Callie Clontz

The Sloan Digital Sky Survey (SDSS)-V Local Volume Mapper (LVM) is a wide-field integral field spectroscopic survey of the southern Milky Way plane, the Magellanic Clouds, and nearby Local Group galaxies. We used early science observations of the whole body of the nearest nuclear cluster, Omega Centauri, to extend the LVM beyond its primary interstellar-medium science case. The wide LVM field allowed us to precisely map ω Cen’s line-of-sight rotation out to ~3 r _HL or 15′, reaching a maximum value of (8.4 ± 0.8) km s ⁻¹ at r ≈ 4.7′. Within the central region, comparisons with existing VLT MUSE oMEGACat data explicitly show that the unresolved light signal is dominated by a small number of bright stars, with an effective sample size of only ~12 per resolution element. Using Gaia DR3 as an external reference, we verified that the SDSS-V’s LVM reduction pipeline recovers integrated stellar fluxes to 1–4% across six magnitudes of surface brightness. Our resulting total spectrum of ω Cen is one of the highest S/N integrated spectra for any globular or nuclear star cluster. We used it to test four widely used SSP template libraries against the resolved age-metallicity ground truth from oMEGACat. All templates recovered an old, metal-poor population. However, even at signal to noise ratio of ~1300, the inferred mean ages and mean [Fe/H] vary by ~7 Gyr and ~0.4 dex, respectively, across libraries and wavelength ranges, reflecting a systematic floor for integrated-light studies of old multi-population systems.

Omega Centauri ( ω Cen) is the most massive and chemically complex star cluster in the Milky Way (MW) and is widely interpreted as the surviving nuclear star cluster of an accreted dwarf galaxy, although the identity of its parent remains uncertain. We test whether the merger remnants Sequoia, Thamnos, and Gaia–Enceladus (GE) originated in a single progenitor, the ω Dwarf, whose nucleus survives as ω Cen. Combining APOGEE and GALAH abundances with Gaia astrometry, we reconstruct the progenitor’s chemical structure using orbital energy as a proxy for premerger radius. The chemically evolved, younger, Al–N-rich population is strongly concentrated toward the center, consistent with formation during or after the merger. By contrast, the primordial population exhibits dwarf-galaxy-like chemistry across all components, supporting a common origin. Its metallicity profile follows an inverted U-shaped radial trend similar to those observed in present-day nucleated dwarf galaxies. The inner regions, traced by ω Cen and Thamnos, are also more α -enhanced than the outskirts, implying shorter and more efficient star formation and supporting a scenario in which the primordial nucleus assembled partly through the inspiral and merger of globular clusters. Although the chemical evidence permits GE to be associated with ω Dwarf, Sequoia and Thamnos alone form a coherent outside-in stripping sequence around ω Cen. Excluding GE yields a mass–metallicity-relation estimate of 6 . 8 − 3.7 + 8.3 × 1 0 8 M ⊙ for ω Dwarf, whereas including GE increases the inferred mass by a factor of ∼3.4, to 2 . 3 − 1.5 + 4.4 × 1 0 9 M ⊙ . Both estimates are consistent with the nuclear-star-cluster–host-galaxy mass relation, identifying Sequoia and Thamnos as the most robust surviving debris of the ω Cen progenitor.

Modern simulations of stellar dynamics in globular clusters peg a dominant role for stellar-mass black holes, but direct evidence for black holes in clusters remains limited. We present the discovery of an astrometric stellar-mass black hole–main sequence star binary in ω Centauri, the most massive Galactic globular cluster, using Hubble Space Telescope data from the oMEGACat project and additional JWST data that span a total of 23 yr. The luminous companion to the black hole is a main-sequence turnoff star, and has a period of 9 4 − 42 + 63 yr, a semimajor axis of 3 1 − 12 + 15 au, and an eccentricity of e = 0.7 2 − 0.13 + 0.08 . Since we observe the binary during periastron, the mass of the black hole is well constrained even though we only observe a partial orbit: the inferred black hole mass is 4.4 6 − 1.01 + 1.22 M _⊙ . We call this black hole oMEGACat BH-2. This is the first astrometric discovery of a stellar-mass black hole in a globular cluster, and is the longest-period black hole binary system yet discovered. The low mass of this black hole is perhaps surprising given the low metallicity of the cluster, and shows that at least some low-mass black holes form at metallicity Z < 10 ⁻³ . We find that the binary is almost certainly dynamically formed and is soft, with an expected binary disruption timescale of ∼800 Myr. While the total number of black hole binaries in ω Centauri is uncertain, we show that existing surveys only cover a small area of parameter space, and that the presence of additional detectable black hole binaries is likely.

