Speaker
Description
Well documented up to redshift z ~ 4, the bimodality between “blue/star-forming" (SF) and “red/quiescent" (Q) galaxies is the statistical expression of the so-called quenching of the star formation. On the other hand, the diversity observed among quiescent galaxies (e.g., in terms of mass or morphology) suggests that the mechanisms involved in the quenching are multiple. For instance, the processes that are at play in the quenching of low-mass galaxies may be quite different from those involved in the quenching of massive galaxies, after billion years on the star formation main sequence (Faber et al. 2007, Schawinski et al. 2014, Moutard et al. 2016b).
In particular, the fact that star formation is observed to stop earlier in more massive galaxies, on average, underlies a downsizing of the quenching that argues for the existence of mass-related quenching processes, and the stellar mass function of SF galaxies shows that such “mass quenching” operates in galaxies with stellar mass reaching the characteristic mass $M^* \sim 10^{10.6}M_\odot$ (Ilbert et al. 2010, Peng et al. 2010, Moutard et al. 2016b). While $M^*$ galaxy quenching has been shown to be quite slow (lasting 1–3.5 Gyrs; e.g., Moutard et al. 2016b, Pandya et al. 2017), several mechanisms able to halt the cold gas supply have been put forth, e.g., due to the heating of the gas via viral shocks within dark-matter halos reaching a critical mass of $M_h \sim 10^{12}M_\odot$ (e.g., Kereš et al. 2005) or via radio-loud active galactic nucleus (AGN) feedback (e.g., Best et al. 2005).
I will present the results we obtained regarding the connection between the quenching of the star formation in evolved and massive galaxies and the presence of radio-loud and X-ray AGNs, by taking advantage of the rest-frame NUV-r vs. r-K colour (or NUVrK) diagram to unambiguously identify $M^*$ quenching galaxies that are in transition in the so-called green valley between SF and Q galaxies.
I will first show that radio-loud AGNs are mostly hosted by already quenched and massive ($M_* > 10^{11}M_\odot$) galaxies, which tends to confirm that their feedback is not the primary cause for $M^*$ galaxy quenching, as suggested previously (e.g., Hickox et al. 2009). More interestingly, I will then discuss the fact that the X-ray AGNs suffering from heavy obscuration of their soft X-ray emission are mostly hosted by $M^*$ galaxies that in the process of quenching, which argues for a quenching scenario that involves mergers of (gas-poor) $M^*$ galaxies after the onset of the quenching process, i.e., a scenario where $M^*$ galaxy mergers are not the cause but rather an aftermath of the quenching process. I will finally discuss how this is consistent with a picture where $M^*$ galaxy quenching happens along cosmic filaments.
