ω Centauri: A MUSE discovery of a counter-rotating core

ω Centauri is considered the most massive globular cluster of the Milky Way and likely the former nuclear star cluster of a galaxy accreted by the Milky Way. It is speculated to contain an intermediate-mass black hole (IMBH) from several dynamical models. However, uncertainties regarding the location of the cluster center or the retention of stellar remnants limit the robustness of the IMBH detections reported so far. In this paper, we derive and study the stellar kinematics from the highest-resolution spectroscopic data yet, using the Multi Unit Spectroscopic Explorer (MUSE) in the narrow field mode (NFM) and wide field mode (WFM). Our exceptional data near the center reveal for the first time that stars within the inner 20" (∼0.5 pc) counter-rotate relative to the bulk rotation of the cluster. Using this dataset, we measure the rotation and line-of-sight velocity dispersion (LOSVD) profile out to 120′′ with different centers proposed in the literature. We find that the velocity dispersion profiles using different centers match well with those previously published. Based on the counter--rotation, we determine a kinematic center and look for any signs of an IMBH using the high-velocity stars close to the center. We do not find any significant outliers >60 km/s within the central 20′′, consistent with no IMBH being present at the center of ω Centauri. A detailed analysis of Jeans' modeling of the putative IMBH will be presented in the next paper of the series

oMEGACat I: MUSE spectroscopy of 300,000 stars within the half-light radius of ω\omega Centauri

Omega Centauri ($\omega$ Cen) is the most massive globular cluster of the Milky Way and has been the focus of many studies that reveal the complexity of its stellar populations and kinematics. However, most previous studies have used photometric and spectroscopic datasets with limited spatial or magnitude coverage, while we aim to investigate it having full spatial coverage out to its half-light radius and stars ranging from the main sequence to the tip of the red giant branch. This is the first paper in a new survey of $\omega$ Cen that combines uniform imaging and spectroscopic data out to its half-light radius to study its stellar populations, kinematics, and formation history. In this paper, we present an unprecedented MUSE spectroscopic dataset combining 87 new MUSE pointings with previous observations collected from guaranteed time observations. We extract spectra of more than 300,000 stars reaching more than two magnitudes below the main sequence turn-off. We use these spectra to derive metallicity and line-of-sight velocity measurements and determine robust uncertainties on these quantities using repeat measurements. Applying quality cuts we achieve signal-to-noise ratios of 16.47/73.51 and mean metallicity errors of 0.174/0.031 dex for the main sequence stars (18 mag $\rm < mag_{F625W}<$22 mag) and red giant branch stars (16 mag $<\rm mag_{F625W}<$10 mag), respectively. We correct the metallicities for atomic diffusion and identify foreground stars. This massive spectroscopic dataset will enable future studies that will transform our understanding of $\omega$ Cen, allowing us to investigate the stellar populations, ages, and kinematics in great detail.Comment: 27 pages, 18 figures, 3 tables, accepted for publication in ApJ, the catalog will be available in the online material of the published articl

Kinematic differences between multiple populations in Galactic globular clusters

Aims. The formation process of multiple populations in globular clusters is still up for debate. These populations are characterized by different light-element abundances. Kinematic differences between the populations are particularly interesting in this respect, because they allow us to distinguish between single-epoch formation scenarios and multi-epoch formation scenarios. We derive rotation and dispersion profiles for 25 globular clusters and aim to find kinematic differences between multiple populations in 21 of them to constrain the formation process. Methods. We split red-giant branch (RGB) stars in each cluster into three populations (P1, P2, P3) for the type-II clusters and two populations (P1 and P2) otherwise using Hubble photometry. We derive the global rotation and dispersion profiles for each cluster by using all stars with radial velocity measurements obtained from MUSE spectroscopy. We also derive these profiles for the individual populations of each cluster. Based on the rotation and dispersion profiles, we calculate the rotation strength in terms of ordered-overrandom motion (v/σ)HL evaluated at the half-light radius of the cluster. We then consistently analyse all clusters for differences in the rotation strength of their populations. Results. We detect rotation in all but four clusters. For NGC 104, NGC 1851, NGC 2808, NGC 5286, NGC 5904, NGC 6093, NGC 6388, NGC 6541, NGC 7078 and NGC 7089 we also detect rotation for P1 and/or P2 stars. For NGC 2808, NGC 6093 and NGC 7078 we find differences in (v/σ)HL between P1 and P2 that are larger than 1σ. Whereas we find that P2 rotates faster than P1 for NGC 6093 and NGC 7078, the opposite is true for NGC 2808. However, even for these three clusters the differences are still of low significance. We find that the strength of rotation of a cluster generally scales with its median relaxation time. For P1 and P2 the corresponding relation is very weak at best. We observe no correlation between the difference in rotation strength between P1 and P2 and cluster relaxation time. The stellar radial velocities derived from MUSE data that this analysis is based on are made publicly available

oMEGACat I: MUSE spectroscopy of 300,000 stars within the half-light radius of ω Centauri