An understanding of the assembly history of the complex star cluster Omega Centauri has long been sought after, with many studies separating the stars on the color–magnitude diagram into multiple groupings across small magnitude ranges. Utilizing the oMEGACat combined astro-photometric and spectroscopic data set, we parse 14 subpopulations from the upper red giant branch to below the main-sequence turnoff. We combine our results with previous works to estimate the age and age spread of each population. We find that the chemically enhanced (P2) populations are all ∼1 Gyr younger (∼11.6 Gyr old) and have significantly higher intrinsic age spreads (0.6 Gyr) than the primordial (P1) populations (∼12.6 Gyr old, 0.3 Gyr spread), with the intermediate (Im) populations falling in between the two. Additionally, we connect for the first time the chromosome diagram to the two-stream age–metallicity relation, allowing us to link the P1 and P2 stars to the distinct star formation tracks, proposed to be in situ and ex situ contributions to the cluster’s assembly. Our results are consistent with some suggested formation models and rule out others, but no current model can explain all observed features of the subpopulations.

We investigate the foreground interstellar medium along the line of sight and intracluster medium of ω Centauri ( ω Cen) by measuring the equivalent width of Na I D absorptions from MUSE observations. The large line-of-sight velocity difference between ω Cen and the foreground enables us to separate Na I D absorption contributed from atomic gas in the interstellar and intracluster medium. We find that small-scale substructures in the foreground Na I D distribution correlate with differential reddening derived from photometric methods. Using an empirical Na I D equivalent width-reddening relation, we determine an average reddening of E ( B − V ) = 0.153 ± 0.003 mag within the half-light radius of ω Cen. However, the Na I D-inferred differential reddening is significantly larger than photometric estimates. This is likely due to scatter in the Na I D-reddening relation. We find no evidence for intracluster atomic gas from spectra of horizontal branch stars, as there is no significant Na I D absorption at ω Cen’s systemic velocity. Given this nondetection, we place the strongest upper limit to date on the intracluster atomic gas column density in ω Cen of ≲2.17 × 10 ¹⁸ cm ⁻² . We also estimate the ionized gas density from pulsar dispersion measure variations, which exceed the atomic gas limit by ∼50 times. Nevertheless, the strong correlation between dispersion measure and foreground Na I D suggests that much or all of this ionized gas resides in the foreground. Given ongoing mass loss from bright giant stars, our findings imply that the intracluster gas accumulation timescale is short and gas removal in the cluster is likely not tied to stripping as ω Cen passes through the galactic disk.

An understanding of the assembly history of the complex star cluster Omega Centauri has long been sought after, with many studies separating the stars on the color-magnitude diagram into multiple groupings across small magnitude ranges. Utilizing the oMEGACat combined astro-photometric and spectroscopic dataset we parse 14 subpopulations from the upper red-giant branch to below the main-sequence turnoff. We combine our results with previous works to estimate the age and age spread of each population. We find that the chemically enhanced (P2) populations are all ~1 Gyr younger (~11.6 Gyr old) and have significantly higher intrinsic age spreads (0.6 Gyr) than the primordial (P1) populations (~12.6 Gyr old, 0.3 Gyr spread), with the intermediate (Im) populations falling in between the two. Additionally, we connect for the first time the Chromosome Diagram to the two-stream age-metallicity relation, allowing us to link the P1 and P2 stars to the distinct star formation tracks, proposed to be in-situ and ex-situ contributions to the cluster's assembly. Our results are consistent with some suggested formation models and rule out others but no current model can explain all observed features of the subpopulations.

