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 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 9.1 − 2.8 + 2.6 × 10 4 M ⊙ (1σ uncertainties). The inferred stellar mass of the cluster is 6.22 − 0.05 + 0.03 × 10 6 M ⊙ , 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 ⊙, 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 ⊙ 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 ⊙) stripped nucleus, and one of the few dynamically detected intermediate-mass BHs.

AstroSat Study of the Globular Cluster NGC 2298: Probable Evolutionary Scenarios of Hot Horizontal Branch Stars

We present the far-UV (FUV) photometry of images acquired with UVIT on AstroSat to probe the horizontal branch (HB) population of the Galactic globular cluster NGC 2298. UV-optical color–magnitude diagrams (CMDs) are constructed for member stars in combination with Hubble Space Telescope UV Globular Cluster Survey data for the central region and Gaia and ground-based photometric data for the outer region. A blue HB (BHB) sequence with a spread and four hot HB stars are detected in all FUV-optical CMDs and are compared with theoretical updated BaSTI isochrones and synthetic HB models with a range in helium abundance, suggesting that the hot HB stars are helium enhanced when compared to the BHB. The estimated effective temperature, radius, and luminosity of HB stars, using the best spectral energy distribution fits, were compared with various HB models. BHB stars span a temperature range from 7500 to 12,250 K. Three hot HB stars have 35,000–40,000 K, whereas one star has around ∼100,000 K. We suggest the following evolutionary scenarios: two stars are likely to be the progeny of extreme HB (EHB) stars formed through an early hot-flasher scenario, one is likely to be an EHB star with probable helium enrichment, and the hottest HB star, which is about to enter the white dwarf cooling phase, could have evolved from the BHB phase. Nevertheless, these are interesting spectroscopic targets to understand the late stages of evolution.

Spectroscopic Confirmation of the Sixth Globular Cluster in the Fornax Dwarf Spheroidal Galaxy*

The Fornax dwarf spheroidal galaxy has an anomalous number of globular clusters, five, for its stellar mass. There is a longstanding debate about a potential sixth globular cluster (Fornax 6) that has recently been “rediscovered” in DECam imaging. We present new Magellan/M2FS spectroscopy of the Fornax 6 cluster and Fornax dSph. Combined with literature data we identify ∼15–17 members of the Fornax 6 cluster, showing that this overdensity is indeed a star cluster and associated with the Fornax dSph. The cluster is significantly more metal-rich (mean metallicity of [ Fe / H ] ¯ = −0.71 ± 0.05) than the other five Fornax globular clusters (−2.5 < [Fe/H] < −1.4) and more metal-rich than the bulk of Fornax. We measure a velocity dispersion of 5.6 − 1.6 + 2.0 km s − 1 corresponding to an anomalously high mass-to-light of 15 < M/L < 258 at 90% confidence when calculated assuming equilibrium. Two stars inflate this dispersion and may be either Fornax field stars or as yet unresolved binary stars. Alternatively, the Fornax 6 cluster may be undergoing tidal disruption. Based on its metal-rich nature, the Fornax 6 cluster is likely younger than the other Fornax clusters, with an estimated age of ∼2 Gyr when compared to stellar isochrones. The chemodynamics and star formation history of Fornax shows imprints of major events such as infall into the Milky Way, multiple pericenter passages, star formation bursts, and/or potential mergers or interactions. Any of these events may have triggered the formation of the Fornax 6 cluster.

Globular Cluster Candidates in the Sagittarius Dwarf Galaxy

Recently, new Sagittarius (Sgr) dwarf-galaxy globular clusters were discovered, which opens the question of the actual size of the Sgr globular cluster population, and therefore on our understanding of the Sgr galaxy formation and accretion history of the Milky Way. Based on Gaia EDR3 and SDSS IV DR16 (APOGEE-2) data sets, we performed an analysis of the color–magnitude diagrams (CMDs) of the eight new Sgr globular clusters found by Minniti et al. from a sound cleaning of the contamination of Milky Way and Sgr field stars, complemented by available kinematic and metal abundance information. The cleaned CMDs and spatial stellar distibutions reveal the presence of stars with a wide range of cluster membership probabilities. Minni 332 turned out to be a younger (<9 Gyr) and more metal-rich ([M/H] ≳ −1.0 dex) globular cluster than M54, the nuclear Sgr globular cluster; as could also be the case of Minni 342, 348, and 349, although their results are less convincing. Minni 341 could be an open cluster candidate (age < 1 Gyr, [M/H] ∼ −0.3 dex), while the analyses of Minni 335, 343, and 344 did not allow us to confirm their physical reality. We also built the Sgr cluster frequency (CF) using available ages of the Sgr globular clusters and compared it with that obtained from the Sgr star formation history. Both CFs are in excellent agreement. However, the addition of eight new globular clusters with ages and metallicities distributed according to the Sgr age–metallicity relationship turns out in a remarkably different CF.

Low-luminosity Galaxies in the Early Universe Have Observed Sizes Similar to Star Cluster Complexes

We compare the sizes and luminosities of faint z = 6–8 galaxies magnified by the Hubble Frontier Fields clusters with star-forming regions, as well as more evolved objects, in the nearby universe. Our high-redshift comparison sample includes 330 z = 6–8 galaxies, for which size measurements were made as part of a companion study where lensing magnifications were estimated from various public models. Accurate size measurements for these sources are complicated by the lens model uncertainties, but other results and arguments suggest that faint galaxies are small, as discussed in a companion study. The measured sizes for sources in our comparison sample range from <50 pc to ∼500 pc. For many of the lowest-luminosity sources, extremely small sizes are inferred, reaching individual sizes as small as 10–30 pc, with several sources in the 10–15 pc range with our conservative magnification limits. The sizes and luminosities are similar to those of single star cluster complexes like 30 Doradus in the lower-redshift universe and—in a few cases—super star clusters. The identification of these compact, faint star-forming sources in the z ∼ 6–8 universe also allows us to set upper limits on the proto-globular cluster luminosity function at z ∼ 6. By comparisons of the counts and sizes with recent models, we rule out (with some caveats) proto-globular cluster formation scenarios favoring substantial (ξ = 10) post-formation mass loss and set useful upper limits on others. Our size results suggest we may be very close to discovering a bona fide population of forming globular clusters at high redshift.