Dense Stellar Systems and Massive Black Holes: Sources of Gravitational Radiation and Tidal Disruptions Dissertation zur Erlangung des akademischen Grades doctor rerum naturalium habilitatus (Dr. rer. nat. habil) in der Wissenschaftsdisziplin “Astronomie” eingereicht als kumulative Dissertation an der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Potsdam von Dr. Pau Amaro Seoane Potsdam, September 4, 2015 Max-Planck-Institut für Gravitationsphysik (Albert-Einstein-Institut) Published online at the Institutional Repository of the University of Potsdam: URN urn:nbn:de:kobv:517-opus4-95439 http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-95439 CONTENTS i introduction 7 1 motivation 9 2 relaxation in galactic nuclei 13 3 stellar dynamics and the cosmic growth of smbhs 29 4 featured articles 43 ii articles 47 5 populating the gc with compact objects 49 6 tidal disruptions in fragmenting discs 61 7 a rapid evolving region 83 8 tidal disruptions of separated binaries 95 9 strong mass segregation: event rate of emris and cusp re-growth 109 10 spin matters: emri event rates 131 11 butterflys and emris 157 12 kicking imbhs off gcs: imris 171 13 imris, tdes and the formation of ultra-compact dwarf galax- ies 195 iii discussion 227 iv acknowledgements 237 1 3 Contents GLOSSARY Acronym Meaning 1 M⊙ 1 Solar Mass = 1.99 × 1030 kg 1 pc 1 parsec ≈ 3.09 × 1016 m 1 Myr/Gyr One million/billion years AGN Active Galactic Nucleus BH Black Hole CO Compact Object (a white dwarf or a neutron star), or a stellar-mass black hole. In general, a collapsed star with a mass 2 [1.4, 10] M⊙ in this work EMRI Extreme Mass Ratio Inspiral GC Galactic Centre GPU Graphics Processing Unit GW/GWs Gravitational Wave/s HB Giant stars in the horizontal branch IMBH Intermediate-Mass Black Hole (M 2 [102, 105] M⊙) IMRI Intermediate Mass Ratio Inspiral MBH Massive Black Hole (M ≈ 106 M⊙) MW Milky Way NB6 Direct-summation N−body6 NS Neutron Star PN post-Newtonian RG Red giant SMBH Super Massive Black Hole (M > 106 M⊙) SNR Signal-To-Noise Ratio SPH Smoothed Particle Hydronamics TDE Tidal Disruption Event UCD Ultra-Compact Dwarf Galaxy 4 Abstract: Massive, dark objects, very likely supermassive black holes (SMBH), are found in the majority of galaxies. The observed tight correlations between their masses and the surrounding stellar system indicate an enigmatic coevolution. Why does the mass of − an object, whose typical size is ∼ 10 7 parsec (pc, for a mass of 106 M⊙) correlates with the stellar properties of the entire galaxy, which can be some 103 pc in size? Are these objects black holes, as predicted by Einstein’s theory of general relativity or do we need alternatives? The answer lies in the event horizon, the single and unique characteristic that defines SMBHs. To probe it, we have at our disposal stars. Nuclear star clusters harboring a massive, dark object with a mass of up to ∼ 107 M⊙ are good testbeds to probe the event horizon of the potential SMBH with stars. The advantage is clear: Stellar dynamics around a massive object is a relatively simple multi-particle problem, and we have data: In our own galaxy we have observations of stars and clouds that are on very close orbits to our SMBH. In other galaxies we have detected about 50 stellar tidal disruption candidates, and this number should rapidly increase with the upcoming transient surveys. One of the most important sources of gravitational waves (GWs) for a space-based detector are extreme-mass-ratio binaries (EMRIs) in the stage where the dynamics are driven by GW emission. These systems are composed of a stellar-mass compact object (a white dwarf, a neutron star, or a stellar-mass black hole) that inspirals into a massive black hole located at a galactic centre. The masses of 2 interest for the small compact object are in the range m = 1 − 10 M⊙, and for the massive black hole in the range M = 105 − 107 M⊙. Then, the mass-ratio for these − − systems is in the interval m = m/M ∼ 10 7 − 10 3. The GW signals emitted by EMRIs are long lasting (months to years) and contain many GW cycles, of the order of 105 during the last year before the small compact object plunges into the massive black hole. Many of these cycles are spent in the neighbourhood of the massive black hole horizon, meaning GWs encode a map of the strong-field region of the massive black hole, the fabric of space and time. These extraordinary features of EMRIs allow for a revolutionary research program, which could lead to understanding different aspects of stellar dynamics in galactic centres, tests of the geometry of BHs, and tests of General Relativity and alternative theories of gravity. In this