N -body simulations of globular clusters in tidal fields: Effects of intermediate-mass black holes

A number of recent observational studies of Galactic globular clusters have measured the variation in the slope of a cluster's stellar mass function α with clustercentric distance r. In order to gather a deeper understanding of the information contained in such observations, we have explored the evolution of α(r) for star clusters with a variety of initial conditions using a large suite of N -body simulations. We have specifically studied how the time evolution of α(r) is affected by initial size, mass, binary fraction, primordial mass segregation, black hole retention, an external tidal field, and the initial mass function itself. Previous studies have shown that the evolution of α G is closely related to the amount of mass loss suffered by a cluster. Hence for each simulation we have also followed the evolution of the slope of the cluster's global stellar mass function, α G , and have shown that clusters follow a well-defined track in the α G -dα(r)/d(ln(r/r m )) plane. The location of a cluster on the α G − dα(r)/d(ln(r/r m )) plane can therefore constrain its dynamical history and, in particular, constrain possible variations in the stellar initial mass function. The α G -dα(r)/d(ln(r/r m )) plane thus serves as a key tool for fully exploiting the information contained in wide field studies of cluster stellar mass functions.

Section: Black Hole Retentionsupporting

confidence: 91%

Radial variation in the stellar mass functions of star clusters

Webb¹,

Vesperini²

2016

“…However, Kruijssen (2009b) notes that the evolution in α will be different for clusters with short dissolution times. Comparisons of Equation 2 to the work of Lutzgendorf et al (2013) suggests that the existence of a massive black hole (greater than 1% of the total cluster mass) at the centre of a cluster may alter our prediction of…”

Section: The Stellar Mass Function -Initial Mass Relationmentioning

confidence: 89%

“…Observations of such massive black holes in globular clusters have yet to be confirmed. Lutzgendorf et al (2013) also finds that the black hole retention fraction can alter the evolution of α, however the added uncertainty in…”

Section: The Stellar Mass Function -Initial Mass Relationmentioning

confidence: 98%

Back to the future: estimating initial globular cluster masses from their present-day stellar mass functions

Webb

Leigh

2015

We use N -body simulations to model the 12 Gyr evolution of a suite of star clusters with identical initial stellar mass functions over a range of initial cluster masses, sizes, and orbits. Our models reproduce the distribution of present-day global stellar mass functions that is observed in the Milky Way globular cluster population. We find that the slope of a star cluster's stellar mass function is strongly correlated with the fraction of mass that the cluster has lost, independent of the cluster's initial mass, and nearly independent of its orbit and initial size. Thus, the mass function -initial mass relation can be used to determine a Galactic cluster's initial total stellar mass, if the initial stellar mass function is known. We apply the mass function -initial mass relation presented here to determine the initial stellar masses of 33 Galactic globular clusters, assuming an universal Kroupa initial mass function. Our study suggests that globular clusters had initial masses that were on average a factor of 4.5 times larger than their present day mass, with three clusters showing evidence for being 10 times more massive at birth.

“…Indeed most of these clusters have small galactocentric radii (RG < 5 kpc) where tidal effects should be most important. Alternatively, a compact cluster of stellar mass black holes might prevent the cores of these clusters from collapsing (Morscher et al 2013;Lützgendorf, Baumgardt & Kruijssen 2013). Indeed, this possibility has been suggested by Mackey et al (2007) to explain the large core radii of young star clusters in the LMC and more recently by Peuten et al (2016) to explain the absence of mass segregation in NGC 6101.…”

Section: The N -Body Modelsmentioning

confidence: 96%

N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes

Baumgardt

2016

Self Cite

208

252

We have determined the masses and mass-to-light ratios of 50 Galactic globular clusters by comparing their velocity dispersion and surface brightness profiles against a large grid of 900 N -body simulations of star clusters of varying initial concentration, size and central black hole mass fraction. Our models follow the evolution of the clusters under the combined effects of stellar evolution and two-body relaxation allowing us to take the effects of mass segregation and energy equipartition between stars selfconsistently into account. For a subset of 16 well observed clusters we also derive their kinematic distances. We find an average mass-to-light ratio of Galactic globular clusters of < M/L V >= 1.98 ± 0.03, which agrees very well with the expected M/L ratio if the initial mass function (IMF) of the clusters was a standard Kroupa or Chabrier mass function. We do not find evidence for a decrease of the average mass-to-light ratio with metallicity. The surface brightness and velocity dispersion profiles of most globular clusters are incompatible with the presence of intermediate-mass black holes (IMBHs) with more than a few thousand M ⊙ in them. The only clear exception is ω Cen, where the velocity dispersion profile provides strong evidence for the presence of a ∼40,000 M ⊙ IMBH in the centre of the cluster.