A. Feldmeier

Surface brightness profile of the Milky Way’s nuclear star cluster

Context. Although the Milky Way nuclear star cluster (MWNSC) was discovered more than four decades ago, several of its key properties have not been determined unambiguously up to now because of the strong and spatially highly variable interstellar extinction toward the Galactic centre. Aims. In this paper we aim at determining the shape, size, and luminosity/mass of the MWNSC. Methods. To investigate the properties of the MWNSC, we used Spitzer/IRAC images at 3:6 and 4:5 m, where interstellar extinction is at a minimum but the overall emission is still dominated by stars. We corrected the 4:5 m image for PAH emission with the help of the IRAC 8:0 m map and for extinction with the help of a [3:6 4:5] colour map. Finally, we investigated the symmetry of the

Limits On Intermediate-Mass Black Holes In Six Galactic Globular Clusters With Integral-Field Spectroscopy

Context. The formation of supermassive black holes at high redshift still remains a puzzle to astronomers. No accretion mechanism can explain the fast growth from a stellar mass black hole to several billion solar masses in less than one Gyr. The growth of supermassive black holes becomes reasonable only when starting from a massive seed black hole with mass on the order of 10 -10 M. Intermediate-mass black holes are therefore an important field of research. Especially the possibility of finding them in the centers of globular clusters has recently drawn attention. Searching for kinematic signatures of a dark mass in the centers of globular clusters provides a unique test for the existence of intermediate-mass black holes and will shed light on the process of black-hole formation and cluster evolution. Aims. We are investigating six galactic globular clusters for the presence of an intermediate-mass black hole at their centers. Based on their kinematic and photometric properties, we selected the globular clusters NGC 1851, NGC 1904 (M 79), NGC 5694, NGC 5824, NGC 6093 (M 80), and NGC 6266 (M 62). Methods. We used integral field spectroscopy to obtain the central velocity-dispersion profile of each cluster. In addition we completed these profiles with outer kinematic points from previous measurements for the clusters NGC 1851, NGC 1094, NGC 5824, and NGC 6093. We also computed the cluster photometric center and the surface brightness profile using HST data. After combining these datasets we compared them to analytic Jeans models. We used varying M/L profiles for clusters with enough data points in order to reproduce their kinematic profiles in an optimal way. Finally, we varried the mass of the central black hole and tested whether the cluster is better fitted with or without an intermediate-mass black hole. Results. We present the statistical significance, including upper limits, of the black-hole mass for each cluster. NGC 1904 and NGC 6266 provide the highest significance for a black hole. Jeans models in combination with a M/L profile obtained from N-body simulations (in the case of NGC 6266) predict a central black hole of M = (3 ± 1) × 10 M for NGC 1904 and M = (2 ± 1) × 10 M for NGC 6266. Furthermore, we discuss the possible influence of dark remnants and mass segregation at the center of the cluster on the detection of an IMBH.

Large scale kinematics and dynamical modelling of the Milky Way nuclear star cluster

Context. Within the central 10 pc of our Galaxy lies a dense cluster of stars. This nuclear star cluster forms a distinct component of the Galaxy, and similar nuclear star clusters are found in most nearby spiral and elliptical galaxies. Studying the structure and kinematics of nuclear star clusters reveals the history of mass accretion and growth of galaxy nuclei and central massive black holes. Aims. Because the Milky Way nuclear star cluster is at a distance of only 8 kpc, we can spatially resolve the cluster on sub-parsec scales. This makes the Milky Way nuclear star cluster a reference object for understanding the formation of all nuclear star clusters. Methods. We have used the near-infrared long-slit spectrograph ISAAC (VLT) in a drift-scan to construct an integral-field spectroscopic map of the central 9:5 8 pc of our Galaxy, and six smaller fields out to 19 pc along the Galactic plane. We use this spectroscopic data set to extract stellar kinematics both of individual stars and from the unresolved integrated light spectrum. We present a velocity and dispersion map from the integrated light spectra and model these kinematics using kinemetry and axisymmetric Jeans models. We also measure radial velocities and CO bandhead strengths of 1375 spectra from individual stars. Results. We find kinematic complexity in the nuclear star clusters radial velocity map including a misalignment of the kinematic position angle by 9 counterclockwise relative to the Galactic plane, and indications for a rotating substructure perpendicular to the Galactic plane at a radius of 20 00 or 0.8 pc. We determine the mass of the nuclear star cluster within r = 4:2 pc to (1.4 +0:6 0:7 ) 10 7 M . We also show that our kinematic data results in a significant underestimation of the supermassive black hole (SMBH) mass. Conclusions. The kinematic substructure and position angle misalignment may hint at distinct accretion events. This indicates that the Milky Way nuclear star cluster grew at least partly by the mergers of massive star clusters. Compared to other nuclear star clusters, the Milky Way nuclear star cluster is on the compact side of the re MNSC relation. The underestimation of the SMBH mass might be caused by the kinematic misalignment and a stellar population gradient. But it is also possible that there is a bias in SMBH mass measurements obtained with integrated light, and this might a ect SMBH mass determinations of other galaxies.