Constraining the helium enhancement in stars is critical for understanding the formation mechanisms of multiple populations in star clusters. However, measuring helium variations for many stars within a cluster remains observationally challenging. We use Hubble Space Telescope photometry combined with MUSE spectroscopic data for over 7200 red giant branch stars in ωCen to measure helium differences between distinct groups of stars as a function of metallicity, separating the impact of helium enhancements from other abundance variations on the pseudo-color (chromosome) diagram. Our results show that stars at all metallicities have subpopulations with significant helium enhancement (ΔY _min ≳ 0.11). We find a rapid increase in helium enhancement across low metallicities ([Fe/H] ≃ −2.05 to [Fe/H] ≃ −1.92), with this enhancement leveling out at ΔY = 0.15 at higher metallicities. The fraction of helium-enhanced stars steadily increases with metallicity from 10% at [Fe/H] ≃ −2.04 to over 90% at [Fe/H] ≃ −1.04. From these measurements, we calculate the total mass in helium produced over the cluster’s lifetime, finding M He tot ∼ 2.7 × 1 0 5 M ⊙ . This study is the first to examine helium enhancement across the full range of metallicities in ωCen, providing new insight into its formation history and additional constraints on enrichment mechanisms.

Omega Centauri (ω Cen) is the Milky Way’s most massive globular cluster and is likely the stripped nucleus of an accreted dwarf galaxy. In this paper, we analyze ω Cen’s kinematics using data from oMEGACat, a comprehensive catalog of ω Cen’s central regions, including 1.4 million proper motion measurements and 300,000 spectroscopic radial velocities. Our velocity dispersion profiles and kinematic maps are consistent with previous work but improve on their resolution, precision, and spatial coverage. The cluster’s 3D dispersion is isotropic in the core, with increasing radial anisotropy at larger radii. The 2D kinematic maps show an elongation of the velocity dispersion field comparable to the flattening observed photometrically. We find good agreement between proper motions and line-of-sight velocity dispersion, and we measure a kinematic distance of 5494 ± 61 pc, the most precise kinematic distance to ω Cen available. The subset of data with precise metallicity measurements shows no correlation between metallicity and kinematics, supporting the picture of well-mixed stellar populations within the half-light radius of ω Cen. Finally, we study the degree of energy equipartition using a large range of stellar masses. We find partial energy equipartition in the center that decreases towards large radii. The spatial dependence of the radial energy equipartition is stronger than the tangential energy equipartition. Our kinematic observations can serve as a new reference for future dynamical modeling efforts that will help to further disentangle the complex mass distribution within ω Cen.

We present age estimates for over 8100 subgiant branch (SGB) stars in Omega Centauri (ω Cen) to study its star formation history. Our large data set, which combines multi-wavelength Hubble Space Telescope photometry with MUSE metallicities, provides an unprecedented opportunity to measure individual stellar ages. We do this by fitting each star’s photometry and metallicity with theoretical isochrones that are embedded with an empirical [C + N + O]–[Fe/H] relation specific to ω Cen. The bulk of the stars have ages between 13 and 10 Gyr, with the mean stellar age being 12.08 ± 0.01 Gyr and the median age uncertainty being 0.68 Gyr. From these ages we construct the most complete age–metallicity relation for ω Cen to date. We find that the mean age of stars decreases with increasing metallicity and find two distinct streams in the age–metallicity plane, hinting at different star formation pathways. We derive an intrinsic spread in the ages of 0.75 ± 0.01 Gyr for the whole cluster, with the age spread showing a clear increase with metallicity. We verify the robustness of our age estimations by varying isochrone parameters and constraining our systematics. We find the C + N + O relation to be the most critical consideration for constraining the age–metallicity relation. We also present an SGB chromosome map with age information. In the future these stellar ages could be combined with chemical abundances to study age differences in subpopulations and uncover the chemical evolution history of this massive nuclear star cluster.