Massive black holes in dwarf galaxies: from AGN feedback to multimessenger signatures

Massive black holes in dwarf galaxies: from AGN feedback to multimessenger signatures

Contrary to the standard lore, there is mounting observational evidence that feedback from active galactic nuclei (AGN) may also play a role at the low-mass end of the galaxy population. We have explored this possibility with a series of high-resolution zoom-in simulations, varying the AGN prescription and supernova energetics.

We find that with the commonly employed Bondi model for black hole growth, black hole accretion in dwarfs is completely degenerate with the black hole seed mass. This motivated us to develop an alternative black hole accretion model that is directly tied to the gas availability in the immediate surroundings of the black hole and does not artificially suppress the growth of low-mass seeds. We also experimented with varying the boost parameter in the Bondi prescription. Both the supply-limited accretion model and the boosted Bondi set-ups lead to efficient accretion onto low-mass black hole seeds in our simulated dwarf system. In fact, there are sufficient amounts of gas to power brief, Eddington-limited accretion episodes in dwarf galaxies.

These episodes have a profound effect on the large-scale outflows, increasing outflow temperatures and velocities, which could be probed by future observations with JWST NIRSpec. The AGN-boosted outflows heat the circumgalactic medium and regulate star formation via maintenance-mode feedback, with the most significant impact at high redshifts, where supernova feedback alone cannot suppress cosmic inflows efficiently (see Fig. 1).

Fig. 1: Gas temperature projections of selected cosmological zoom-in simulations across a range of redshifts. Streamlines are also overplotted, with blue indicating inflows and red indicating outflows. The AGN runs can suppress the cosmic inflows at much higher redshifts and continuous AGN activity may keep the dwarf in a quenched state.

Finally, we investigated possible multi-messenger signatures of black hole mergers. The dwarf galaxy used for our zoom-ins lives in a relatively quiet environment, however, it experiences a high-redshift minor merger at z=4 that delivers significant amounts of fresh gas. For reasonable assumptions on the secondary black hole mass, we would expect this merger event to be observable by LISA with a signal-to-noise ratio greater than 10. Crucially, this merger event would also result in a bright EM counterpart, with the AGN X-ray luminosities for the majority of simulation set-ups explored peaking just after the merger at z=4, which may be observable by future X-ray missions such as AXIS or Lynx.

Project title: Modelling the multi-messenger signatures of massive black hole formation mechanisms with next-generation cosmological simulations

Project number: dp209

DiRAC resources used: CSD3 (Data Intensive Service, Cambridge) & COSMA (Memory Intensive Service, Durham)