Nadine Neumayer

The NGC 404 Nucleus: Star Cluster and Possible Intermediate-mass Black Hole

We examine the nuclear morphology, kinematics, and stellar populations in nearby S0 galaxy NGC 404 using a combination of adaptive optics assisted near-IR integral-field spectroscopy, optical spectroscopy, and Hubble Space Telescope imaging. These observations enable study of the NGC 404 nucleus at a level of detail possible only in the nearest galaxies. The surface brightness profile suggests the presence of three components: a bulge, a nuclear star cluster (NSC), and a central light excess within the cluster at radii < 3 pc. These components have distinct kinematics with modest rotation seen in the NSC and counter-rotation seen in the central excess. Molecular hydrogen emission traces a disk with rotation nearly orthogonal to that of the stars. The stellar populations of the three components are also distinct, with half of the mass of the NSC having ages of ~1 Gyr (perhaps resulting from a galaxy merger), while the bulge is dominated by much older stars. Dynamical modeling of the stellar kinematics gives a total NSC mass of 1.1 × 107 M ☉. Dynamical detection of a possible intermediate-mass black hole (BH) is hindered by uncertainties in the central stellar mass profile. Assuming a constant mass-to-light ratio, the stellar dynamical modeling suggests a BH mass of <1 × 105 M ☉, while the molecular hydrogen gas kinematics are best fitted by a BH with a mass of 4.5+3.5 –2.0 × 105 M ☉. Unresolved and possibly variable dust emission in the near-infrared and active galactic nucleus-like molecular hydrogen emission-line ratios do suggest the presence of an accreting BH in this nearby LINER galaxy.

Resolving the innermost parsec of Centaurus A at mid-infrared wavelengths ?

Context. To reveal the origin of mid-infrared radiation from the core of Centaurus A, we carried out interferometric observations with the MID-infrared Interferometer (MIDI) at ESO’s VLTI telescope array. Aims. Observations were obtained with four baselines between unit telescopes of the VLTI, two of them roughly along the radio axis and two orthogonal to it. The interferometric measurements are spectrally resolved with λ/∆λ = 30 in the wavelength range 8 to 13 µm. Their resolution reaches 15 mas at the shortest wavelengths. Supplementary observations were obtained in the near-infrared with the adaptive optics instrument NACO, and at mm wavelengths with SEST and JCMT. Methods. The mid-infrared emission from the core of Centaurus A is dominated by an unresolved point source (<10 mas). Observations with baselines orientated perpendicular to the radio jet reveal an extended component which can be interpreted as a geometrically thin, dusty disk, the axis of which is aligned with the radio jet. Its diameter is about 0.6 pc. It contributes between 20% (at λ � 8 µm) and 40% (at λ � 13 µm) to the nuclear flux from Centaurus A and contains dust at about 240 K. We argue, that the unresolved emission is dominated by a synchrotron source. Its overall spectrum is characterized by an Fν ∼ ν −0.36 power-law which cuts off exponentially towards high frequencies at νc = 8 × 10 13 Hz and becomes optically thick at ν<ν 1 � 45 GHz. Results. Based on a Synchrotron Self Compton (SSC) interpretation for the γ-ray emission, we find a magnetic field strength of 26 µT and a maximum energy of relativistic electrons of γc = Ec/mec 2 = 8500. Near γc, the acceleration time scale is τacc = 4 days, in good agreement with the fastest flux variations, observed at X-ray frequencies. Our SSC model argues for a Doppler factor δ � 1w hich – together with the jet-counter jet ratio of the radio jets on parsec scale – results in an upper limit for the bulk Lorentz factor Γjet < 2.5, at variance with the concept of a “mis-directed BL Lac object”. Conclusions. We estimate a thermal luminosity of the core, Pth � 1.3 × 10 34 W = 1.5 × 10 −4 × LEdd, intermediate between the values for highly efficiently accreting AGN (e.g. Seyfert galaxies) and those of typical FR I radio galaxies. This luminosity, which is predominantly released in X-rays, is most likely generated in an Advection Dominated Accretion Flow (ADAF) and seems just sufficient to heat the dusty disk.