Omega Centauri (ω Cen) is the most massive globular cluster of the Milky Way and has been the focus of many studies that reveal the complexity of its stellar populations and kinematics. However, most previous studies have used photometric and spectroscopic datasets with limited spatial or magnitude coverage, while we aim to investigate it having full spatial coverage out to its half-light radius and stars ranging from the main sequence to the tip of the red giant branch. This is the first paper in a new survey of ω Cen that combines uniform imaging and spectroscopic data out to its half-light radius to study its stellar populations, kinematics, and formation history. In this paper, we present an unprecedented MUSE spectroscopic dataset combining 87 new MUSE pointings with previous observations collected from guaranteed time observations. We extract spectra of more than 300,000 stars reaching more than two magnitudes below the main sequence turn-off. We use these spectra to derive metallicity and line-of-sight velocity measurements and determine robust uncertainties on these quantities using repeat measurements. Applying quality cuts we achieve signal-to-noise ratios of 16.47/73.51 and mean metallicity errors of 0.174/0.031 dex for the main sequence stars (18 mag < magF625W < 22 mag) and red giant branch stars (16 mag < magF625W <10 mag), respectively. We correct the metallicities for atomic diffusion and identify foreground stars. This massive spectroscopic dataset will enable future studies that will transform our understanding of ω Cen, allowing us to investigate the stellar populations, ages, and kinematics in great detail

Measuring Masses and Densities of Nuclear Star Clusters in the Nearest Galaxies

Galaxy nuclei contain both black holes (BHs) and nuclear star clusters (NSCs). Low-mass galaxies are typically dominated by NSCs. Measurements of NSC mass density suggest a scaling relation exists between them and their host galaxy mass, but very few NSC masses are dynamically estimated. We will present density profiles and dynamical masses for 29 nearby galaxies within 10 Mpc based on HST imaging and accompanying infrared spectroscopy from GNIRS and XSHOOTER. We quantify the NSC profiles by determining the effective radius and Sersic index. These profiles are then combined with their central velocity dispersions, derived using CO bandheads, to get dynamical mass density estimates for the NSCs and upper limits on any BHs. These measurements increase the number of dynamical measurements of NSCs by a factor of 3. The resulting scaling relations can be used to disentangle the formation scenarios of the NSCs. In addition, the measurement of central densities will help determine the expected rate of tidal disruption events by central BHs in galactic nuclei

Detection of enhanced central mass-to-light ratios in low-mass early-type galaxies: Evidence for black holes?

We present dynamical measurements of the central mass-To-light ratio (M/L) of a sample of 27 low-mass early-Type galaxies. We consider all galaxies with 9.7 &lt; log( &lt; 10.5 in our analysis, selecting out galaxies with available high-resolution Hubble Space Telescope (HST) data, and eliminating galaxies with significant central color gradients or obvious dust features. We use the HST images to derive mass models for these galaxies and combine these with the central velocity dispersion values from data to obtain a central dynamical M/L estimate. These central dynamical are higher than dynamical derived at larger radii and stellar population estimates of the galaxy centers in ∼80% of galaxies, with a median enhancement of ∼14% and a statistical significance of 3.3σ. We show that the enhancement in the central M/L is best described either by the presence of black holes in these galaxies or by radial initial mass function variations. Assuming a black hole model, we derive black hole masses for the sample of galaxies. In two galaxies, NGC 4458 and NGC 4660, the data suggest significantly overmassive black holes, while in most others only upper limits are obtained. We also show that the level of M/L enhancements we see in these early-Type galaxy nuclei are consistent with the larger enhancements seen in ultracompact dwarf galaxies (UCDs), supporting the scenario where massive UCDs are created by stripping galaxies of these masses

First direct dynamical detection of a dual supermassive black hole system at sub-kiloparsec separation

