Category: AGNs

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Reionization with galaxies and active galactic nuclei

In this work we investigate the properties of the sources that reionized the intergalactic medium (IGM) in the high-redshift Universe. Using a semi-analytical model aimed at reproducing galaxies and black holes in the first ∼1.5 Gyr of the Universe, we revisit the relative role of star formation and black hole accretion in producing ionizing photons that can escape into the IGM. Both star formation and black hole accretion are regulated by supernova feedback, resulting in black hole accretion being stunted in low-mass haloes. We explore a wide range of combinations for the escape fraction of ionizing photons (redshift-dependent, constant, and scaling with stellar mass) from both star formation ( ⟨fsfesc⟩ ) and AGN ( fbhesc ) to find: (I) the ionizing budget is dominated by stellar radiation from low stellar mass ( M∗<10^9M⊙ ) galaxies at z > 6 with the AGN contribution (driven by Mbh>10^6M⊙ black holes in M∗≳10^9M⊙ galaxies) dominating at lower redshifts; (II) AGN only contribute 10−25 per cent to the cumulative ionizing emissivity by z = 4 for the models that match the observed reionization constraints; (III) if the stellar mass dependence of ⟨fsfesc⟩ is shallower than fbhesc , at z < 7 a transition stellar mass exists above which AGN dominate the escaping ionizing photon production rate; (IV) the transition stellar mass decreases with decreasing redshift. While AGN dominate the escaping emissivity above the knee of the stellar mass function at z ∼ 6.8, they take-over at stellar masses that are a tenth of the knee mass by z = 4.

https://ui.adsabs.harvard.edu/abs/2020MNRAS.495.3065D/abstract

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Unveiling early black hole growth with multifrequency gravitational wave observations

Third-generation ground-based gravitational wave interferometers, like the Einstein Telescope (ET), Cosmic Explorer, and the Laser Interferometer Space Antenna (LISA), will detect coalescing binary black holes over a wide mass spectrum and across all cosmic epochs. We track the cosmological growth of the earliest light and heavy seeds that swiftly transit into the supermassive domain using a semi-analytical model for the formation of quasars at z = 6.4, 2, and 0.2, in which we follow black hole coalescences driven by triple interactions. We find that light-seed binaries of several 102M⊙ are accessible to ET with a signal-to-noise ratio (S/N) of 10-20 at 6 < z < 15. They then enter the LISA domain with larger S/N as they grow to a few 10^4M⊙ . Detecting their gravitational signal would provide first time evidence that light seeds form, grow, and dynamically pair during galaxy mergers. The electromagnetic emission of accreting black holes of similar mass and redshift is too faint to be detected even for the deepest future facilities. ET will be our only chance to discover light seeds forming at cosmic dawn. At 2 < z < 8, we predict a population of ‘starved binaries’, long-lived marginally growing light-seed pairs, to be loud sources in the ET bandwidth (S/N > 20). Mergers involving heavy seeds ( ∼10^5−10^6M⊙ ) would be within reach up to z = 20 in the LISA frequency domain. The lower z model predicts 11.25 (18.7) ET (LISA) events per year, overall.

https://ui.adsabs.harvard.edu/abs/2021MNRAS.500.4095V/abstract

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Cosmic archaeology with massive stellar black hole binaries

The existence of massive stellar black hole binaries (MBHBs), with primary black hole (BH) masses ≥ 31 Mo, was proven by the detection of the gravitational wave (GW) event GW150914 during the first LIGO/Virgo observing run (O1), and successively confirmed by seven additional GW signals discovered in the O1 and 02 data. By adopting the galaxy formation model GAMESH coupled with binary population synthesis (BPS) calculations, here we investigate the origin of these MBHBs by selecting simulated binaries compatible in mass and coalescence redshifts. We find that their cosmic birth rates peak in the redshift range 6.5 < 2 < 10, regardless of the adopted BPS.
These MBHBs are then old systems forming in low-metallicity (Z~ [0.01-0.1] Zo), low-stellar-mass galaxies, before the end of cosmic reionization, i.e. significantly beyond the peak of cosmic star formation. GW signals generated by coalescing MBHBs open up new possibilities to probe the nature of stellar populations in remote galaxies, at present too faint to be detected by available electromagnetic facilities.

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