The nature of the red disk-like galaxies at high redshift: dust attenuation vs. intrinsically red stellar populations.

We investigate the nature of the disk-like galaxies with “red” colours (i.e. Ic - K > 4 or J - K > 2.3), discovered at redshift 0.7 < z < 3.2, by combining models of radiative transfer of the stellar and scattered radiation through different dusty interstellar media with stellar population evolutionary synthesis models. Reproducing the observed optical/near-infrared colours suggests that high-z, red disk-like galaxies have declining star-formation rates with e-folding times as short as ∼ 3 Gyr. Being “red” does not necessarily imply having luminosity-weighted old (i.e. > 1 Gyr) ages and/or being very dusty, since the contribution to the bolometric luminosity of the intermediate-age (i.e. between 0.2 and 1–2 Gyr) stellar populations is relevant. In particular, this is due to the thermally pulsating Asymptotic-Giant-Branch stars, with intrinsically red rest-frame V - K colours. The winds of these intermediate-age stars are expected to contribute substantially to the enrichment of the interstellar medium of their host disk-like galaxy with carbonaceous dust (e.g. the Policyclic Aromatic Hydrocarbons). Finally, our models of dusty, star-forming disks barely show Rc - K > 5.3, and only for an extremely limited region of the explored parameter space, whatever their redshift. Hence, Rc - K-selected galaxies at 0.7 < z < 3.2 most probably are either systems with a bulge (and, thus, potential hosts of an active galactic nucleus), maybe old and passively evolving, or starbursts/mergers. © 2005 American Institute of Physics [ABSTRACT FROM AUTHOR]