Implications of the nanoHertz Gravitational-Wave Background for Galactic Feedback and Massive Black Hole Growth

Abstract

We investigate how pulsar timing array (PTA) measurements of the nanoHertz gravitational-wave background (GWB) can constrain models for the growth history of supermassive black holes (SMBHs) and how active galactic nucleus (AGN) and stellar feedback models can affect GWB predictions. Feedback regulates supermassive black hole (SMBH) growth, altering the black hole mass function (BHMF). Using BHMFs drawn from multiple cosmological simulation suites including IllustrisTNG, MillenniumTNG, Simba, and CAMELS, and combining these with a quasar-based SMBH binary population framework, we predict the resulting GWB amplitude under a range of different stellar and AGN feedback prescriptions. We find that the choice of both stellar and AGN feedback models alters the high-mass end of the BHMF and changes the predicted GWB amplitude by up to a factor of 2 for the fiducial simulations and a factor 10 for extreme feedback variations in CAMELS. Models with inefficient or absent AGN feedback produce abundant SMBHs and yield GWB amplitudes consistent with PTA data, yet fail in producing realistic galaxies. Fiducial models of AGN and stellar feedback suppress SMBH growth too much and under-predict the expected signal, an effect which could possibly be mitigated by more realistic black hole seeding and growth prescriptions. The mismatch between the GWB amplitudes predicted by cosmological simulations and those observed by PTAs suggests that SMBH growth is more efficient or occurs earlier than captured by current models. This demonstrates that PTA measurements provide a powerful new probe of feedback physics and the SMBH population.

Figures (7)

• Figure 1: Graphic illustrating the pipeline used to obtain the GWB amplitude from the simulation BHMFs. We explore different simulation models (left-side), MillenniumTNG/IllustrisTNG and Simba, and variations of said models using the CAMELS simulation suites. These different simulations predict different BHMFs which are used in our semi-analytic model to calculate the characteristic strain spectrum (right-side), described in Section \ref{['sec:GWB']}.
• Figure 2: The $z=0$ BHMF predicted by the Simba simulation model when implementing different AGN feedback modes. The volume, resolution, cosmology, and all other astrophysics are the same in these simulations. This illustrates how much variation can occur in the predicted BHMF due to AGN feedback. Variations on the high mass end ($M_{BH} \gtrsim 10^8 M_\odot$, marked by the shaded grey region) of the mass function will most impact the gravitational wave background amplitude with higher mass SMBHs contributing more.
• Figure 3: The BHMF (integrated from $z=0$ to 2) predicted by the IllustrisTNG (left) and Simba (right) simulations for different box sizes. MillenniumTNG, TNG300, TNG100, TNG50, and CAMELS-TNG/Simba are run in a box with co-moving side length 500, 205, 75, 35, and 25 Mpc/h respectively. The Simba simulations are run in a box with co-moving side length as labeled in the legend. The dashed horizontal line corresponds to where mass bins contain fewer than 10 SMBHs matched to the simulations by color. Both simulation models struggle to produce a well sampled high mass ($M_{BH} > 10^{9} M_\odot$) BHMF in a box with side length 25 Mpc/h, i.e. in the CAMELS simulations. These results imply that a box size length of at least $35$ Mpc/h for IllustrisTNG and $50$ Mpc/h for Simba is required to produce a reasonable sampled BHMF for $M_{BH} > 10^{9} M_\odot$. For the CAMELS simulations, the shaded region corresponds to the 16th-to-84th percentiles from the CV set.
• Figure 4: The predicted BHMF at $z=0$ from the CAMELS-TNG simulation 1P set for parameter variations controlling galactic feedback. The name of the parameter varied is labeled in the top right of each plot. The legend indicates what color corresponds to the parameter value with red and blue being the largest and smallest value for that parameter respectively. The grey dashed line marks where the mass bins contain fewer than 10 SMBHs. Some galactic feedback parameters produce large variations in the predicted BHMF while others have minimal effect. Changes to the high mass end of the BHMF are expected to impact the predicted GWB amplitude while changes to the low mass end are not. Descriptions of the parameters can be found in Table \ref{['tab:TNGsummary']}.
• Figure 5: The same as Figure \ref{['fig:TNG_1P']} but for the CAMELS-Simba simulation suite. Descriptions of the parameters can be found in Table \ref{['tab:SIMBAsummary']}.