The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy

arXiv:2304.06904 · astro-ph.GA · Submitted 2023-04-14 · Read on arXiv

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Introduction to the show: ident: Astrophysics Radio. Generated commentary on the latest astrophysics papers.

Vera: Today's paper: "The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy".

Jocelyn: The gist We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters and two newly discovered open clusters at low Galactic latitudes to determine their kinematics,

Vera: First, who's behind it and why it matters.

Paper summary: Vera: We just covered how this paper confirms the identities of six star clusters using M2FS spectroscopy to get detailed data on their velocities and chemical compositions. The main point is that they are not just random stellar overdensities found by Gaia DR2; they are real objects >

Jocelyn: They use the M2FS system to select members, specifically for Gran three they selected stars with line-of-sight velocities between eight two km s minus one and nine seven km s minus one > <ref:2304.06904#pg1>

Subrahmanyan: That velocity selection is a way to confirm membership because the stellar parameters they get from Teff and log g also reinforce the idea that these stars belong together as a single population >

Vera: And for Gran three specifically, the members selected from that velocity range were consistent with single stellar populations, which aligns with its description as an old, metal-poor globular cluster > <ref:2304.06904#pg1>

Jocelyn: For Garro one they found that the members had metallicities that were consistent with a near circular orbit in the outer disk and its classification remains ambiguous because of the age estimate being around four gigayears > <ref:2304.06904#pg1>

Subrahmanyan: That ambiguity is what makes it interesting for structure formation studies; we need to know if it's an open cluster or a globular cluster because that affects how we model the stellar population >

Vera: And they use Gaia astrometry to compute the orbits, and for Gran four they find an eccentric orbit with a eccentricity of about zero point six three > <ref:2304.06904#pg1>

Jocelyn: That orbit has a small pericenter around eight kiloparsecs and a large apocenter around thirty-three point nine kiloparsecs >

Subrahmanyan: Those orbital parameters for Gran four are very suggestive of those proposed LMS-one/Wukong and or Helmi streams merger events, which is what links these local objects to the larger galactic environment > <ref:2304.06904#pg1>

Vera: So, the paper's thesis is that spectroscopic confirmation and orbital modeling provide the necessary data to determine the kinematics and chemical properties of these candidates, which reveals their true nature >

Jocelyn: And it matters because it shows that Gaia astrometry, when paired with high-resolution spectroscopy from M2FS, can help us identify star clusters we might otherwise miss >

Subrahmanyan: And for the world this means we can refine our models of how these local structures evolve within the Milky Way potential, using these six objects as test cases >

Conclusion: Vera: So to wrap up on "The Kinematics, Metallicities, and Orbits of Six Recently Discovered Galactic Star Clusters with Magellan/M2FS Spectroscopy," the authors have successfully confirmed three globular clusters and three open clusters using their combined data sets.

Jocelyn: The main implication is that this combination of spectroscopic measurements and astrometry gives us a much more complete picture of the kinematics and chemistry of these nearby star cluster candidates >

Subrahmanyan: It solidifies the understanding that these objects aren't just scattered stars but have specific, traceable orbits within our galaxy, which helps us refine our models for structure formation >

Vera: For an observer listening, it means we can start to see how local groups connect to the larger galactic architecture using these detailed kinematic and chemical constraints >

Jocelyn: It really shows that the census of star clusters is still quite incomplete and that tools like Gaia are powerful but require follow-up work from telescopes like M2FS to get the full picture >

Subrahmanyan: So, this work helps us test how local clusters behave in a way that informs how we model structure formation on larger scales >

McWilliams Center for Cosmology at Carnegie Mellon University · Institute for Astronomy at the University of Edinburgh · Institute of Astronomy at the University of Cambridge · Kavli Institute for Cosmology at the University of Cambridge · Harvard-Smithsonian Center for Astrophysics · Department of Astronomy at the University of Michigan, Ann Arbor, Michigan State University, Department of Physics at the University of Santa Barbara, Santa Barbara California State University, Lowell Observatory

astro-ph.GA

Submitted: 2023-04-14

Updated: 2023-09-08

Comments: 20 pages, 15 figures, accepted to MNRAS, associated data products available at https://doi.org/10.5281/zenodo.7809128