International audienceWe investigated whether the two recently discovered nuclei in NGC 7727 both host a super-massive black hole (SMBH). We used the high spatial resolution mode of the integral-field spectrograph MUSE on the VLT in adaptive optics mode to resolve the stellar kinematics within the sphere of influence of both putative black holes. We combined the kinematic data with an HST-based mass model and used Jeans models to measure their SMBH mass. We report the discovery of a dual SMBH system in NGC 7727. We detect a SMBH in the photometric center of the galaxy in Nucleus 1, with a mass of MSMBH = 1.54−0.15+0.18 × 108 M⊙. In the second nucleus, which is 500 pc offset from the main nucleus, we also find a clear signal for a SMBH with a mass of MBH = 6.33−1.40+3.32 × 106 M⊙. Both SMBHs are detected at high significance. The off-axis nature of Nucleus 2 makes modeling the system challenging; however, a number of robustness tests suggest that a black hole is required to explain the observed kinematics. The SMBH in the offset Nucleus 2 makes up 3.0% of its total mass, which means its SMBH is over-massive compared to the MBH − MBulge scaling relation. This confirms it as the surviving nuclear star cluster of a galaxy that has merged with NGC 7727. This discovery is the first dynamically confirmed dual SMBH system with a projected separation of less than a kiloparsec and the nearest dynamically confirmed dual SMBH at a distance of 27.4 Mpc. The second Nucleus is in an advanced state of inspiral, and it will eventually result in a 1:24 mass ratio SMBH merger. Optical emission lines suggest Nucleus 2 is a Seyfert galaxy, making it a low-luminosity Active Galactic Nuclei. There are likely many more quiescent SMBHs as well as dual SMBH pairs in the local Universe that have been missed by surveys that focus on bright accretion signatures

Detection of A ∼100,000 M Black Hole in M31’s Most Massive Globular Cluster: A Tidally Stripped Nucleus

We investigate the presence of a central black hole (BH) in B023-G078, M31's most massive globular cluster. We present high-resolution, adaptive-optics assisted, integral-field spectroscopic kinematics from Gemini/NIFS that shows a strong rotation ($\sim$20 km/s) and a velocity dispersion rise towards the center (37 km/s). We combine the kinematic data with a mass model based on a two-component fit to $HST$ ACS/HRC data of the cluster to estimate the mass of a putative BH. Our dynamical modeling suggests a $>$3$\sigma$ detection of a BH component of 9.1$^{+2.6}_{-2.8}\times$10$^4$ M$_\odot$ (1$\sigma$ uncertainties). The inferred stellar mass of the cluster is 6.22$^{+0.03}_{-0.05}\times$10$^6$ M$_\odot$, consistent with previous estimates, thus the BH makes up 1.5% of its mass. We examine whether the observed kinematics are caused by a collection of stellar mass BHs by modeling an extended dark mass as a Plummer profile. The upper limit on the size scale of the extended mass is 0.56 pc (95% confidence), which does not rule out an extended mass. There is compelling evidence that B023-G078 is the tidally stripped nucleus of a galaxy with a stellar mass $>$10$^9$ M$_{\odot}$, including its high mass, two-component luminosity profile, color, metallicity gradient, and spread in metallicity. Given the emerging evidence that the central BH occupation fraction of $>$10$^9$ M$_{\odot}$ galaxies is high, the most plausible interpretation of the kinematic data is that B023-G078 hosts a central BH. This makes it the strongest BH detection in a lower mass ($<$10$^7$ M$_{\odot}$) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs

Nearby Early-type Galactic Nuclei at High Resolution: Dynamical Black Hole and Nuclear Star Cluster Mass Measurements

We present a detailed study of the nuclear star clusters (NSCs) and massive black holes (BHs) of four of the nearest low-mass early-type galaxies: M32, NGC 205, NGC 5102, and NGC 5206. We measure the dynamical masses of both the BHs and NSCs in these galaxies using Gemini/NIFS or VLT/SINFONI stellar kinematics, Hubble Space Telescope (HST) imaging, and Jeans anisotropic models. We detect massive BHs in M32, NGC 5102, and NGC 5206, while in NGC 205, we find only an upper limit. These BH mass estimates are consistent with previous measurements in M32 and NGC 205, while those in NGC 5102 and NGC 5206 are estimated for the first time and both found to be &lt;106 M ☉. This adds to just a handful of galaxies with dynamically measured sub-million M ☉ central BHs. Combining these BH detections with our recent work on NGC 404's BH, we find that 80% (4/5) of nearby, low-mass ({10}9{--}{10}10 M ☉ {σ }\star ̃ 20{--}70 km s-1) early-type galaxies host BHs. Such a high occupation fraction suggests that the BH seeds formed in the early epoch of cosmic assembly likely resulted in abundant seeds, favoring a low-mass seed mechanism of the remnants, most likely from the first generation of massive stars. We find dynamical masses of the NSCs ranging from 2 to 73 × 106 M ☉ and compare these masses to scaling relations for NSCs based primarily on photometric mass estimates. Color gradients suggest that younger stellar populations lie at the centers of the NSCs in three of the four galaxies (NGC 205, NGC 5102, and NGC 5206), while the morphology of two are complex and best fit with multiple morphological components (NGC 5102 and NGC 5206). The NSC kinematics show they are rotating, especially in M32 and NGC 5102 (V/{σ }\star ̃ 0.7)