The MEGaN project I. Missing formation of massive nuclear clusters and tidal disruption events by star clusters - massive black hole interactions

We investigated the evolution of a massive galactic nucleus hosting a super-massive black hole (SMBH) with mass $M_\mathrm{SMBH}=10^8 \mathrm{M}_\odot$ surrounded by a population of 42 heavy star clusters (GCs). Using direct $N$-body modelling, we show here that the assembly of an NSC through GCs orbital decay and merger is efficiently inhibited by the tidal forces exerted from the SMBH. The GCs mass loss induced by tidal forces causes a significant modification of their mass function, leading to a population of low-mass ($<10^4$) clusters. Nonetheless, the GCs debris accumulated around the SMBH give rise to well-defined kinematical and morphological properties, leading to the formation of a disk-like structure. Interestingly, the disk is similar to the one observed in the M31 galaxy nucleus, which has properties similar to our numerical model. The simulation produced a huge amount of data, which we used to investigate whether the GC debris deposited around the SMBH can enhance the rate of tidal disruption events (TDEs) in our galaxy inner density distribution. Our results suggest that the GCs disruption shapes the SMBH neighbourhoods leading to a TDE rate of $\sim 2 \times 10^{-4}$yr$^{-1}$, a value slightly larger than what expected in previous theoretical modelling of galaxies with similar density profiles and central SMBHs. The simulation presented here is the first of its kind, representing a massive galactic nucleus and its star cluster population on scales $\sim 100$ pc.Comment: 15 pages, 10 figures, 4 tables. Accepted for publication in MNRA

The MEGaN project II. Gravitational waves from intermediate mass- and binary black holes around a supermassive black hole

We investigate the evolution of intermediate-mass (IMBHs), stellar (BHs) and binary black holes (BHBs), deposited near a supermassive black hole (SMBH) by a population of massive star clusters. Stellar BHs rapidly segregate around the SMBH, driving the formation of extreme mass-ratio inspirals that coalesce at a rate $\Gamma= 0.02-0.2$ yr$^{-1}$ Gpc$^{-3}$ at redshift $z=0$. A few IMBHs orbiting the SMBH favour the formation of massive pairs that coalescence within a Hubble time, being the merger rate for this channel $\Gamma =0.03$ yr$^{-1}$ Gpc$^{-3}$. Recoiling kicks post-merger can eject the remnant from the galaxy centre, especially in dwarf galaxies. Our results suggest that this mechanism can lead to up to $10^5$ ejected SMBH within 1 Gpc. An IMBH co-existing with a few single and binary BHs in the same cluster can affect significantly their evolution, either driving binary disruption, yielding to intermediate-mass ratio inspirals (merger rate $\Gamma =9.5$ yr$^{-1}$ Gpc$^{-3}$), or boosting BHBs coalescence ($\Gamma =2-8$ yr$^{-1}$ Gpc$^{-3}$). In a few simulations, the SMBH boosts BHBs coalescence, leading this process to a merger rate $\Gamma =1$ yr$^{-1}$ Gpc$^{-3}$. We note that BHBs experiencing a merger in a galactic nucleus can be erroneously estimated $\sim 30\%$ heavier than it really is because of the Doppler shift of the wave frequency as caused by the rapid motion around the SMBH. All our simulations are carried out using an $N$-body code tailored to treat close encounters and post-Newtonian dynamics, that includes also the galaxy field and dynamical friction in the particles' equation of motion.Comment: 23 pages, 13 Figures, 5 tables. Accepted for publication in MNRAS. This version matches the accepted on

Lack of nuclear clusters in dwarf spheroidal galaxies: implications for massive black holes formation and the cusp/core problem

One of the leading scenarios for the formation of nuclear star clusters in galaxies is related to the orbital decay of globular clusters (GCs) and their subsequent merging, though alternative theories are currently debated. The availability of high-quality data for structural and orbital parameters of GCs allows us to test different nuclear star cluster formation scenarios. The Fornax dwarf spheroidal (dSph) galaxy is the heaviest satellite of the Milky Way and it is the only known dSph hosting five GCs, whereas there are no clear signatures for the presence of a central massive black hole. For this reason, it represents a suited place to study the orbital decay process in dwarf galaxies. In this paper, we model the future evolution of the Fornax GCs simulating them and the host galaxy by means of direct N-body simulations. Our simulations also take into account the gravitational field generated by the MilkyWay. We found that if the Fornax galaxy is embedded in a standard cold dark matter halo, the nuclear cluster formation would be significantly hampered by the high central galactic mass density. In this context, we discuss the possibility that infalling GCs drive the flattening of the galactic density profile, giving a possible alternative explanation to the so-called cusp/core problem. Moreover, we briefly discuss the link between GC infall process and the absence of massive black holes in the centre of dSphs