The nature of the red disc-like galaxies at high redshift: dust attenuation and intrinsically red stellar populations

Abstract

We investigate which conditions of dust attenuation and stellar populations allow models of dusty, continuously star-forming, bulge-less disc galaxies at 0.8 ≲ z ≲ 3.2 to meet the different colour selection criteria of high-z ‘red’ galaxies (e.g. RC−K > 5.3, IC−K > 4, J−K > 2.3). As a main novelty, we use stellar population models that include the thermally pulsating asymptotic giant branch (TP-AGB) phase of stellar evolution. The star formation rate of the models declines exponentially as a function of time, the e-folding time being longer than 3 Gyr. In addition, we use calculations of radiative transfer of the stellar and scattered radiation through different dusty interstellar media in order to explore the wide parameter space of dust attenuation. We find that synthetic discs can exhibit red optical/near-infrared colours because of reddening by dust, but only if they have been forming stars for at least ∼1 Gyr. Extremely few models barely exhibit RC−K > 5.3, if the inclination i= 90° and if the opacity 2 ×τV≳ 6. Hence, RC−K-selected galaxies at 1 ≲ z ≲ 2 most probably are either systems with an old, passively evolving bulge or starbursts. Synthetic discs at 1 ≲ z ≲ 2 exhibit 4 < IC−K < 4.8, if they are seen edge on (i.e. at i∼ 90°) and if 2 ×τV≳ 0.5. This explains the large fraction of observed, edge-on disc-like galaxies with Ks < 19.5 and F814W-Ks≳ 4. Finally, models with 2 ≲ z ≲ 3.2 exhibit 2.3 < J−K < 3, with no bias towards i∼ 90° and for a large range in opacity (e.g. 2 ×τV > 1 for i∼ 70°). In conclusion, red disc-like galaxies at 0.8 ≲ z ≲ 3.2 may not necessarily be dustier than nearby disc galaxies (with 0.5 ≲ 2 ×τV ≲ 2) and/or much older than ∼1 Gyr. This result is due both to a realistic description of dust attenuation and to the emission contribution by TP-AGB stars, with ages of 0.2 to 1–2 Gyr and intrinsically red colours.