How do supermassive black holes co-evolve with their host galaxy – the perspective of cosmological simulations

This is a full-time semester project carried out during the third semester of my master studies in astrophysics.

Overview

Most galaxies contain a supermassive black hole (SMBH) between a few millions to a few hundreds of million times the mass of the Sun. These SMBH accrete mass at the very center of their host galaxy, a process that emits tremendous amount of light. This is called a quasar, which luminosity can outshine that of its host galaxy.

But how do the properties of quasars relate to the ones of their host galaxies, in particular their mass properties? Do quasars power galaxies? Or are they formed only in the most massive galaxies?

Many of the answers to those questions rely on relations between the mass of SMBH and their host galaxy and their evolution with redshift. We propose to use existing hydrodynamical cosmological simulations to establish such relations, with particular focus on the dark matter content of quasar host galaxies, as this is now a measurable quantity thanks to gravitational lensing. We will use the hydrodinamical simulations from the IllustrisTNG project which offers three different sizes of box for the simulations : 50, 100 and 300Mpc.

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IllustrisTNG

Results We study in particular three scaling relations : the black hole - stellar mass 𝑀• − 𝑀∗, the black hole - velocity dispersion 𝑀• − 𝜎 and black hole - dark matter halo mass 𝑀• − 𝑀𝐷 𝑀 . We conduct these studies starting at redshift 𝑧 = 0 and then focusing on their redshift evolution and the impact of the type of galaxies and type of TNG simulations considered. Our analysis indicate that TNG100 is able to retrieve a correlation in good agreement with observations at 𝑧 = 0 for the 𝑀• − 𝑀∗ relation. However the simulation produces a tighter relation than the observations and has difficulties recreating galaxies with both high-mass black hole and stellar mass typically present in observational datasets. TNG100 also produces correlations for 𝑀• − 𝜎 and 𝑀• − 𝑀𝐷𝑀 but the discrepancies with the observed relations are more pronounced. The simulated 𝑀• − 𝜎 relation is not steep enough and is also off-setted with respect to 𝜎, with 𝜎 being too low compared to observations. The 𝑀• − 𝑀𝐷𝑀 correlation is in good agreement for 𝑀∗ < 1012 with a slope of ∼ 2.7 and ∼ 2.6 for observations and simulations respectively. Galaxies with 𝑀∗ > 1012 however present a discrepancy in the slopes, with a slope of ∼ 0.34 and ∼ 0.76 for observations and simulations respectively. Our findings also show that for the time evolution of the 𝑀• − 𝑀∗ relation, galaxies with a stellar mass smaller than ∼ 109.5𝑀⊙ at redshift 𝑧 ∼ 7 present two phases of BH growth. During the first phase they practically don’t grow while the stellar mass rapidly increases, then they grow efficiently and galaxies faithfully follow the observed relation on the 𝑀• − 𝑀∗ diagram. We also found that the final stellar mass of a galaxy is not correlated to its initial stellar mass as galaxies with a similar initial stellar mass can end up having a broad range of final stellar mass. Finally, for the 𝑀• − 𝑀∗ correlation, all simulations and type of galaxies considered give similar results in agreement with observations, with TNG100 giving the best result. As for the 𝑀• − 𝜎 relation, it seems that no simulation is able to produce results in agreement with the observations, although TNG300 results are the closest to observations. As for the 𝑀• − 𝑀𝐷𝑀 relation, we see how the simulations struggle to produce results in agreement with observations when galaxies are splitted into two dark matter mass bins. However all simulations are in good agreement with observations when the galaxies are considered all together.

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IllustrisTNG

References

About this project

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