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The Search for Intermediate-Mass Black Holes in Globular Clusters

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The Search for Intermediate-Mass Black Holes in Globular Clusters Astronomical Science The Search for Intermediate-mass Black Holes in Globular Clusters Nora Lützgendorf1 lution on all scales. Supermassive black cess to explain the rapid growth of Markus Kissler-Patig1 holes (SMBHs) have masses from a mil- SMBHs is through the merger of smaller Tim de Zeeuw1, 2 lion up to several billions of solar masses seed black holes of intermediate mass Holger Baumgardt 3 and are situated in the centres of massive (between 100 and 10 000 solar masses, Anja Feldmeier1 galaxies. When a SMBH accretes the e.g., Ebisuzaki et al., 2001). Karl Gebhardt 4 surrounding material it emits tremendous Behrang Jalali5 amounts of energy. This energy can be Numerical simulations have shown that Nadine Neumayer1 observed at all wavelengths and is detect- intermediate-mass black holes can form Eva Noyola 6 able at large distances. The most active in runaway mergers of stars in a dense galaxies are quasars and have been a environment, such as young star clusters. puzzle to astronomers since their discov- Also, ultraluminous X-ray sources at off- 1 ESO ery in the early 1960s. Their presence at centre positions in galaxies provide strong 2 Sterrewacht Leiden, Leiden University, large redshifts indicates that they existed evidence of massive black holes in stel- the Netherlands at a time when the Universe was only one lar clusters (e.g., Soria et al., 2011). The 3 School of Mathematics and Physics, billion years old or less, at a very early globular clusters observed today are as University of Queensland, Brisbane, stage of structure formation (Fan, 2006) old as their host galaxy: could they have Australia when the first galaxies were just forming. delivered seed black holes, the missing 4 Astronomy Department, University of How could a black hole with a mass of a link, in the crucial early stage of galaxy Texas at Austin, USA billion or more solar masses have formed formation? 5 I. Physikalisches Institut, Universität zu so early in the history of the Universe, Köln, Germany when galaxies were not yet fully evolved? A further motivation for studying IMBHs 6 Instituto de Astronomia, Universidad is that observations have shown a cor- Nacional Autonoma de Mexico (UNAM), Black holes can grow in two ways: relation between the masses of SMBHs Mexico through the accretion of surrounding and the velocity dispersion of their host material and by merging with other black galaxies (see Figure 1 and e.g., Ferrarese holes. Even if a black hole accretes at & Merritt, 2000; and Gebhardt et al., Intermediate-mass black holes (IMBHs) the highest possible rate (the Eddington 2000). Extrapolating this relation to the fill the gap between supermassive rate) for a billion years, it would not reach mass ranges of IMBHs yields a velocity and stellar-mass black holes and they one million solar masses if it started with dispersion of between 10 and 20 km/s, could act as seeds for the rapid growth a one solar mass seed. Observations such as those observed from the stars of supermassive black holes in early have also shown that SHMBs do not con- in globular clusters. The origin and inter- galaxy formation. Runaway collisions of stantly accrete at the efficiency of the pretation of this relation is still under massive stars in young and dense stel- Eddington rate. Thus, the preferred pro- debate and it is not known whether it lar clusters could have formed IMBHs that are still present in the centres of 10 globular clusters. We have resolved the 10 central dynamics of a number of glob- ular clusters in our Galaxy using the FLAMES integral-field spectrograph at the VLT. Combining these data with photometry from HST and comparing 10 8 them to analytic models, we can detect the rise in the velocity dispersion pro- file that indicates a central black hole. Our homogeneous sample of globu- ) ी lar cluster integral-field spectroscopy (M 10 6 allows a direct comparison between BH clusters with and without an IMBH. M Ω Cen The missing link? NGC 6388 Figure 1. The MBH– σ 4 relation for supermas- 10 G1 Black holes have fascinated humanity for M 15 sive black holes in gal- axies is shown. Globular almost three centuries. Objects of pure 47 Tuc Globular Clusters clusters and dwarf gal- gravity forming singularities in space time axies lie at the extrapo- were first postulated by John Michell lation of the relation in 1783 and still seem enigmatic to physi- towards lower black 102 hole masses. The points cists today, fuelling a strong drive to 10 100 for a few globular clus- understand their nature, growth and evo- σ (km/s) ters are plotted. The Messenger 147 – March 2012 21 Astronomical Science Lützgendorf N. et al., The Search for Intermediate-mass Black Holes UT2INTEGRATION IN OZPOZ ARGUS ARRANGEMENT Figure 2. A schematic of the VLT FLAMES ARGUS integral field unit 300 μm in the Ozpoz fibre posi- Sky Spectr tioner is shown (centre). button 15 The ARGUS fibre geom- Tumbler subslits etry and the allocation ograph Argus mm 2 of fibres into sub­slits head 4. feeding the GIRAFFE spectrometer is shown at right. 