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Central Massive Objects: the Stellar Nuclei – Black Hole Connection

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Central Massive Objects: the Stellar Nuclei – Black Hole Connection Astronomical News Report on the ESO Workshop Central Massive Objects: The Stellar Nuclei – Black Hole Connection held at ESO Garching, Germany, 22–25 June 2010 Nadine Neumayer1 Eric Emsellem1 1 ESO An overview of the ESO workshop on black holes and nuclear star clusters is presented. The meeting reviewed the status of our observational and the- oretical understanding of central mas- sive objects, as well as the search for intermediate mass black holes in globu- lar clusters. There will be no published proceedings, but presentations are available at http://www.eso.org/sci/ meetings/cmo2010/program.html. This workshop brought together a broad international audience in the combined fields of galaxy nuclei, nuclear star clus­ ters and supermassive black holes, to confront state­of­the art observations with cutting­edge models. Around a hundred participants from Europe, North and South America, as well as East Asia and Figure 1. Workshop participants assembled outside made up of several populations of stars. Australia gathered for a three­day meet­ ESO Headquarters in Garching. The existence of very young O and WR ing held at ESO Headquarters in Garching, stars in the central few arcseconds Germany (see Figure 1). The sessions around the black hole is puzzling. The were of very high quality, with many very – Are intermediate mass black holes currently favoured solution to this paradox lively, interesting and fruitful discussions. formed in nuclear clusters/globular of youth is in situ star formation in infalling All talks can be found online on the web clusters? gas clouds. This view is also supported page of the workshop1. – How do the central massive objects by the fact that the Milky Way nuclear star relate to their host galaxies? cluster is rotating (Rainer Schödel). Res­ The key scientific questions for this work­ onant relaxation can explain the warp and shop were: In the course of the workshop we walked the co­/counter­rotating dichotomy of – What is the evolutionary/causal con­ through these science questions, start­ the young stars (Bence Kocsis). However, nection between nuclear clusters and ing from the best studied example of a the existence of S stars very close to black holes? supermassive black hole and its surround­ the black hole is even more surprising. – What can the Galactic Centre tell us ing nuclear star cluster, at the heart of These are ordinary B stars with an age of about the “nuclear cluster–black hole” our own Galaxy. We indicate the authors about 108 years. Their observed proper­ connection? of the contributions, which are high­ ties can be best explained by a binary – Where do we stand observationally for lighted in this summary, so that they can disruption scenario, called the Hills mech­ black holes, nuclear clusters and inter­ be traced in the presentations online1. anism (Alessia Gualandris). mediate mass black holes? – What do theoretical models tell us Where is the expected stellar cusp at the about star formation in the extreme The Galactic Centre black hole and Galactic Centre? The formation timescale gravitational potential near the black nuclear star cluster might be longer than 1010 years, i.e. the hole and under the extreme stellar nuclear cluster is not old enough to have densities in galactic centres? The known orbits of 30 stars around the formed a cusp (Holger Baumgardt). The – Do we understand the feeding of the central radio source in our Galaxy make deficit of old stars around Sgr A* could central parsec? How are nuclear Sgr A* the best case for a supermassive be explained by the collision and destruc­ 6 clusters replenished with fresh gas? black hole, with a mass of 4.3 × 10 MA tion of giants with main sequence stars – What do theoretical models tell us (Stefan Gillessen). Figure 2 shows a fit to or stellar mass black holes (Melvyn Davies). about dynamics, evolution and migra­ the orbit of one of these stars (S2). The Another possibility might be that a fraction tion of nuclear star clusters in galaxy supermassive black hole resides in a very of the stars get disrupted and ac creted 7 centres? massive star cluster of about 3 × 10 MA, onto the central black hole. The Messenger 141 – September 2010 37 Astronomical News Neumayer N., Emsellem E., Report on the Workshop “Central Massive Objects” 1994 luminosity objects. Nuclear clusters are Greene et al. submitted 0.175 1994 9 09 similar but more compact stellar systems 20 Elliptical ., 1992 al than ultra­compact dwarf galaxies, et 0.15 ekin the latter having comparatively elevated Gült mass­to­light ratios (Michael Hilker). Speckle 0.125 Using spectrographs assisted by adap­ 3.5 m ) 8 ी ) pos err: M tive optics, the stellar populations and / BH NGC 4258 0.1 2 mas kinematics of both types of systems can M 9 new