2022, Pechetti, Renuka, Seth, Anil, Kamann, Sebastian, Caldwell, Nelson, Strader, Jay, den Brok, Mark, Luetzgendorf, Nora, Neumayer, Nadine, Voggel, Karina

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 show a strong rotation (∼20 km s-1) and a velocity dispersion rise toward the center (37 km s-1). 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 >3σ detection of a BH component of (1σ uncertainties). The inferred stellar mass of the cluster is , 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 >109 M o˙, 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 >109 M o˙ 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 (<107 M o˙) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.

2022, Kacharov, Nikolay, Alfaro-Cuello, Mayte, Neumayer, Nadine, Lützgendorf, Nora, Watkins, Laura L., Mastrobuono-Battisti, Alessandra, Kamann, Sebastian, van de Ven, Glenn, Seth, Anil C., Voggel, Karina T., Georgiev, Iskren Y., Leaman, Ryan, Bianchini, Paolo, Böker, Torsten, Mieske, Steffen

We present comprehensive multicomponent dynamical models of M54 (NGC 6715), the nuclear star cluster of the Sagittarius (Sgr) dwarf galaxy, which is undergoing a tidal disruption in the Milky Way halo. Previous papers in this series used a large MUSE mosaic data set to identify multiple stellar populations in the system and study their kinematic differences. Here, we use Jeans-based dynamical models that fit the population properties (mean age and metallicity), spatial distributions, and kinematics simultaneously. They provide a solid physical explanation for our previous findings. Population-dynamical models deliver a comprehensive view of the whole system, and allow us to disentangle the different stellar populations. We explore their dynamical interplay and confirm our previous findings about the build-up of Sgr’s nuclear cluster via contributions from globular cluster stars, Sgr inner field stars, and in situ star formation. We explore various parameterizations of the gravitational potential and show the importance of a radially varying mass-to-light ratio for the proper treatment of the mass profile. We find a total dynamical mass within M54's tidal radius (∼75 pc) of 1.60 ± 0.07 × 106 M ⊙ in excellent agreement with N-body simulations. Metal-poor globular cluster stars contribute about 65% of the total mass or 1.04 ± 0.05 × 106 M ⊙. Metal-rich stars can be further divided into young and intermediate-age populations, which contribute 0.32 ± 0.02 × 106 M ⊙ (20%) and 0.24 ± 0.02 × 106 M ⊙ (15%), respectively. Our population-dynamical models successfully distinguish the different stellar populations in Sgr’s nucleus because of their different spatial distributions, ages, metallicities, and kinematic features.