Melanie Habouzit | Research

A bright future for massive black holes

The coming multi-messenger era will be pivotal to detect massive black holes closest to the redshift of their formation. JWST, Euclid, Roman, Athena will significantly increase our ability to detect the electromagnetic emission of lower-mass and/or higher-redshift black holes than possible today (current detections lie below the black curve), while LISA will detect gravitational waves from coalescing black holes at any redshift. For the first, we foresee the discovery of massive black holes' origins.
The figures below illustrate the parameter space of upcoming and future missions (from Volonteri, Habouzit, Colpi, Nature Physics Reviews, 2021) and today's theoretical models for black hole formation.


The population of massive black holes in large-scale cosmological simulations
Supermassive black holes in cosmological simulations I: Mbh-Mstar relation and black hole mass function - Habouzit+21
Supermassive black holes in cosmological simulations II: The AGN population and predictions for upcoming X-ray missions - Habouzit+22

Studying the formation and evolution of galaxies requires capturing highly nonlinear processes spanning a large dynamical range and the physics of the dark and baryonic components of matter. For that reason, a large effort has been carried in the community over the last few years to produce cosmological simulations of side length 100 comoving Mpc with about 1 kpc spatial resolution. They are a powerful resource to statistically study thousands of galaxies with stellar masses in the range 1e9-1e11 Msun over time.
We conducted an exhaustive in-depth analysis of the MBH population produced in the Illustris, TNG100, TNG300, Horizon-AGN, EAGLE, and SIMBA simulations. Our work highlights the discrepancies between simulations (see below for the Mbh-Mstar diagram), and assesses the agreement with current observations. The papers also present new observational perspectives to constrain simulation subgrid models.


High-redshift faint quasars and JWST
Co-evolution of massive black holes and their host galaxies: discrepancies from six cosmological simulations and the key role of JWST - Habouzit, Onoue, Banados, Neeleman, Angles-Alcazar, Walter, Pillepich, Dave, Jahnke, Dubois, MNRAS, 2022

Characterising the mass of massive black holes and the stellar mass of their host galaxies is extremely difficult beyond the low-redshift Universe. As such, current constraints on the Mbh-Mstar relation stop at z=2. Finding constraints at higher redshift is crucial to understand the build-up of the co-evolution between the black holes and their galaxies.
We analyzed the black hole population produced by six cosmological simulations and showed that the black hole mass offsets (at fixed stellar mass) of high-redshift black holes with respect to the local Universe vary from one simulation to another. In other words, some simulations produce more massive black holes at high redshift than at z=0, and some other simulations produce instead less massive black holes.
Moreover, we demonstrated that key constraints on the black hole to stellar mass relation can be obtained by a population of faint quasars (as defined in the figure below) characterised by JWST. The constraints from bright quasars would not be representative of the entire black hole population.


Formation of the high-redshift quasars
On the formation of the first quasars - Valiante, Agarwal, Habouzit, Pezzulli, PASA Review, 2017

The tightest constraint on the formation of supermassive black holes in the early Universe is the observation of very bright and massive black holes, called quasars. Some of them are observed at z>6, when the Universe is younger than 1Gyr. This population of quasars is shown in the left figure, and different theoretical growth histories for these quasars are shown in the right panel.


In a PASA review, we described the chemical environment of such powerful objects, and we review the different complementary methods that have been used to investigate the formation of supermassive black holes and their evolution through cosmic times (i.e., semi-analytical models and/or cosmological simulations).

Formation of supermassive black holes and their evolution through cosmic times
Blossoms from black holes seeds: properties and early growth regulated by supernova feedback - Habouzit, Volonteri, Dubois, MNRAS, 2017

Massive black holes inhabit local galaxies, including the Milky Way and some dwarf galaxies. Black hole formation, occurring at early cosmic times, must account for the properties of black holes in today’s galaxies, notably why some galaxies host a black hole, and others do not.
We have developed an implementation of black hole formation in dense, low-metallicity environments, as advocated by models invoking the collapse of the first generation of stars, or of dense nuclear star clusters. The seed masses are computed one-by-one on-the-fly, based on the star formation rate and the PopIII stellar initial mass function. This self-consistent method to seed black holes allows us to study the distribution of black holes in a cosmological context and their evolution over cosmic times. The following figures show the gas density map and the corresponding gas metallicity map of one of the simulations. Pink circles represent halos where a black hole was formed, and the radii of the circles are proportional to the mass of the black holes.


