THE VELOCITY DISPERSION PROFILE OF NGC 6388 FROM RESOLVED-STAR SPECTROSCOPY: NO EVIDENCE OF A CENTRAL CUSP AND NEW CONSTRAINTS ON THE BLACK HOLE MASS (original) (raw)
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Kinematic signature of an intermediate-mass black hole in the globular cluster NGC 6388
Astronomy & Astrophysics, 2011
Context. Intermediate-mass black holes (IMBHs) are of interest in a wide range of astrophysical fields. In particular, the possibility of finding them at the centers of globular clusters has recently drawn attention. IMBHs became detectable since the quality of observational data sets, particularly those obtained with HST and with high resolution ground based spectrographs, advanced to the point where it is possible to measure velocity dispersions at a spatial resolution comparable to the size of the gravitational sphere of influence for plausible IMBH masses. Aims. We present results from ground based VLT/FLAMES spectroscopy in combination with HST data for the globular cluster NGC 6388. The aim of this work is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and to compare the results with the expected value predicted by the M • − σ scaling relation. Methods. The spectroscopic data, containing integral field unit measurements, provide kinematic signatures in the center of the cluster while the photometric data give information of the stellar density. Together, these data sets are compared to dynamical models and present evidence of an additional compact dark mass at the center: a black hole. Results. Using analytical Jeans models in combination with various Monte Carlo simulations to estimate the errors, we derive (with 68% confidence limits) a best fit black-hole mass of (17 ± 9) × 10 3 M and a global mass-to-light ratio of M/L V = (1.6 ± 0.3) M /L .
The Open Cluster NGC 7789. I. Radial Velocities for Giant Stars
Publications of the Astronomical Society of the Pacific, 1998
A total of 597 radial-velocity observations for 112 stars in the ∼1.6 Gyr old open cluster NGC 7789 have been obtained since 1979 with the radial velocity spectrometer at the Dominion Astrophysical Observatory. The mean cluster radial velocity is −54.9 ± 0.12 km s −1 and the dispersion is 0.86 km s −1 , from 50 constant-velocity stars selected as members from this radial-velocity study and the proper motion study of McNamara and Solomon (1981). Twenty-five stars (32%) among 78 members are possible radial-velocity variable stars, but no orbits are determined because of the sparse sampling. Seventeen stars are radialvelocity non-members, while membership estimates of six stars are uncertain. There is a hint that the observed velocity dispersion falls off at large radius. This may due to the inclusion of long-period binaries preferentially in the central area of the cluster. The known radial-velocity variables also seem to be more concentrated toward the center than members with constant velocity. Although this is significant at only the 85% level, when combined with similar result of Raboud & Mermilliod (1994) for three other clusters, the data strongly support the conclusion that mass segregation is being detected.
Astronomical Journal, 2010
New radial velocity measurements for previously known and newly confirmed globular clusters (GCs) in the nearby massive galaxy NGC 5128 are presented. We have obtained spectroscopy from LDSS-2/Magellan, VIMOS/VLT, and Hydra/CTIO from which we have measured the radial velocities of 218 known, and identified 155 new, GCs. The current sample of confirmed GCs in NGC 5128 is now 605 with 564 of these having radial velocity measurements. We have performed a new kinematic analysis of the GC system that extends out to 45 arcmin in galactocentric radius. We have examined the systemic velocity, projected rotation amplitude and axis, and the projected velocity dispersion of the GCs as functions of galactocentric distance and metallicity. Our results indicate that the metal-poor GCs have a very mild rotation signature of (26 pm 15) km/s. The metal-rich GCs are rotating with a higher, though still small signature of (43 pm 15) km/s around the isophotal major axis of NGC 5128 within 15 arcmin. Their velocity dispersions are consistent within the uncertainties and the profiles appear flat or declining within 20 arcmin. We note the small sample of metal-rich GCs with ages less than 5 Gyr in the literature appear to have different kinematic properties than the old, metal-rich GC subpopulation. The mass and mass-to-light ratios have also been estimated using the GCs as tracer particles for NGC 5128. Out to a distance of 20 arcmin, we have obtained a mass of (5.9 pm 2.0) x 10^(11) M_(sun) and a mass-to-light ratio in the B-band of 16 M_(sun)/L_(B,sun). Combined with previous work on the ages and metallicities of its GCs, as well as properties of its stellar halo, our findings suggest NGC 5128 formed via hierarchical merging over other methods of formation, such as major merging at late times.