DOI: 10.1093/mnras/stad2760

License: http://creativecommons.org/licenses/by/4.0/

Importance score: 78/100

The gist: The gist We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters and two newly discovered open clusters at low Galactic latitudes to determine their kinematics,

Key concepts

Magellan/M2FS Spectroscopy: This technique involves using a telescope (Magellan) equipped with a fiber system (M2FS) to take high-resolution spectra of stars. By analyzing these spectra, scientists measure crucial stellar properties like temperature, surface gravity, and chemical abundances ([Fe/H], [Mg/Fe]), which helps determine the cluster's age and origin.
Gaia Astrometry: Gaia is a space mission that precisely measures the positions and motions (proper motions) of stars. This data is used to calculate the 3D kinematics, systemic proper motion, and orbital paths of star clusters like Gran 3 and Gran 4, revealing how they move within the Milky Way's gravitational field.
Orbital Properties: This refers to the path a star cluster takes through space around the Galactic center. The study calculated these orbits using a galactic potential model. For instance, Gran 4 has an eccentric orbit suggesting it might have been involved in past merger events with other stellar streams, providing clues about its formation history.
Globular Cluster vs. Open Cluster: These are two types of star clusters distinguished by their size and age. Globular clusters are typically very old and dense, often found in the Galactic bulge or halo. Open clusters are younger and less dense, usually located in the disk. The paper uses metallicity ([Fe/H]) and age estimates to help classify Garro 01 as possibly an open cluster despite its similarity to a globular cluster.

Terminology

Metallicity — 1× in this paper · explained in 3 episodes

Summary

The gist We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters and two newly discovered open clusters at low Galactic latitudes to determine their kinematics, chemical properties, and orbital properties

Discovery and Candidate Identification

The search for stellar overdensities was carried out in 2018 after the release of the Gaia DR2 using the satellite detection pipeline broadly based on the methods presented in Koposov et al. (2008, 2015), but extended into space of proper motions The algorithm consists of several steps, including looping over all proper motions where mualpha − Xi < 1 and mudelta − Xj < 1 where Xi,Xj span the range of proper motions from-15 to 15 mas/year with 1 mas/year steps. The identification of overdensities is based on a stellar count map and a significance map where the significance (or the Z-score) in each pixel is assigned as Z = F−1(Pnull(≥ N)) where F−1 is the inverse of the CDF of a normal distribution. The application of this algorithm to Gaia DR2 in 2018 yielded a few hundred significant distinct overdensities, and around 30 objects were deemed to be likely real dwarf galaxies or globular clusters and were selected for further inspection.

Spectroscopic Follow-up and Membership

Spectroscopic observations were obtained using the Michigan/Magellan Fiber System (M2FS) at the 6.5-m Magellan/Clay Telescope at Las Campanas Observatory, Chile. The spectra are processed and modeled using a library of synthetic template spectra computed on a regular grid of stellar atmospheric parameters: effective temperature (Teff), surface gravity (log g), metallicity ([Fe/H]) and magnesium abundance ([Mg/Fe]). Good quality measurements are considered stars with S/N > 0 and sigmavlos > 5 km s−1, and stars with S/N > 2 to be good quality [Fe/H] measurements. For Gran 3, the M2FS members are identified by selecting stars in the 82 < vlos < 97 km s−1. The stellar parameters ([Fe/H], Teff, and log g) of the members identified from velocities further reinforce their membership and they are consistent with single stellar populations.