masers from AO, 8/10 m ( pos err: Kuo et al., in prep. be probed today (Mariya Lyubenova) log seconds < 500 µas rc 0.075 to test, for example, whether or not they c (a 7 Speckle host massive black holes. De NGC 1068 0.05 10 m pos err: 1 mas IC 2560 0.025 no VLBI Feeding, star formation and feedback Circinus 6 2 2.25 2.5 The question of how galaxy nuclei are 0 2002 log ( /km/s) σ* BH–bulge scaling relations are not universal replenished with fresh gas was discussed on the third day of the conference. There 0.05 0.025 0 –0.025 –0.05 –0.075 Figure 3. The relation between black hole mass RA (arcseconds) are different mechanisms at work that and bulge velocity dispersion is shown for the maser make gas lose angular momentum and galaxies presented by Greene et al. (2010). The Figure 2. Result of the combined orbit fit for the maser galaxies trace a population of low­mass sys­ drive it from large to small scales (mergers, Galactic Centre star S2. Blue: NTT/VLT measure­ tems whose black holes lie below the M–σ relation bars, unstable gravitational discs, three­ ments. Red: Keck measurements. The black line defined by elliptical galaxies (red line). shows the Keplerian fit (after Gillessen et al., 2009). armed spirals, turbulent viscosity and magnetic stress), all inferred from observa­ tions and simulations (e.g., Francisco Black holes and their scaling relations Nuclear star clusters and their relation to Müller­Sanchez, Nozomu Kawakatu). black holes However it is still unclear which mechanism Currently, dynamical black hole mass dominates under which circumstances determinations exist for about 50 galax­ Nuclear star clusters are very common. (Witold Maciejewski, Tessel van der Laan, ies. These correlate tightly with the overall They are found in spirals, S0s and dwarf Gaëlle Dumas, Rainer Beck). properties of their host galaxy’s bulge, elliptical galaxies with an occupation e.g., its velocity dispersion σ. However, fraction of about 50–75 %. Nuclear clus­ Observations show a lag between the the low­mass end of the black hole–host ters are compact and massive, with half­ central starburst and AGN phase, due to galaxy scaling relations is still not well­ light radii of typically ~ 3–5 parsec, and a transition of fast supernova to slower 6 7 sampled. Jens Thomas reported on the masses of 10 –10 MA, and they show mass­loss winds (Richard Davies). The ongoing SINFONI programme to fill in complex star formation histories (Jakob mass loss from surrounding stars seems the underpopulated regions in the M–σ Walcher). Generally, nuclear clusters to be sufficient to grow a nuclear disc relation using stellar kinematic modell­ are seen in late­type galaxies and black and to cause accretion (Marc Schartmann). ing. Karl Gebhardt pointed out that get­ holes in early­type galaxies, but often We were reminded however that we still ting any black hole mass to better than these two components coincide (Alister do not understand how black holes can 20 % accuracy is difficult and that taking Graham). For the nearest nuclear star be fed (Norman Murray, Rainer Beck), systematic effects into account is very clusters, the stellar and gas kinematics and how AGN feedback works, although important. What counts when setting up are spatially well­resolved, and enable a number of scenarios have been stud­ the scaling relations is the robustness of the dynamical detection of black holes ied with detailed numerical simulations the assumed uncertainties. inside the star clusters (Anil Seth, Nadine (Vincenzo Antonuccio­Delogu, Chris Neumayer). Some nuclear star clusters Power). Feedback from star formation Jenny Greene presented accurate black also show signatures of an AGN, making could also be an important (and competi­ hole mass determinations in galaxies with the co­existence of a black hole indisput­ tive!) contributor in regulating the feeding megamasers, along with measurements able (Aaron Barth, Joseph Shields). itself: in the Milky Way it seems to mostly of their stellar velocity dispersion. These be acting via radiative pressure, but it galaxies lie below the M–σ relation, show­ Nuclear star clusters observed in nearby remains to be seen if this is true also in ing a large scatter (see Figure 3). Moreover, (early­type) galaxies (in the Virgo and denser environments near the centre. active galactic nuclei (AGN) diagnostics Fornax clusters) seem to have properties help to measure black hole masses at the that vary continuously along the luminos­ low­mass end. The future for black hole ity sequence (Laura Ferrarese). Although Are intermediate mass black holes detections looks bright! Upcoming tran­ they represent a tiny fraction of the total formed in nuclear/globular clusters? sient surveys may be able to detect many light in a galaxy they can be rather promi­ tidal disruption events of stars around low nent above the extrapolation of the outer Theoretically, in young star clusters after mass black holes (Linda Strubbe).
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