The active and passive populations of Extremely Red Objects

TL;DR

This study uses advanced galaxy models to reproduce and analyze the properties of Extremely Red Objects, revealing the significant impact of TP-AGB stars and predicting observational strategies to better understand their star formation activity.

Contribution

The paper demonstrates that incorporating TP-AGB stars in galaxy spectra models significantly increases the predicted number of red active galaxies and provides detailed predictions for future IR observations.

Findings

Model accurately reproduces number counts and redshift distributions of EROs.

Including TP-AGB stars increases the number of red active objects.

Deep Far-IR observations are crucial for constraining star formation rates.

Abstract

[abridged] The properties of galaxies with the reddest observed R-K colors (Extremely Red Objects, EROs), including their apparent division into passive and obscured active objects with roughly similar number densities, are a known challenge for models of galaxy formation. We produce mock catalogues generated by interfacing the predictions of the semi-analytical MORGANA model for the evolution of galaxies in a Lambda-CDM cosmology with the spectro-photometric + radiative transfer code GRASIL and Infrared (IR) template library to show that the model correctly reproduces number counts, redshift distributions and active fractions of R-K>5 sources. We test the robustness of our results against different dust attenuations and, most importantly, against the inclusion of TP-AGB stars in Simple Stellar Populations used to generate galaxy spectra, and find that the inclusion of TP-AGBs has a relevant effect, in that it allows to increase by a large factor the number of very red active objects at all color cuts. We find that though the most passive and the most obscured active galaxies have a higher probability of being selected as EROs, many EROs have intermediate properties and the population does not show bimodality in specific star formation rate (SSFR). We predict that deep observations in the Far-IR, from 100 to 500 micron, are the most efficient way to constrain the SSFR of these objects; we give predictions for future Herschel observations. Finally, we test whether a simple evolutionary sequence for the formation of z=0 massive galaxies, going through a sub-mm-bright phase and then a ERO phase, are typical in this galaxy formation model. We find that this sequence holds for ~25 per cent of z=0 massive galaxies, while the model typically shows a more complex connection between sub-mm, ERO and massive galaxies. [abridged]