Speaker
Description
The formation and growth of Supermassive Black Holes (BHs) of mass MBH = 10^(6 - 10) Msun, have always challenged our understanding of modern astronomy. Indeed, some of these objects were already in place at very early cosmic times, which makes us wonder how they assembled so quickly, and what happens once they reach such high masses that early in the history of the Universe.
The focus of this talk is the BH - galaxy assembly in the most massive galaxies. We chose to explore the Boötes field, making use of multi-wavelength data to target efficiently both Active Galactic Nuclei (AGN) in the X-ray with Chandra, and the host galaxy with optical photometry to build their Spectral Energy Distribution (SED). By doing so, we assess how often massive galaxies host a growing BH, and how growth is governed in these extreme objects. Is it still on-going ? If so, how many of these objects are still growing and at which rate ? What are the properties of the galaxies hosting these extreme active BHs ?
Through SED fitting, we measured stellar mass and star formation rate, linking these galaxy properties to BH accretion rate using X-ray proxies. We derived AGN fractions and specific BH accretion rate distributions as a function of stellar mass and redshift to localise BH growth in time and in galaxy population. We traced back BH and galaxy growth overtime and assess how much and when did these objects grow since z<4, across a wide galaxy population.
We found that, while AGN seem preferentially located in moderately massive galaxies of log(Mstar) > 10 Msol, we do not find a stellar mass dependence in AGN fraction or accretion rate distribution above this mass threshold. Our BH-galaxy growth tracks reveal that while most BH mass has been accumulated since z=4 for lower mass BHs, the assembly of the most massive BHs is more complex, with little to no mass gain since z=4. This implies that rapid and intense growth episodes prior to z=4 were necessary to form these massive BHs.
Thanks to the wide field Chandra imaging, our results provided robust constrains in the high mass and high luminosity regime. These results highlight the potential of large-scale X-ray surveys, such as eROSITA, to achieve significant advancements in this field.