habilitation I present my main results in this line of research. Part I INTRODUCTION 1 M O T I VAT I O N The centre-most part of a galaxy, its nucleus consists of a cluster of a few 107 to a few 108 stars surrounding the DCO, assumed from now onward to be a MBH, with a size of a few pc. The nucleus is naturally expected to play a major role in the interaction between the DCO and the host galaxy, as we mentioned before. In the − nucleus, stellar densities in excess of 106 pc 3 and relative velocities of order a few 100 − to a few 1000 km s 1 are reached. In these exceptional conditions, unlike anywhere else in the bulk of the galaxy, collisional effects come into play. These include 2- body relaxation, i.e., mutual gravitational deflections, and genuine contact collisions between stars. This means that, if a star happens to pass very close to the MBH, some part of it or all of it may be torn apart because of the so-called tidal gravity of the central object. The difference in gravitational forces on points diametrically separated on the star alter its shape, from its initial approximately spherical architecture to an ellipsoidal one and, in the end, the star is disrupted. These processes may contribute significantly to the mass of the MBH. Tidal disrup- tions trigger phases of bright accretion that may reveal the presence of a MBH in an otherwise quiescent, possibly very distant, galaxy. In Figure 1 we give an intuitive image of a tidal disruption of an extended star, where distortions due to gravitational-lens have not been taken into consideration. In Figure 2, on the left we show a Chandra X-ray image of J1242-11 with a scale of 40 arcsec on a side. This figure pinpoints one of the most extreme variability events ever detected in a galaxy. One plausible explanation for the extreme brightness of the ROSAT source could be accretion of stars on to a super-massive black hole. On the right we have its optical companion piece, obtained with the 1.5 m Danish telescope at ESO/La Silla. The right circle indicates the position of the Chandra source in the centre of the brighter galaxy. These processes may contribute significantly to the mass of the MBH [30, 53]. Tidal disruptions trigger phases of bright accretion that may reveal the presence of a MBH in an otherwise quiescent, possibly very distant, galaxy [29, 34]. On the other hand, stars can be swallowed whole if they are kicked directly through the horizon of the MBH (so-called direct plunges) or gradually inspiral due to the emis- sion of GWs The latter process, known as an “ Extreme Mass Ratio Inspiral” (EMRI) is one of the main objects of interest for eLISA [Evolved Laser Interferometer Space Antenna 3]. A compact object, such as a star so dense that it will not be disrupted 9 motivation Figure 1: Schematic representation of the tidal disruption process. In the first panel on the left, an extended star approaches the central MBH. As soon as the star feels the overwhelming tidal forces acting on it, the initial spherical architec- ture becomes spheroidal and the star stars to be torn apart. On the third panel the star is totally ripped and about 50% of it accreted on to the MBH. Credits: ESA and Stefanie Komossa. Illustration credit M. Weiss. 10 motivation Figure 2: Optical and X-ray images of RX J1242-11. Credits: (left) ESO/MPE/S.Komossa and (right) NASA/CXC/MPE, [37]. by the tidal forces of the MBH, (say, a neutron star, a white dwarf or a small stellar black hole), is able to approach very close to the central MBH. When the compact ob- ject comes very close to the MBH, a large amount of orbital energy is radiated away, causing the semi-major axis shrink. This phenomenon will be repeated thousand of times as the object inspirals until is swallowed by the central MBH. The “doomed” object spends many orbits around the MBH before it is swallowed. When doing so, it radiates energy which can be conceptualised as a snapshot con- taining detailed information about space-time and all the physical parameters which characterise the binary, the MBH and the small stellar black hole: their masses, spins, inclination and their sky position. The emitted GWs encode a map of the space-time. If we can record and decode it, then we will be able to test the theory that massive dark objects are indeed Kerr black holes as the theory of general relativity predicts, and not exotic objects such as boson stars. This would be the ultimate test of general relativity. The detection of such an EMRI will allow us to do very exciting science: EMRIs will give us measurements of the masses and spins of BHs to an accuracy which is beyond that of any other astrophysical technique.