Supernova feedback is believed to be able to regulate the stellar content of low-mass galaxies. Therefore, we have tested several strengths of supernova feedback on the formation and evolution of supermassive black holes. We show the coevolution of black holes and their host galaxies in the following figures, on the left the simulation with a weak supernova feedback, i.e. thermal or kinetic feedback, on the right a stronger feedback, i.e. with prevention of the cooling after a supernova event.


The simulation with the strongest supernova feedback produces a population of galaxies, and a population of black holes, in better agreement with observational constraints. The simulated black holes connect to the observational local sample of black holes at z=0. We also find a good agreement with the bolometric and xray luminosity functions derived from observations, and a good agreement on the number of high-redshift AGN candidates found in the CDFS surveys.

Investigating the number density of direct collapse black holes
On the number density of direct collapse black hole seeds - Habouzit, Volonteri, Latif, Dubois, Peirani, MNRAS, 2016

The model of direct collapse is appealing as it leads to the formation of large mass black hole seeds, of about 10^4-6 Msun, which eases explaining how quasars at z=6-7 are powered by black holes with masses >10^9 Msun. However, direct collapse BH formation appears to be rare, as the conditions required by the scenario are metal-free gas, the presence of a strong photo-dissociating Lyman-Werner flux, and large inflows of gas at the center of the halo sustained for 10-100 Myr.
We have performed several cosmological hydrodynamical simulations that cover a large range of box sizes and resolutions, thus allowing us to understand the impact of several physical processes on the distribution of direct collapse black holes.
We identified halos where direct collapse could happen, and derived the number density of black holes. The figure below summarizes our findings, as well as results from other previous studies (in grey and black symbols).

moi Under optimistic assumptions, we found that for the direct collapse to account for the presence of black holes in normal galaxies, the critical Lyman-Werner flux required for direct collapse must be about two orders of magnitude lower than predicted by 3D simulations that include detailed chemical models. However, when supernova feedback is relatively weak, enough direct collapse black holes to explain z=607 quasars can be obtained for Lyman-Werner fluxes about one order of magnitude lower than found in 3D simulations.

Formation and evolution of black holes with primordial non-Gaussianities density perturbations
Black hole formation and growth with non-Gaussian primordial density perturbations - Habouzit, Volonteri, Nishimichi, Peirani, Dubois, Mamon, Silk, Chevallard, MNRAS, 2016

In this project, we have explored the possibility that positively skewed scale-dependent non-Gaussian primordial fluctuations may ease the assembly of massive black holes. These perturbations produce more low-mass halos at high redshift, thus altering the production of metals and ultraviolet flux, believed to be important factors in black hole formation. Additionally, a higher number of progenitors and of nearly equal-mass halo mergers would boost the mass increase provided by BH-BH mergers and merger-driven accretion.

We used a set of two cosmological simulations, with either Gaussian or scale-dependent non-Gaussian primordial fluctuations to perform a proof-of-concept experiment to estimate how black hole formation and growth are altered. We have modeled black hole formation and growth in a post-processing semi-analytical fashion.
The figure shows an illustration of the large scale environment of the two same halos (top panels: halo 1, bottom panels: halo 2), in the two simulations (left panels: Gaussian initial perturbations universe, right panels: simulation with primordial non-Gaussianities). Shaded areas show regions under different intensities of photo-dissociating flux. Dots show neighborhood halos, where direct collapse black holes could potentially form.


We found that the fractions of halos where black holes form are almost identical in the two simulations, but that non-Gaussian primordial perturbations increase the total number density of black holes by a factor of 2.
We also evolved BHs using merger trees extracted from the simulations and found that both the mean BH mass and the number of the most massive BHs at z = 6.5 are up to twice the values expected for Gaussian primordial density fluctuations.