On the connection between ULX and IMBHs
On the connection between ULX and IMBHs
On the connection between ULX and IMBHs
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Overview of <strong>ULX</strong>:➢ Ultraluminous X-ray sources (<strong>ULX</strong>) are extra-galactic,off-center X-ray sources with X-ray luminosities inexcess of 2⋅10 39erg/sec.➢ If <strong>the</strong> radiation is emitted isotropically, <strong>the</strong>y wouldexceed <strong>the</strong> Eddington luminosity of stellar-mass blackholes by many orders of magnitude.➢ They are <strong>the</strong>refore c<strong>and</strong>idates for intermediate-massblack holes (<strong>IMBHs</strong>).➢ In addition, <strong>the</strong>re is an apparent <strong>connection</strong> <strong>between</strong><strong>ULX</strong> <strong>and</strong> massive star formation regions <strong>and</strong> (possibly)young star clusters.
Formation of <strong>IMBHs</strong> in Young Clusters➢ Observations indicate that <strong>the</strong>remight be a <strong>connection</strong> <strong>between</strong> <strong>ULX</strong><strong>and</strong> star clusters.➢ Matsumoto et al. (2001) for examplefound a bright X-ray source at <strong>the</strong>center of <strong>the</strong> starburst galaxy M82with an Eddington luminositycorresponding to a black hole ofseveral hundred solar masses.➢ Optical follow-up observationsshowed that <strong>the</strong> position of thissource coincides with that of ayoung luminous star cluster.
Formation of <strong>IMBHs</strong> in Young Clusters➢ McCrady et al. (2003) used <strong>the</strong> HST<strong>and</strong> Keck telescopes to determine <strong>the</strong>density profile <strong>and</strong> total masses for anumber of young star clusters in M82.➢ MGG-11 was <strong>the</strong> most concentrated(half-light radius 1.2 pc) <strong>and</strong> secondheaviest cluster in <strong>the</strong>ir sample (M=3.5⋅10 5 M Sun).➢ In addition it appears to have a top-heavy IMF.
Formation of <strong>IMBHs</strong> in Young Clusters➢ The runaway merging leads to<strong>the</strong> formation of a single objectof more than1000 M Sunwithin afew Myrs.➢ This object could form an inter-mediate-mass black hole whichcreates <strong>the</strong> X-ray radiation.➢ Simulations of o<strong>the</strong>r clustersshow that only MGG-11 isconcentrated enough to undergorunaway merging, in agreementwith <strong>the</strong> fact that only MGG-11contains a <strong>ULX</strong>.(from Portegies Zwart et al. 2004)
<strong>On</strong>e question remaining is how to create <strong>the</strong> <strong>ULX</strong> ?
How to create a <strong>ULX</strong> out of an IMBH ?Madhusudhan et al. (2005):➢ Modeled <strong>the</strong> accretion of gas onto anIMBH from a captured donor star.➢ They found that in order to explain<strong>ULX</strong> luminosities, one needs massivestars in orbits with 6-30 R Sunaround<strong>the</strong> IMBH.
How to create a <strong>ULX</strong> out of an IMBH ?Hopman et al. (2003):➢ Performed semi-analytic calculations toinvestigate <strong>the</strong> tidal capture of stars byan IMBH.➢ Found that 10% of <strong>IMBHs</strong> in MGG-11type clusters tidally capture a star.Blecha et al. (2005):➢ Studied encounters of low-mass <strong>IMBHs</strong>with binaries in a cluster core.➢ Found a high probability for <strong>the</strong> captureof stars but only a very lowprobability that <strong>the</strong>se stars will become<strong>ULX</strong> sources.
Tidal heating:➢A star passing inside=r M BHtM S1 /3R Stidally disrupted by <strong>the</strong> black hole.of a black hole is➢Outside this radius, <strong>the</strong> star is tidally deformed by <strong>the</strong>black hole. The energy needed for <strong>the</strong> deformation istaken out of <strong>the</strong> orbital energy.➢As a result, <strong>the</strong> orbit shrinks <strong>and</strong> <strong>the</strong> star circularisesnear <strong>the</strong> black hole.➢In a star cluster, <strong>the</strong> inspiraling star has encounters witho<strong>the</strong>r stars which can scatter it away from <strong>the</strong> inspiralorbit.➢In addition, <strong>the</strong> inspiral must be slow since o<strong>the</strong>rwise <strong>the</strong>star is heated <strong>and</strong> disrupted during <strong>the</strong> inspiral.
Our runs (Baumgardt et al. 2005, astro-ph/0511752):➢ We have performed N-body simulations of <strong>the</strong> formation ofan IMBH in <strong>the</strong> center of MGG-11 <strong>and</strong> <strong>the</strong> later evolution of<strong>the</strong> cluster up to an age of 12 Myrs.➢ Our simulations included stellar evolution, tidal disruptionof stars <strong>and</strong> tidal heating of stars which pass near to anIMBH.➢ Our simulations also contained perturbations tocircularised stars from passing perturbers.Results:1) We found significant black hole growth. Depending on <strong>the</strong>mass-radius relation adopted for <strong>the</strong> runaway star, <strong>IMBHs</strong>with masses in <strong>the</strong> range 1000 to 4000 were formed.M Sun
2) A stellar cusp forms around <strong>the</strong> IMBH <strong>and</strong> due to masssegregation,massive stars are enhanced in this cusp:
3) Within <strong>the</strong> 9 Myrs of our simulations, every IMBH disruptedon average 25 stars <strong>and</strong> experienced 3 successful inspirals.4) Due to perturbations from cusp stars, successful inspiralsoriginate predominantly from low-a orbits.
5) For about 40% all inspiraled stars, <strong>the</strong> maximum energyrate pumped into <strong>the</strong>m was less than <strong>the</strong>ir Eddingtonluminosity, i.e. <strong>the</strong> stars are likely to survive <strong>the</strong> inspiral.
6) The mass transfer from <strong>the</strong> tidally circularised stars onto<strong>the</strong> IMBH is enough to power a <strong>ULX</strong> for several Myrs onaverage. Our model <strong>the</strong>refore provides an explanation for<strong>the</strong> <strong>ULX</strong> in M82 <strong>and</strong> streng<strong>the</strong>ns <strong>the</strong> <strong>connection</strong> <strong>between</strong><strong>ULX</strong> <strong>and</strong> <strong>IMBHs</strong>.