The mass of the black hole in Centaurus A from SINFONI AO-assisted integral-field observations of stellar kinematics

We present a determination of the mass of the supermassive black hole (BH) and the nuclear stellar orbital distribution of the elliptical galaxy Centaurus A (Cen A) (NGC 5128) using high-resolution integral-field observations of the stellar kinematics. The observations were obtained with SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared) at the European Southern Observatory Very Large Telescope in the near-infrared (IR) (K band), using adaptive optics (AO) to correct for the blurring effect of the Earths atmosphere. The data have a spatial resolution of 0.17 arcsec full width at half-maximum and high signal-to-noise ratios (S/N) ≥ 80 per spectral pixel so that the shape of the stellar line-of-sight velocity distribution can be reliably extracted. We detect clear low-level stellar rotation, which is counter-rotating with respect to the gas. We fit axisymmetric three-integral dynamical models to the data to determine the best-fitting values for the BH mass M BH = (5.5 ± 3.0) x 10 7 M ⊙ (3σ errors) and (M/L) K = (0.65 ± 0.15) in solar units. These values are in excellent agreement with previous determinations from the gas kinematics, and in particular with our own published value, extracted from the same data. This provides one of the cleanest gas versus stars comparisons of M BH determination, due to the use of integral-field data for both dynamical tracers and due to a very well-resolved BH sphere of influence R BH ≈ 0.70 arcsec. We derive an accurate profile of the orbital anisotropy, and carefully test its reliability using spherical Jeans models with radially varying anisotropy. We find an increase in the tangential anisotropy close to the BH, but the spatial extent of this effect seems restricted to the size of R BH instead of that of R b ≈ 3.9 arcsec of the core in the surface brightness profile, contrary to detailed predictions of current simulations of the binary BH scouring mechanism. More realistic simulations would be required to draw conclusions from this observation.

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.

On central black holes in ultra-compact dwarf galaxies

Context. The dynamical mass-to-light (M/L) ratios of massive ultra-compact dwarf galaxies (UCDs) are about 50% higher than predicted by stellar population models. Aims. Here we investigate the possibility that these apparently elevated M/L ratios of UCDs are caused by a central black hole (BH) that heats up the internal motion of stars. We focus on a sample of ∼50 extragalactic UCDs from the literature for which velocity dispersions and structural parameters have been measured. Methods. To be self-consistent in our BH mass estimates, we first redetermine the dynamical masses and M/L ratios of our sample UCDs, using up-to-date distance moduli and a consistent treatment of aperture and seeing effects. On average, the homogeneously redetermined dynamical mass and M/L ratios agree to within 5% with previous literature results. We calculate the ratio Ψ= (M/L)dyn/(M/L)pop between the dynamical and the stellar population M/L for an assumed age of 13 Gyr. Ψ > 1 indicates an elevated dynamical M/L ratio, suggesting dark mass on top of a canonical stellar population of old age. For all UCDs with Ψ > 1w e estimate the mass of a hypothetical central black hole needed to reproduce the observed integrated velocity dispersion Results. Massive UCDs (M > 10 7 M� )h ave an average Ψ= 1.7 ± 0.2, implying notable amounts of dark mass in them. We find that, on average, central BH masses of 10–15% of the UCD mass can explain these elevated dynamical M/L ratios. The implied BH masses

Are nuclear star clusters the precursors of massive black holes

We present new upper limits for black hole masses in extremely late type spiral galaxies. We confirm that this class of galaxies has black holes with masses less than 106M⊙, if any. We also derive new upper limits for nuclear star cluster masses in massive galaxies with previously determined black hole masses. We use the newly derived upper limits and a literature compilation to study the low mass end of the global-to-nucleus relations. We find the following. (1) The MBH-σ relation cannot flatten at low masses, but may steepen. (2) The MBH-Mbulge relation may well flatten in contrast. (3) The MBH-Sersic n relation is able to account for the large scatter in black hole masses in low-mass disk galaxies. Outliers in the MBH-Sersic n relation seem to be dwarf elliptical galaxies. When plotting MBH versus MNC we find three different regimes: (a) nuclear cluster dominated nuclei, (b) a transition region, and (c) black hole-dominated nuclei. This is consistent with the picture, in which black holes form inside nuclear clusters with a very low-mass fraction. They subsequently grow much faster than the nuclear cluster, destroying it when the ratio MBH/MNC grows above 100. Nuclear star clusters may thus be the precursors of massive black holes in galaxy nuclei.

Masses and scaling relations for nuclear star clusters, and their co-existence with central black holes

Galactic nuclei typically host either a Nuclear Star Cluster (NSC, prevalent in galaxies with masses

Measuring the Mass of the Central Black Hole in the Bulgeless Galaxy NGC 4395 from Gas Dynamical Modeling

\lesssim 10^{10}M_\odot

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

) or a Massive Black Hole (MBH, common in galaxies with masses