The formation of the most diffuse giant galaxy cores in the Universe

September 01, 2018

Supermassive black holes (SMBH) of up to tens of billon solar masses are hiding in the centers of giant elliptical galaxies. At the same time, these galaxies have ‘missing’ nuclear light as the stellar densities at their cores are much lower than in other giant galaxies. A team of researchers at the University of Helsinki and the astronomical Max Planck Institutes in Garching have used a newly developed simulation technique to investigate the origin of this ‘missing’ light with realistic galaxy models. When two massive elliptical galaxies merge, many central stars are expelled during the final coalescence of the stellar nuclei and their SMBHs. This new model can explain the simultaneous formation of the most diffuse giant galaxy cores as well as other observed core properties such as decoupled rotation and anisotropic stellar velocity distributions.

Massive elliptical galaxies are not just the largest – with up to 1013 solar masses – they also have markedly different properties than their smaller siblings. At their centres they harbour supermassive black holes (SMBHs) with typical masses of 0.1% of the total stellar mass of the galaxy – i.e. these SMBHs can easily exceed billions of solar masses. Also the properties of the stars in the centres of these galaxies are very special. The observed surface densities are much lower than for other giant galaxies, and instead of steep central cusps, these galaxies have very flat density cores. In addition, in many cases the stars in the central regions are predominantly moving on circular orbits, with a conspicuous lack of stars on more radial orbits. Furthermore, the central region as a whole is often rotating quite disconnected from the rest of the galaxy – a property termed decoupled rotation.

The reason behind these differing properties might be merger events - merging elliptical galaxies can be commonly observed in the sky. Already, numerical models have indicated that low-density cores can form when two elliptical galaxies merge. The coalescing nuclei with the SMBHs eject stars from the galaxy centres in a process called ‘SMBH scouring’. Reliable models for this process require very accurate simulation codes in order to correctly resolve the small-scale gravitational interaction of the forming binary black holes with the surrounding stars and the final merger of the SMBHs. Earlier studies so far have typically been limited to relatively low particle numbers as well as idealized galaxy models, and often did not simulate the final merger of the two SMBHs.

A team of researchers from the University of Helsinki and MPA/MPE have developed a novel simulation method called KETJU – the Finnish word for chain. This simulation technique allows for much larger and more accurate simulations. The KETJU code combines a hierarchical tree method on large-scales with a modern regularization procedure on small-scales. This allows for the accurate computation of the gravitational forces on kilo-parsec scales in the galactic halo down to the milli-parsec scales where the binary SMBHs emit gravitational waves and finally merge. The simulated elliptical galaxy mergers are also more realistic, as they now include the massive and extended dark matter halo component, in addition to the central stellar component.

The study demonstrates that a central low-density core can form rapidly on a timescale of ~ 30 Myr – but only in cases where merging binary SMBHs are found in the centre of the galaxy. Over this timescale, stars with a total mass similar to the combined mass of the two SMBHs are ejected from the galaxy. In the absence of central SMBHs the central region keeps its high stellar density and no stars are ejected (Fig. 1). The ejection process is stronger for more massive black holes, in good agreement with observations. The simulations can even explain the very extended core region of the very massive galaxy NGC1600 (Fig. 2). For its stellar mass this galaxy has an unusually massive black hole with an accompanying very large low-density core region. 

However, the merging black holes affect not only the stellar density of the cores but also the kinematics of the stars in the central region. After the ejection process, the remaining stars are mostly moving on circular orbits and do not come close to the central SMBH binary. Stars on more radial orbits have already before experienced strong interactions with the central SMBH binary and have been kicked out as a result. Again, this process is found to be stronger for more massive SMBHs and agrees well with observational estimates. Finally, the study also shows that massive SMBH binaries can give rise to rotation of the core region. In the case presented here the core is even counter-rotating (Fig. 3). This type of decoupled or misaligned rotation is commonly observed in many massive elliptical galaxies with both SMBHs and low density cores.

The team was able to demonstrate that all major photometric and kinematic properties of the centres of massive elliptical galaxies, such as low density cores, velocity anisotropies, and decoupled rotation, can be explained by a single process: the dynamical evolution and eventual coalescence of SMBHs in a galaxy merger. This process can explain the origin of even the most diffuse galaxy cores in the Universe. In a follow-up study the researchers will investigate the gravitational wave emission signals from the final stages of the SMBH mergers.

Thorsten Naab for the research team:

Antti Rantala & Peter Johansson (University of Helsinki, Finland)

Thorsten Naab & Matteo Frigo (Max Planck Institute for Astrophysics, Garching, Germany)

Jens Thomas (Max Planck Institute for extraterrestrial Physics, Garching, Germany)


We acknowledge support from the Finnish supercomputing center: CSC-IT Center for Science and the Max Planck Supercomputing and Data Facility.



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