Abstract
ABSTRACT Quasars at $z = 6$ are powered by accretion on to supermassive black holes with masses $M_{\rm BH} \sim 10^9 \rm \, M_{\odot}$. Their rapid assembly requires efficient gas inflow into the galactic nucleus, sustaining black hole accretion at a rate close to the Eddington limit, but also high central star formation rates. Using a set of cosmological ‘zoom-in’ hydrodynamic simulations performed with the moving mesh code Arepo, we show that $z = 6$ quasar host galaxies develop extremely tightly bound stellar bulges with peak circular velocities $300\!-\!500 \, \rm km \, s^{-1}$ and half-mass radii ${\approx}0.5 \, \rm kpc$. Despite their high binding energy, we find that these compact bulges expand at $z \, \lt \, 6$, with their half-mass radii reaching ${\approx}5 \, \rm kpc$ by $z = 3$. The circular velocity drops by factors of ≈2 from their initial values to $200\!-\!300 \, \rm km \, s^{-1}$ at $z \, \approx \, 3$ and the stellar profile undergoes a cusp-core transformation. By tracking individual stellar populations, we find that the gradual expansion of the stellar component is mainly driven by fluctuations in the gravitational potential induced by bursty AGN feedback. We also find that galaxy size growth and the development of a cored stellar profile does not occur if AGN feedback is ineffective. Our findings suggest that AGN-driven outflows may have profound implications for the internal structure of massive galaxies, possibly accounting for their size growth, the formation of cored ellipticals as well as for the saturation of the MBH–σ⋆ seen at high-velocity dispersions σ⋆.
Highlights
Detections of bright quasars at z > 6 show that supermassive black holes with masses MBH 109 M assemble in less than a Gyr after the Big Bang (e.g. Fan et al 2004; Willott et al 2010; Mortlock et al 2011; Venemans et al 2013; Gallerani et al 2017)
We have unambiguously shown that AGN feedback is directly responsible for the gradual expansion of the compact stellar bulges that form in massive galaxies at z > 3
We suggest that massive galaxy growth occurs in different phases: (i) At high-z, these systems initially go through a phase in which gas is accreted onto the nucleus at high rates, powering black hole accretion and the formation of a compact ≈ 500 pc stellar bulge
Summary
Detections of bright quasars at z > 6 show that supermassive black holes with masses MBH 109 M assemble in less than a Gyr after the Big Bang (e.g. Fan et al 2004; Willott et al 2010; Mortlock et al 2011; Venemans et al 2013; Gallerani et al 2017). The efficient in-fall of gas into the galactic nucleus necessary to sustain black hole growth, likely triggers powerful central starbursts and the formation of compact stellar bulges characterised by extreme stellar velocity dispersions σ∗. In their high resolution cosmological simulations, Dubois et al (2012) find galaxies with stellar bulges with σ∗ > 600 km s−1 measured within ≈ 500 pc embedded within haloes with virial mass Mvir 1012 M at z > 6. The compact sizes are attributed to their special location at the intersection between
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