Star formation in accretion discs and SMBH growth

Accretion discs around active galactic nuclei (AGNs) are potentially unstable to star formation at large radii. We note that when the compact objects formed from some of these stars spiral into the central supermassive black hole (SMBH), there is no radiative feedback and therefore the accretion rate is not limited by radiation forces. Using a set of accretion disc models, we calculate the accretion rate on to the central SMBH in both gas and compact objects. We find that the time-scale for an SMBH to double in mass can decrease by factors ranging from similar to 0.7 to as low as similar to 0.1 in extreme cases, compared to gas accretion alone. Our results suggest that the formation of extremely massive black holes at high redshift may occur without prolonged super-Eddington gas accretion or very massive seed black holes. We comment on potential observational signatures as well as implications for other observations of AGNs.