# Rejuvenated galaxies with very old bulges at the origin of the bending of the main sequence and of the "green valley"

## TL;DR

This study reveals that the bending of the star-forming main sequence is largely caused by rejuvenation of galaxies with very old bulges and young disks, challenging the idea that bulge formation occurs late internally.

## Contribution

It provides detailed analysis of bulge and disk star formation histories, showing bulges are very old and suggesting early bulge formation rather than late internal development.

## Key findings

- Bulges are maximally old, nearing the universe's age at observation.
- Disks are relatively young, averaging about 1.5 Gyr.
- Main sequence bending is mainly due to galaxy rejuvenation, not bulge formation.

## Abstract

We investigate the nature of star-forming galaxies with reduced specific star formation rate (sSFR) and high stellar masses, those `green valley' objects that seemingly cause a reported bending, or flattening, of the star-forming main sequence. The fact that such objects host large bulges recently led some to suggest that the internal formation of bulges was a late event that induced the sSFRs of massive galaxies to drop in a slow downfall, and thus the main sequence to bend. We have studied in detail a sample of 10 galaxies at $0.45<z<1$ with secure SFR from Herschel, deep Keck optical spectroscopy, and HST imaging from CANDELS allowing us to perform multi-wavelength bulge to disc decomposition, and to derive star formation histories for the separated bulge and disc components. We find that the bulges hosted in these systems below main sequence are virtually all maximally old, with ages approaching the age of the Universe at the time of observation, while discs are young ($\langle$ T$_{50}\rangle \sim 1.5$ Gyr). We conclude that, at least based on our sample, the bending of the main sequence is, for a major part, due to rejuvenation, and we disfavour mechanisms that postulate the internal formation of bulges at late times. The very old stellar ages of our bulges suggest a number density of Early Type Galaxies at $z=1-3$ higher than actually observed. If confirmed, this might represent one of the first direct validations of hierarchical assembly of bulges at high redshifts.