Gravitational waves from coalescing massive black holes in young dense clusters

HST observations reveal that young massive star clusters form in gas-rich environments like the Antenn{\ae} galaxy which will merge in collisional processes to form larger structures. These clusters amalgamate and if some of these clusters harbour a massive black hole in their centres, they can become a strong source of gravitational waves when they coalesce. In order to understand the dynamical processes that are into play in such a scenario, one has to carefully study the evolution of the merger of two of such young massive star clusters and more specifically their respective massive black holes. This will be a promising source of gravitational waves for both, LISA and the proposed Big Bang Observer (BBO), whose first purpose is to search for an inflation-generated gravitational waves background in the frequency range of $10^{-1}-1$ Hz. We used high-resolution direct summation $N-$body simulations to study the orbital evolution of two colliding globular clusters with different initial conditions. Even if the final eccentricity is almost negligible when entering the bandwidth, it will suffice to provide us with detailed information about these astrophysical events.Comment: Based on contribution at the Sixth International LISA Symposium, 7 page

Stability and evolution of super-massive stars (SMS)

Highly condensed gaseous objects with masses larger than 5x10^4 M_sun are called super-massive stars. In the quasistationary contraction phase, the hydrostatic equilibrium is determined by radiation pressure and gravitation. The global structure is that of an n=3 polytrope at the stability limit. Small relativistic corrections for example can initiate a free fall collapse due to the 'post Newtonian' instability. Since the outcome of the final collapse -A super-massive black hole or hypernova- depends sensitively on the structure and the size of the object, when the instability sets in, it is important to investigate in more detail the contraction phase of the SMS. If the gaseous object is embedded in a dense stellar system, the central star cluster, the interaction and coupling of both components due to dynamical friction changes the energy balance and evolution of the SMS dramatically. Dynamical friction between stars and gas, which can be estimated semi-analytically (see Just et al. 1986), has three different effects on the two-component system. We discuss in which evolutionary stages and parameter range these interaction processes are relevant and how they can influence the stability and evolution of the SMS.Comment: 6 pages, 1 figure, needs eas.cls (included). EAS Publ. Series, Vol. 10 EDP, Paris in pres

Revealing the formation of stellar-mass black hole binaries: The need for deci-Hertz gravitational wave observatories

The formation of compact stellar-mass binaries is a difficult, but interesting problem in astrophysics. There are two main formation channels: In the field via binary star evolution, or in dense stellar systems via dynamical interactions. The Laser Interferometer Gravitational-Wave Observatory (LIGO) has detected black hole binaries (BHBs) via their gravitational radiation. These detections provide us with information about the physical parameters of the system. It has been claimed that when the Laser Interferometer Space Antenna (LISA) is operating, the joint observation of these binaries with LIGO will allow us to derive the channels that lead to their formation. However, we show that for BHBs in dense stellar systems dynamical interactions could lead to high eccentricities such that a fraction of the relativistic mergers are not audible to LISA. A non-detection by LISA puts a lower limit of about $0.005$ on the eccentricity of a BHB entering the LIGO band. On the other hand, a deci-Hertz observatory, like DECIGO or Tian Qin, would significantly enhance the chances of a joint detection, and shed light on the formation channels of these binaries.Comment: Submitte

The fragmenting past of the disk at the Galactic Center : The culprit for the missing red giants

Since 1996 we have known that the Galactic Center (GC) displays a core-like distribution of red giant branch (RGB) stars starting at ~ 10", which poses a theoretical problem, because the GC should have formed a segregated cusp of old stars. This issue has been addressed invoking stellar collisions, massive black hole binaries, and infalling star clusters, which can explain it to some extent. Another observational fact, key to the work presented here, is the presence of a stellar disk at the GC. We postulate that the reason for the missing stars in the RGB is closely intertwined with the disk formation, which initially was gaseous and went through a fragmentation phase to form the stars. Using simple analytical estimates, we prove that during fragmentation the disk developed regions with densities much higher than a homogeneous gaseous disk, i.e. "clumps", which were optically thick, and hence contracted slowly. Stars in the GC interacted with them and in the case of RGB stars, the clumps were dense enough to totally remove their outer envelopes after a relatively low number of impacts. Giant stars in the horizontal branch (HB), however, have much denser envelopes. Hence, the fragmentation phase of the disk must have had a lower impact in their distribution, because it was more difficult to remove their envelopes. We predict that future deeper observations of the GC should reveal less depletion of HB stars and that the released dense cores of RGB stars will still be populating the GC.Comment: 5 pages, no figures, accepted for publication ApJ Lett

A rapid evolving region in the Galactic Center: Why S-stars thermalize and more massive stars are missing

