Star formation and quenching among the most massive galaxies at z~1.7

TL;DR

This study analyzes massive galaxies at z~1.7, revealing the importance of accurate de-blending for SFR measurement, and finds a close link between bulge growth, AGN activity, and star formation quenching.

Contribution

It provides a detailed object-by-object analysis of massive galaxies at high redshift, highlighting the role of morphology and AGN in star formation quenching.

Findings

Approximately 45% of galaxies are on the Main Sequence.

About 55% are sub-MS with significantly lower SFRs.

Nearly all bulge-dominated MS galaxies host an AGN.

Abstract

We have conducted a detailed object-by-object study of a mass-complete (M*>10^11 M_sun) sample of 56 galaxies at 1.4 < z < 2 in the GOODS-South field, showing that an accurate de-blending in MIPS/24um images is essential to properly assign to each galaxy its own star formation rate (SFR), whereas an automatic procedure often fails. This applies especially to galaxies with SFRs below the Main Sequence (MS) value, which may be in their quenching phase. After that, the sample splits evenly between galaxies forming stars within a factor of 4 of the MS rate (~45%), and sub-MS galaxies with SFRs ~10-1000 times smaller (~55%). We did not find a well defined class of intermediate, transient objects below the MS, suggesting that the conversion of a massive MS galaxy into a quenched remnant may take a relatively short time (<1 Gyr), though a larger sample should be analyzed in the same way to set precise limits on the quenching timescale. X-ray detected AGNs represent a ~30% fraction of the sample, and are found among both star-forming and quenched galaxies. The morphological analysis revealed that ~50% of our massive objects are bulge-dominated, and almost all MS galaxies with a relevant bulge component host an AGN. We also found sub-MS SFRs in many bulge-dominated systems, providing support to the notion that bulge growth, AGN activity and quenching of star formation are closely related to each other.