 370 HCM 6A: The available Lya and continuum observations indicate two possible solutions: 1) a young burst or ongoing constant SF with non-negligible extinction or 2) a composite young+"old" stellar population. In the first case one obtains a best fit A_V ~ 0.5-1.8 at a 1 sigma level. In consequence we obtain SFR ~ 11-41 Msun/yr, higher than earlier estimates, and we estimate a fairly high total luminosity (L ~ (1-4)*10^{11} Lsun) for this galaxy, in the range LIRG. A Lya transmission of ~23-90% is estimated from our best fit models. Other properties 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 micron.