科研之友
微信
新浪微博
Facebook
分享链接
摘要
We present an analysis of two strongly lensed galaxies at z = 6.56 and z similar to 7 for which multi-band photometric and spectroscopic observations are available. For one source, the data include recent Hubble Space Telescope (HST) and Spitzer observations. Using a spectral energy distribution (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 (SF) history and mass) and their intrinsic Lyman alpha (Ly alpha) emission. The following main results are obtained for the individual galaxies.
(i) Triple arc in Abell 2218, probable z similar to 7 galaxy. The most likely redshift of this source is z similar to 6.0-7.2, taking into account both our photometric determination and lensing considerations. SED fits indicate generally a low extinction [E(B - V) less than or similar to 0.05], but do not strongly constrain the SF history. Best fits have typical-ages of similar to 3-400 Myr. A reasonable maximum age of 250-650 Myr (1 sigma interval) can be estimated. However, the apparent 4000-angstrom break observed recently from the combination of (Infrared Array Camera) IRAC/Spitzer and HST observations can also be reproduced equally well with the template of a young (similar to 3-5 Myr) burst where strong rest-frame optical emission lines enhance the 3.6- and 4.5- mu m fluxes. The estimated star formation rate (SFR) is typically similar to 1 M circle dot yr(-1) for a Salpeter initial mass IMF) from 1 to 100 M circle dot(,) in agreement with previous estimates. The unknowns on the age and SF history could easily explain the apparent absence of Lya in this galaxy.
(ii) Abell 370HCM 6A. The available Ly alpha and continuum observations indicate basically 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, E(B - V) similar to 0.25, or A(V) similar to 0.5-1.8 at a 1 sigma level. In consequence, we obtain SFR similar to 11- 41 M-circle dot yr(-1), higher than earlier estimates, and we estimate a fairly high total luminosity [L similar to (1 - 4) x 10(11) L circle dot] for this galaxy, in the range of luminous infrared galaxies. A Lya transmission of similar to 23-90 per cent is estimated from our best-fitting models. Other properties (age and SF history) remain largely unconstrained. In case of composite stellar populations, the SFR, mass and luminosity estimate are lower. The two scenarios may be distinguishable with IRAC/Spitzer observations at 3.6 and 4.5 mu m.
- 出版日期2005-9-21
