Properties of simulated galaxies and supermassive black holes in cosmic voids

Abstract

Cosmic voids, the under-dense regions of the cosmic web, are widely used to constrain cosmology. Voids contain few, isolated galaxies, presumably expected to be less evolved and preserving memory of the pristine Universe. We use the cosmological hydrodynamical simulation Horizon-AGN coupled to the void finder {\sc \texttt{VIDE}} to investigate properties of galaxies in voids at z=0. We find that, closer to void centers, low-mass galaxies are more common than their massive counterparts. At fixed dark matter halo mass, they have smaller stellar masses than in denser regions. The star formation rate of void galaxies diminishes when approaching void centers, but their sSFR slightly increases, suggesting that void galaxies form stars more efficiently with respect to their stellar mass. We find that this can not only be attributed to the prevalence of low-mass galaxies. The inner region of voids also predominantly host low-mass BHs. However, the BH mass to galaxy mass ratios resemble those of the whole simulation at z=0. Our results suggest that even if the growth channels in cosmic voids are different than in denser environments, voids grow their galaxies and BHs in a similar way. While a large fraction of the BHs have low Eddington ratios, we find that 20\% could be observed as AGN with log10 L=41.5-42.5 erg/s in hard X-ray (2-10 keV). These results pave the way to future work with larger next-generation hydro simulations, aiming to confirm our findings and prepare the application on data from upcoming large surveys such as PFS, Euclid and WFIRST.