Title: The effects of super-Eddington accretion and feedback on the growth of early supermassive black holes and galaxies

Authors: Filip Huško, Cedric G. Lacey, William J. Roper, Joop Schaye, Jemima Mae Briggs, Matthieu Schaller

First Author’s Institution: Leiden Observatory, Leiden University, Leiden, the Netherlands

Status: submitted to Monthly Notices of the Royal Astronomical Society [open access]

Problems of Growing Black Holes

Astronomers believe that almost every galaxy in the observable universe contains a supermassive black hole (SMBH) – a black hole at least a million times more massive than the sun (or more succinctly, a million solar masses). Recent observations from telescopes like the James Webb Space Telescope (JWST) indicate that SMBHs existed just a few million years after the Big Bang.

This poses a problem with our understanding of how black holes grow. We believe that black holes would have formed from the deaths of the first generation of stars, similar to how black holes form in the modern universe. However, based on estimates of their masses, it’s not clear that black holes could form and grow to supermassive scales in such a short period of time.

Black holes grow via mergers and accretion, where black holes “suck up” surrounding matter. For accretion, the material loses gravitational potential energy as it falls in, and some of that energy is converted into radiation (i.e., photons). These photons can interact with other infalling material and exert a pressure on it that slows down the infall rate. The point where the radiation pressure balances with the infall rate is called the Eddington limit, which is a theoretical limit to how quickly black holes can grow via accretion.

Based on the Eddington limit, we don’t understand how black holes from the first generation of stars could have accreted up to supermassive scales in just a few hundred million years. Astronomers think that accretion above the Eddington limit (called super- or hyper-Eddington accretion) may be possible, but not for extended periods of time. Additionally, we don’t fully understand how the radiation produces other features like jets and feedback into the black hole’s environment. Today’s authors use simulations of a galaxy cluster to study hyper-Eddington accretion onto a SMBH and its interplay with jets and feedback from the SMBH.

Black Hole Simulations

The authors run cosmological simulations that emulate the formation and evolution of galaxies, placing black holes at their centers. In their simulations, the authors account for accretion from a thin accretion disk that is not capped at the Eddington limit, along with the thermal feedback it produces. They also have models that contain jets from super-Eddington and sub-Eddington accretion.

Today’s authors study a massive galaxy protocluster and focus on a particular dark matter halo with a massive black hole in the center, shown in Figure 1. This halo has not undergone complex mergers (involving three or more galaxies), is the most massive halo in its region, and has been growing for a significant period of time. This halo undergoes a merger at a redshift of about 7.5 and a smaller merger at about 5.5 The effects of these mergers can be seen in the top panels of Figure 1. The black hole in the halo starts with a mass of 10⁴ solar masses when the halo has a mass of 10¹⁰ solar masses, which occurs at a redshift of about 17.

Messy Eaters

The results of the black hole’s growth and accretion is shown in Figure 2, looking at its growth over time (top left), its accretion rate relative to the Eddington rate (top right), its mass relative to the stellar mass of the halo (bottom left), and the black hole’s/halo’s path through the black hole mass – stellar mass phase space (bottom right). Different models are shown in different colors indicated by the legend in the top left panel. Note that redshift decreases to zero as time moves forward, so time increases from right to left on these plots.

The black hole grows much more rapidly when super-Eddington accretion is allowed, but the final mass is lower. This is because the feedback during the super-Eddington phase reduces how much material is available to accrete, so the black hole undergoes very little growth until the second merger at a redshift of about 5.5. This growth is tempered when jets are enabled, leading to even lower final masses. The jets further disrupt the supply of material feeding the SMBH, causing it to accrete more slowly. This effect is also seen in the top right panel, where the SMBH briefly accretes at super-Eddington rates before being quenched. However, the SMBH grows more rapidly when the second merger occurs, briefly accreting at super-Eddington rates again.

Figure 3 shows visualizations of the jets and feedback from the black hole in the different models.When super-Eddington accretion is enabled, the bubbles and outflows appear larger and at earlier times, though a bit smaller at the end of the simulation. This could be because the black hole ends up a little less massive when allowing for super-Eddington accretion (see the top left panel of Figure 2) because the feedback limits the amount of material available to accrete. Once jets are enabled, the outflows appear significantly larger at all times. Additionally, the feedback has a greater sense of directionality with jets, whereas it is more spherical without.

Overall, today’s authors find that allowing for super-Eddington accretion and jets in this regime can improve the realism of simulations and may provide better comparisons to observations. They note that the inclusion of jets primarily impacts the final mass of the black hole and the shape of the outflows, but other properties of the galaxy (such as its stellar mass) are not significantly impacted. They also note that there are more accurate models of accretion from accretion disks when black holes accrete far below the Eddington rate, but they don’t expect this to have a significant impact on their results.

Astrobite edited by Abbé Whitford

Featured image credit: Wikimedia Commons