The existence of "S-stars" within a distance of 1" from SgrA$^*$ contradicts our understanding of star formation, due to the forbiddingly violent environment. A suggested possibility is that they form far and have been brought in by some fast dynamical process, since they are young. Nonetheless, all conjectured mechanisms either fail to reproduce their eccentricities --without violating their young age-- or cannot explain the problem of "inverse mass segregation": The fact that lighter stars (the S-stars) are closer to SgrA$^*$ and more massive ones, Wolf-Rayet (WR) and O-stars, are farther out. In this Letter we propose that the responsible for both, the distribution of the eccentricities and the paucity of massive stars, is the Kozai-Lidov-{\em like} resonance induced by a sub-parsec disk recently discovered in the Galactic center. Considering that the disk probably extended to smaller radius in the past, we show that in as short as (a few) $10^6$ years, the stars populating the innermost 1" region would redistribute in angular-momentum space and recover the observed "super-thermal" distribution. Meanwhile, WR and O-stars in the same region intermittently attain ample eccentricities that will lead to their tidal disruptions by the central massive black hole. Our results provide new evidences that SgrA$^*$ was powered several millions years ago by an accretion disk as well as by tidal stellar disruptions.Comment: 5 pages, two figures, accepted for publication ApJ Lett

The loss-cone problem in dense nuclei

We address the classical problem of star accretion onto a supermassive central gaseous object in a galactic nucleus. The resulting supermassive central gas-star object is assumed to be located at the centre of a dense stellar system for which we use a simplified model consisting of a Plummer model with an embedded density cusp using stellar point masses. From the number of stars belonging to the loss-cone, which plunge onto the central object on elongated orbits from outside, we estimate the accretion rate taking into account a possible anisotropy of the surrounding stellar distribution. The total heating rate in the supermassive star due to the loss-cone stars plunging onto it is estimated. This semi-analytical study, revisiting and expanding classical paper's work, is a starting point of future work on a more detailed study of early evolutionary phases of galactic nuclei. It merits closer examination, because it is one of the key features for the link between cosmology and galaxy formation.Comment: 9 pages, 6 figures, MNRAS in pres

Colliding red giants in galactic nuclei: Shocks, jets, impact on the ISM, X- and gamma-rays, neutrinos, fusion ignition and afterglow

In galactic nuclei, stellar densities are so high that stars can physically collide with each other. In this work we focus on the collision of red giants and in particular on the formation of non-thermal processes through collisions and their properties. We analytically address these points by evaluating head-on collisions but also take into account scenarios with a deviation from the radial orbit, which we treat in a perturbative fashion. The collisions produce internal shocks with supersonic Mach numbers. Almost immediately, jet-like structures with important Lorentz factors form. The debris from the collision produces another shock wave which, when interacting with the interstellar medium of a galactic nucleus, leads to particle acceleration. We estimate the background flux in X- and gamma rays created by the background of these collisions by deriving the spectral index within a radius of 100 Mpc and find that they are high. Additionally, we make an estimate of the neutrino production and find about $10^{11}$ neutrinos per square meter per second for a collision at 100 Mpc from Earth. Also, we derive that there is a non-negligible chance to ignite fusion during the collision, due to the squeezing of the material. We investigate the possibility that the degenerate cores collide with each other, leading to a high afterglow luminosity, and find that it is non-negligible, although this should be addressed with dedicated numerical simulations. Colliding red giants in galactic nuclei trigger a plethora of high-energy phenomena, and have a particular gravitational wave emission associated, as shown by us, so that their detection will allow us to rule out alternatives.Comment: 30 pages, no figures, submitted. Abstract abridge

The gravitational capture of compact objects by massive black holes

The gravitational capture of a stellar-mass compact object (CO) by a supermassive black hole is a unique probe of gravity in the strong field regime. Because of the large mass ratio, we call these sources extreme-mass ratio inspirals (EMRIs). In a similar manner, COs can be captured by intermediate-mass black holes in globular clusters or dwarf galaxies. The mass ratio in this case is lower, and hence we refer to the system as an intermediate-mass ratio inspiral (IMRI). Also, sub-stellar objects such as a brown dwarf, with masses much lighter than our Sun, can inspiral into supermassive black holes such as Sgr A* at our Galactic centre. In this case, the mass ratio is extremely large and, hence, we call this system ab extremely-large mass ratio inspirals (XMRIs). All of these sources of gravitational waves will provide us with a collection of snapshots of spacetime around a supermassive black hole that will allow us to do a direct mapping of warped spacetime around the supermassive black hole, a live cartography of gravity in this extreme gravity regime. E/I/XMRIs will be detected by the future space-borne observatories like LISA. There has not been any other probe conceived, planned or even thought of ever that can do the science that we can do with these inspirals. We will discuss them from a viewpoint of relativistic astrophysics.Comment: Submitted, 83 pages. Invited chapter for the "Handbook of Gravitational Wave Astronomy" (Eds. C. Bambi, S. Katsanevas and K. Kokkotas; Springer Singapore, 2021