Stellar populations of quasar host galaxies with MFICA decomposition

Galaxy evolution theories require co-evolution between accreting supermassive black holes (SMBH) and galaxies to explain many properties of the local galaxy population, yet observational evidence for the mechanisms driving this co-evolution is lacking. The recent star formation histories of the host galaxies of accreting SMBHs [active galactic nuclei (AGNs)] can help constrain the processes that feed SMBHs and halt star formation in galaxies, but are difficult to obtain for the most luminous AGNs (quasars). We introduce Mean-Field Independent Component Analysis (MFICA) to decompose quasar spectra and obtain recent star formation histories of their host galaxies. Applying MFICA to quasar spectra from the Sloan Digital Sky Survey DR7 Quasar Catalogue in the redshift range 0.16 ≤ z ≤ 0.76, we find that 53 per cent of quasar host galaxies are star-forming, 17 per cent lie in the green-valley, and only 5 per cent are quiescent. This contrasts with 14, 11, and 74 per cent of a mass-matched control sample that are star-forming, green-valley, and quiescent, respectively. We find that ∼ 25 per cent of quasars are hosted by post-starburst galaxies, an excess of 28 ± 1 compared to our control sample. While the heterogeneity of recent star formation histories implies multiple SMBH feeding mechanisms, the excess of post-starburst host galaxies demonstrates the link between accreting SMBHs and a recent starburst followed by rapid quenching. Given that massive post-starburst galaxies are predominantly caused by gas-rich major mergers, our results indicate that 30–50 per cent of quasars originate from merger-induced starbursts.