6.6 mm Array of 22 × 14 channels truly holds at the lower mass end. The hole. Obtaining velocities and velocity and Planetary Camera 2 (WFPC2) in the study of IMBHs and their environments dispersions in the centres of globular clus- I­band (F814W) filter in May 1998 (GO­ might shed light on many astrophysical ters, however, is very difficult due to 6804, PI: F. Fusi Pecci) and retrieved from problems, helping us to develop a better the high stellar densities in these regions. the European HST archive. The choice understanding of black hole formation Resolving individual stars in the centre of filter is related to our spectral observa- and galaxy evolution. and measuring spectra with ground- tions, which are obtained in the wave- based telescopes requires adaptive op­­ length range of the calcium triplet, a prom- The idea that IMBHs might exist in globu- tics, extraordinary weather conditions inent absorption feature around 860 nm lar clusters was first proposed decades and time-consuming observations. For and ideally suited to the measurement ago. Many observational attempts to find these reasons we used a different method of radial velocities. By measuring the rela- direct evidence of a black hole in globu- — integrated light measurements (Noyola, tive shift of the calcium triplet through lar clusters have followed. However, most 2010; Lützgendorf, 2011). absorption line fitting, we obtained a ve ­ studies have only observed the light locity for each spectrum. The final veloc- profiles in the centre of the clusters and We used the large integral field unit ity map for NGC 2808 is also shown in compared these with models. A central ARGUS, a mode of the FLAMES facility Figure 3. Velocity maps are instructive black hole does produce a prominent mounted on Unit Telescope 2 of the Very to look at and good for identifying sus- cusp in the surface brightness profile of Large Telescope (VLT). Figure 2 shows picious features, but in order to compare a globular cluster (Bahcall & Wolf, 1976; schematically the properties of the instru- our data to model predictions we need Baumgardt et al., 2005), but this signature ment. ARGUS consists of a 22 × 14 array the velocity dispersion profile. It is can be confused with the one expected of microlenses (called spaxels for spatial the velocity dispersion σ, or the second from a core-collapse process. For this pixels) with a sampling of 0.52 arcsec- moment of the velocity (the quadratic reason, it is crucial to study the kinematic onds per microlens and a total aperture sum of the expectation value of the veloc- profile in addition to the light profile. of 11.5 by 7.3 arcseconds on the sky; ity and the velocity dispersion), which this field allows the core of a globular holds information about the mass distri- Our group set out to search for the kine- cluster to be covered in a few pointings. bution. matic signatures of intermediate-mass A more detailed description of FLAMES black holes in Galactic globular clusters and the ARGUS mode can be found in In order to derive a velocity dispersion by trying to understand which of our Pasquini (2002) and Kaufer (2003). For profile, we used radial bins around the observed globular clusters host an inter- the observations the GIRAFFE spectro- centre of the cluster and combined all mediate-mass black hole at their centre graph was set to the low spectral resolu- the spectra included in each bin. The and which do not. With a sample of tion mode LR8 (coverage 820–940 nm absorption lines of the resulting spectra ten clusters, our goal is to understand at a spectral resolution of 10 400) and the are broadened due to the different veloci- whether the presence of an IMBH corre- ARGUS unit was pointed to different ties of the individual stars contributing lates with any of the cluster properties. positions, each of them containing three to the spaxels. Measuring the width of exposures of 600 s with 0.5 arcsecond these lines yields the velocity dispersion dithering, to cover the entire core radius. for each bin, thus a velocity dispersion Mapping the integrated light The position angle of the long axis of profile as a function of radius. Great care the integral field unit was varied from 0 to is taken to measure the true velocity A central black hole in a dense stellar 135 degrees in order to achieve maximum dispersion and not artefacts of the instru- system, such as a globular cluster, coverage of the cluster centre.
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