Astrophysical Journal, 2007
We have used a combination of high resolution (HST ACS-HRC, ACS-WFC, and WFPC2) and wide-field (ESO-WFI) observations of the galactic globular cluster NGC 6388 to derive its center of gravity, projected density profile, and central surface brightness profile. While the overall projected profiles are well fit by a King model with intermediate concentration (c=1.8) and sizable core radius (rc=7"), a significant power law (with slope \alpha=-0.2) deviation from a flat core behavior has been detected within the inner 1 arcsecond. These properties suggest the presence of a central intermediate mass black hole. The observed profiles are well reproduced by a multi-mass isotropic, spherical model including a black hole with a mass of ~5.7x10^3 Msol.
Gas Kinematics and the Black Hole Mass at the Center of the Radio Galaxy NGC 4335
The Astronomical Journal, 2002
We investigate the kinematics of the central gas disk of the radio-loud elliptical galaxy NGC 4335, derived from HST/STIS long-slit spectroscopic observations of Hα+[N II] along 3 parallel slit positions. The observed mean velocities are consistent with a rotating thin disk. We model the gas disk in the customary way, taking into account the combined potential of the galaxy and a putative black hole with mass M • , as well as the influence on the observed kinematics of the point spread function and finite slit width. This sets a 3σ upper limit of 10 8 M ⊙ on M • . The velocity dispersion at r 0.5 ′′ is in excess of that predicted by the thin rotating disk model. This does not invalidate the model, if the excess dispersion is caused by localized turbulent motion in addition to bulk circular rotation. However, if instead the dispersion is caused by the BH potential then the thin disk model provides an underestimate of M • . A BH mass M • ∼ 6 × 10 8 M ⊙ is inferred by modeling the central gas dispersion as due to an isotropic spherical distribution of collisionless gas cloudlets. The stellar kinematics for NGC 4335 are derived from a ground-based (WHT/ISIS) long-slit observation along the galaxy major axis. A two-integral model of the stellar dynamics yields M • 3×10 9 M ⊙ . However, there is reason to believe that this model overestimates M • .
The Space Motion of the Globular Cluster NGC 6397
The Astrophysical Journal, 2007
As a by-product of high-precision, ultradeep stellar photometry in the Galactic globular cluster NGC 6397 with the Hubble Space Telescope, we are able to measure a large population of background galaxies whose images are nearly pointlike. These provide an extragalactic reference frame of unprecedented accuracy, relative to which we measure the most accurate absolute proper motion ever determined for a globular cluster. We find mas yr Ϫ1 and mas yr Ϫ1 . We note that the formal statistical errors m cos d p 3.56 ע 0.04 m p Ϫ17.34 ע 0.04 a d quoted for the proper motion of NGC 6397 do not include possible unavoidable sources of systematic errors, such as cluster rotation. These are very unlikely to exceed a few percent. We use this new proper motion to calculate NGC 6397's UVW space velocity and its orbit around the Milky Way and find that the cluster has made frequent passages through the Galactic disk.
Dynamical Measurements of Black Hole Masses in Four Brightest Cluster Galaxies at 100 Mpc
2012
We present stellar kinematics and orbit superposition models for the central regions of four brightest cluster galaxies, based upon integral-field spectroscopy at Gemini, Keck, and McDonald Observatories. Our integral-field data span radii from <100 pc to tens of kiloparsecs, comparable to the effective radius of each galaxy. We report black hole masses, M • , of 2.1 +1.6 −1.6 × 10 10 M for NGC 4889, 9.7 +3.0 −2.5 × 10 9 M for NGC 3842, and 1.3 +0.5 −0.4 × 10 9 M for NGC 7768, with errors representing 68% confidence limits. For NGC 2832, we report an upper limit of M • < 9.0 × 10 9 M . Our models of each galaxy include a dark matter halo, and we have tested the dependence of M • on the model dark matter profile. Stellar orbits near the center of each galaxy are tangentially biased, on comparable spatial scales to the galaxies' photometric cores. We find possible photometric and kinematic evidence for an eccentric torus of stars in NGC 4889, with a radius of nearly 1 kpc. We compare our measurements of M • to the predicted black hole masses from various fits to the relations between M • and stellar velocity dispersion (σ ), luminosity (L), or stellar mass (M ). Still, the black holes in NGC 4889 and NGC 3842 are significantly more massive than all σ -based predictions and most L-based predictions. The black hole in NGC 7768 is consistent with a broader range of predictions.