The nuclear cluster of the Milky Way: our primary testbed for the interaction of a dense star cluster with a massive black hole

This article intends to provide a concise overview, from an observational point-of-view, of the current state of our knowledge of the most relevant properties of the Milky Ways nuclear star cluster (MWNSC). The MWNSC appears to be a typical specimen of nuclear star clusters, which are found at the centers of the majority of all types of galaxies. Nuclear clusters represent the densest and most massive stellar systems in the present-day Universe and frequently coexist with central massive black holes. They are therefore of prime interest for studying stellar dynamics and the MWNSC is the only one that allows us to obtain data on milli-parsec scales. After discussing the main observational constraints, we start with a description of the overall structure and kinematics of the MWNSC, then focus on a comparison to extragalactic systems, summarize the properties of the young, massive stars in the immediate environment of the Milky Ways central black hole, Sagittarius\,A*, and finally focus on the dynamics of stars orbiting the black hole at distances of a few to a few tens of milli parsecs.

Indication for an intermediate-mass black hole in the globular cluster NGC 5286 from kinematics

Context. Intermediate-mass black holes (IMBHs) fill the gap between stellar-mass black holes and supermassive black holes (SMBHs). The existence of the latter is widely accepted, but there are only few detections of intermediate-mass black holes (10-10M ) so far. Simulations have shown that intermediate-mass black holes may form in dense star clusters, and therefore may still be present in these smaller stellar systems. Also, extrapolating the M- σ scaling relation to lower masses predicts intermediate-mass black holes in systems with σ ∼ 10-20 km s such as globular clusters. Aims. We investigate the Galactic globular cluster NGC 5286 for indications of a central intermediate-mass black hole using spectroscopic data from VLT/FLAMES*, velocity measurements from the Rutgers Fabry Perot at CTIO, and photometric data from HST/ACS. Methods. We compute the photometric center, a surface brightness profile, and a velocity-dispersion profile. We run analytic spherical and axisymmetric Jeans models with different central black-hole masses, anisotropy, mass-to-light ratio, and inclination. Further, we compare the data to a grid of N-body simulations without tidal field. Additionally, we use one N-body simulation to check the results of the spherical Jeans models for the total cluster mass. Results. Both the Jeans models and the N-body simulations favor the presence of a central black hole in NGC 5286 and our detection is at the 1-to 1.5-σ level. From the spherical Jeans models we obtain a best fit with black-hole mass M = (1.5 ± 1.0) × 10 M. The error is the 68% confidence limit from Monte Carlo simulations. Axisymmetric models give a consistent result. The best fitting N-body model is found with a black hole of 0.9% of the total cluster mass (4.38 ± 0.18) × 10 M , which results in an IMBH mass of M = (3.9 ± 2.0) × 10 M. Jeans models give values for the total cluster mass that are lower by up to 34% due to a lower value of M/L. Our test of the Jeans models with N-body simulation data shows that the discrepancy in the total cluster mass has two reasons: The influence of a radially varying M/L profile, and underestimation of the velocity dispersion as the measurements are limited to bright stars, which have lower velocities than fainter stars. We conclude that detection of IMBHs in Galactic globular clusters remains a challenging task unless their mass fractions are above those found for SMBHs in nearby galaxies.

Intermediate-mass black holes in globular clusters: observations and simulations

The study of intermediate-mass black holes (IMBHs) is a young and promising field of research. If IMBHs exist, they could explain the rapid growth of supermassive black holes by acting as seeds in the early stage of galaxy formation. Formed by runaway collisions of massive stars in young and dense stellar clusters, intermediate-mass black holes could still be present in the centers of globular clusters, today. Our group investigated the presence of intermediate-mass black holes for a sample of 10 Galactic globular clusters. We measured the inner kinematic profiles with integral-field spectroscopy and determined masses or upper limits of central black holes in each cluster. In combination with literature data we further studied the positions of our results on known black-hole scaling relations (such as M - σ) and found a similar but flatter correlation for IMBHs. Applying cluster evolution codes, the change in the slope could be explained with the stellar mass loss occurring in clusters in a tidal field over its life time. Furthermore, we present results from several numerical simulations on the topic of IMBHs and integral field units (IFUs). We ran N-body simulations of globular clusters containing IMBHs in a tidal field and studied their effects on mass-loss rates and remnant fractions and showed that an IMBH in the center prevents core collapse and ejects massive objects more rapidly. These simulations were further used to simulate IFU data cubes. For the specific case of NGC 6388 we simulated two different IFU techniques and found that velocity dispersion measurements from individual velocities are strongly biased towards lower values due to blends of neighboring stars and background light. In addition, we use the Astrophysical Multipurpose Software Environment (AMUSE) to combine gravitational physics, stellar evolution and hydrodynamics to simulate the accretion of stellar winds onto a black hole.

M • -σ Relation for intermediate-mass black holes in globular clusters

Context. For galaxies hosting supermassive black holes (SMBHs), it has been observed that the mass of the central black hole (M) tightly correlates with the effective or central velocity dispersion (σ) of the host galaxy. The origin of this M-σ scaling relation is assumed to lie in the merging history of the galaxies, but many open questions about its origin and the behavior in different mass ranges still need to be addressed. Aims. The goal of this work is to study the black-hole scaling relations for low black-hole masses, where the regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is entered. Methods. We collected all existing reports of dynamical black-hole measurements in GCs, providing black-hole masses or upper limits for 14 candidates. We plotted the black-hole masses versus different cluster parameters including total mass, velocity dispersion, concentration, and half-mass radius. We searched for trends and tested the correlations to quantify their significance using a set of different statistical approaches. For correlations with a high significance we performed a linear fit, accounting for uncertainties and upper limits. Results. We find a clear correlation between the mass of the IMBH and the velocity dispersion of the GC. As expected, the total mass of the GC then also correlates with the mass of the IMBH. While the slope of the M-σ correlation differs strongly from the one observed for SMBHs, the other scaling relations M-M, and M-L are similar to the correlations in galaxies. Significant correlations of black-hole mass with other cluster properties were not found in the present sample.

The M_BH - sigma relation for intermediate-mass black holes in globular clusters

Context. For galaxies hosting supermassive black holes (SMBHs), it has been observed that the mass of the central black hole (M) tightly correlates with the effective or central velocity dispersion (σ) of the host galaxy. The origin of this M-σ scaling relation is assumed to lie in the merging history of the galaxies, but many open questions about its origin and the behavior in different mass ranges still need to be addressed. Aims. The goal of this work is to study the black-hole scaling relations for low black-hole masses, where the regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is entered. Methods. We collected all existing reports of dynamical black-hole measurements in GCs, providing black-hole masses or upper limits for 14 candidates. We plotted the black-hole masses versus different cluster parameters including total mass, velocity dispersion, concentration, and half-mass radius. We searched for trends and tested the correlations to quantify their significance using a set of different statistical approaches. For correlations with a high significance we performed a linear fit, accounting for uncertainties and upper limits. Results. We find a clear correlation between the mass of the IMBH and the velocity dispersion of the GC. As expected, the total mass of the GC then also correlates with the mass of the IMBH. While the slope of the M-σ correlation differs strongly from the one observed for SMBHs, the other scaling relations M-M, and M-L are similar to the correlations in galaxies. Significant correlations of black-hole mass with other cluster properties were not found in the present sample.

Structure of the nuclear stellar cluster of the Milky Way galaxy

Nuclear star clusters are unambiguously detected in about 50–70% of spiral and spheroidal galaxies. They have typical half-light radii of 2–5 pc, dynamical mass ranging from 10 6 – 10 7 M ⊙ , are brighter than globular clusters, and obey similar scaling relations with host galaxies as supermassive black holes. The nuclear stellar cluster (NSC) which surrounds Sgr A*, the SMBH at the center of our galaxy, is the nearest nuclear cluster to us, and can be resolved to scales of milliparsecs. The strong and highly variable extinction towards the Galactic center makes it very hard to infer the intrinsic properties of the NSC (structure and size). We attempt a new way to infer its properties by using Spitzer MIR images in a wavelength range 3–8 μm where the extinction is at a minimum, and the NSC clearly stands out as a separate structure. We present results from our analysis, including extinction-corrected images and surface brightness profiles of the central few hundred parsecs of the Milky Way.

M dot-sigma relation for intermediate-mass black holes in globular clusters

Context. For galaxies hosting supermassive black holes (SMBHs), it has been observed that the mass of the central black hole (M) tightly correlates with the effective or central velocity dispersion (σ) of the host galaxy. The origin of this M-σ scaling relation is assumed to lie in the merging history of the galaxies, but many open questions about its origin and the behavior in different mass ranges still need to be addressed. Aims. The goal of this work is to study the black-hole scaling relations for low black-hole masses, where the regime of intermediate-mass black holes (IMBHs) in globular clusters (GCs) is entered. Methods. We collected all existing reports of dynamical black-hole measurements in GCs, providing black-hole masses or upper limits for 14 candidates. We plotted the black-hole masses versus different cluster parameters including total mass, velocity dispersion, concentration, and half-mass radius. We searched for trends and tested the correlations to quantify their significance using a set of different statistical approaches. For correlations with a high significance we performed a linear fit, accounting for uncertainties and upper limits. Results. We find a clear correlation between the mass of the IMBH and the velocity dispersion of the GC. As expected, the total mass of the GC then also correlates with the mass of the IMBH. While the slope of the M-σ correlation differs strongly from the one observed for SMBHs, the other scaling relations M-M, and M-L are similar to the correlations in galaxies. Significant correlations of black-hole mass with other cluster properties were not found in the present sample.