The Black Hole Mass in the Brightest Cluster Galaxy NGC 6086
Astrophysical Journal - ASTROPHYS J, 2011
We present the first direct measurement of the central black hole mass, M •, in NGC 6086, the Brightest Cluster Galaxy (BCG) in A2162. Our investigation demonstrates for the first time that stellar-dynamical measurements of M • in BCGs are possible beyond the nearest few galaxy clusters. We observed NGC 6086 with laser guide star adaptive optics and the integral-field spectrograph (IFS) OSIRIS at the W. M. Keck Observatory and with the seeing-limited IFS GMOS-N at Gemini Observatory North. We combined the IFS data sets with existing major-axis kinematics and used axisymmetric stellar orbit models to determine M • and the R-band stellar mass-to-light ratio, M sstarf/LR . We find M • = 3.6+1.7 -1.1 × 109 M sun and M sstarf/L R = 4.6+0.3 -0.7 M sun L sun -1 (68% confidence) from models using the most massive dark matter halo allowed within the gravitational potential of the host cluster. Models fitting only IFS data confirm M • ~ 3 × 109 M sun and M sstarf/LR ~ 4 M sun L sun -1, with w...
Kinematics of a globular cluster with an extended profile: NGC 5694
Monthly Notices of the Royal Astronomical Society, 2014
We present a study of the kinematics of the remote globular cluster NGC 5694 based on GIRAFFE@VLT medium resolution spectra. A sample of 165 individual stars selected to lie on the Red Giant Branch in the cluster Color Magnitude Diagram was considered. Using radial velocity and metallicity from Calcium triplet, we were able to select 83 bona-fide cluster members. The addition of six previously known members leads to a total sample of 89 cluster giants with typical uncertainties 1.0 km/s in their radial velocity estimates. The sample covers a wide range of projected distances from the cluster center, from ∼ 0.2 to 6.5 23 half-light radii (r h). We find only very weak rotation, as typical of metal-poor globular clusters. The velocity dispersion gently declines from a central value of σ = 6.1 km/s to σ 2.5 km/s at ∼ 2 7.1r h , then it remainins flat out to the next (and last) measured point of the dispersion profile, at ∼ 4 14.0r h , at odds with the predictions of isotropic King models. We show that both isotropic single-mass non-collisional models and multi-mass anisotropic models can reproduce the observed surface brightness and velocity dispersion profiles.
Dynamics of the NGC 4636 globular cluster system
Astronomy & Astrophysics, 2006
Context. We present the first dynamical study of the globular cluster system of NGC 4636. It is the southernmost giant elliptical galaxy of the Virgo cluster and is claimed to be extremely dark matter dominated, according to X-ray observations. Aims. Globular clusters are used as dynamical tracers to investigate, by stellar dynamical means, the dark matter content of this galaxy. Methods. Several hundred medium resolution spectra were acquired at the VLT with FORS 2/MXU. We obtained velocities for 174 globular clusters in the radial range 0. 90 < R < 15. 5, or 0.5−9 R e in units of effective radius. Assuming a distance of 15 Mpc, the clusters are found at projected galactocentric distances in the range 4 to 70 kpc, the overwhelming majority within 30 kpc. The measured line-of-sight velocity dispersions are compared to Jeans-models. Results. We find some indication of a rotation of the red (metal-rich) clusters about the minor axis. Out to a radius of 30 kpc, we find a roughly constant projected velocity dispersion for the blue clusters of σ ≈ 200 km s −1. The red clusters are found to have a distinctly different behavior: at a radius of about 3 , the velocity dispersion drops by ∼50 km s −1 to about 170 km s −1 , which then remains constant out to a radius of 7. The cause might be the steepening of the number density profile at ∼3 observed for the red clusters. Using only the blue clusters as dynamical tracers, we perform Jeans-analyses for different assumptions of the orbital anisotropy. Enforcing the model dark halos to be of the NFW type, we determine their structural parameters. Depending on the anisotropy and the adopted M/L-values, we find that the dark matter fraction within one effective radius can vary between 20% and 50%, with most a probable range between 20% and 30%. The ambiguity of the velocity dispersion in the outermost bin is a main source of uncertainty. A comparison with cosmological N-body simulations reveals no striking disagreement. Conclusions. Although the dark halo mass still cannot be strongly constrained, NGC 4636 does not seem to be extremely dark matter dominated. The derived circular velocities are also consistent with Modified Newtonian Dynamics.