Kinematics and Chemical Properties

The kinematics and chemical properties of each cluster were determined by utilizing Gaia astrometry. For Gran 3, it is an old, metal-poor globular cluster located in the Galactic bulge on a retrograde orbit with a mean metallicity of [Fe/H] = −1.83. Gran 4 is an old, metal-poor globular cluster with a halo-like orbit that happens to be passing through the Galactic plane, and its orbital properties are consistent with the proposed LMS-1/Wukong and/or Helmi streams merger events. Garro 01 is metal-rich ([Fe/H] = −0.30) and on a near circular orbit in the outer disk but its classification as an open cluster or globular cluster is ambiguous. Gaia 9 and Gaia 10 are among the most distant known open clusters at RGC ∼ 18, 21.2 kpc and most metal-poor with [Fe/H] ∼ −0.50, −0.34 for Gaia 9 and Gaia 10, respectively.

Orbital Properties

The orbits of the six star clusters were computed using the gala package and compared to other Milky Way globular clusters using the default MilkyWayPotential Galactic potential from Price-Whelan (2017). Gran 3 is on a near circular orbit (e ∼ 0.12) in the inner bulge (RGC ∼ 2.7 kpc) that is confined to the plane of the disk (zmax ∼ 1.8 kpc). Gran 4 has an eccentric orbit (e ∼ 0.63) with a small pericenter (rperi ∼ 8 kpc), large apocenter (rapocenter ∼ 33.9 kpc), and is a candidate member of the LMS-1/Wukong and/or Helmi stream merger events. Garro 01 has a circular orbit (ecc ∼ 0.16) that is confined to the Galactic midplane (zmax ∼ 1.3 kpc) at a relatively large Galactocentric radius (∼ 10 − 13 kpc).

Cluster Classification and Comparison

Garro et al. (2020) classify Garro 01 as a globular cluster based on its close similarity to the globular cluster 47 Tuc but several magnitudes fainter. The age of a star cluster can be key for determining its origin as a globular cluster or open cluster, and the best estimate for the age between 2 − 13 Gyr is 4 Gyr, suggesting Garro 01 is an open cluster. Gaia 9 and Gaia 10 are among the most metal-poor open clusters known and have some of the largest angular momentum in the z-direction. The properties of Garro 01 are consistent with the Galactic metallicity gradient as traced by open clusters, and its classification is ambiguous.

Internal Dynamics and Spatial Distribution

The spatial distribution of each star cluster was constructed using a larger proper motion selected sample from the Gaia DR3 catalog based on the systemic proper motion found from our spectroscopic sample. The Plummer and King fits along with the binned stellar profile of all six clusters are shown in Figure 8, and for the open clusters, we find that Gaia 9 and Gaia 10 are among the most distant (RGC ∼ 18, 21.2 kpc) and most metal-poor open clusters known. The results for the six clusters are included in Table 2.

Conclusion

We have independently discovered three globular clusters (Gran 3/Patchick 125, Gran 4, Garro 01) and three open clusters (Gaia 9, Gaia 10, LP 866) with Gaia astrometry. We find Gran 3 is an old, metal-poor globular cluster on a retrograde orbit trapped within the Galactic bulge. Gran 4 is an old, metal-poor globular cluster with a halo-like orbit that is passing though the Galactic mid-plane. Garro 01 is a metal-rich star cluster on an outer disk-like orbit, and its classification remains ambiguous. Both Gran 3 and Gran 4 have evidence for radially declining velocity dispersion profiles, and there is inconclusive evidence for rotation in the globular clusters. We have confirmed Gaia 9, Gaia 10, and LP 866 as open clusters and identified spectroscopic members from our M2FS spectroscopy. All three open clusters are on circular, disk-like orbits. The Milky Way star cluster population remains incomplete and Gaia astrometry has revolutionised our understanding of star clusters.

Acknowledgements

We thank the referee for their helpful comments. ABP is supported by NSF grant AST-1813881. M.G.W. acknowledges support from NSF grants AST-1813881 and AST-1909584. SK was partially supported by NSF grants AST-1813881 and AST-1909584. EO was partially supported by NSF grant AST-1815767. NC is supported by NSF grant AST-1812461. MM was supported by U.S. National Science Foundation (NSF) grants AST-1312997, AST-1726457 and AST-1815403. IUR acknowledges support from NSF grants AST1613536, AST-1815403, AST-2205847, and PHY-1430152 (Physics Frontier Center/JINA-CEE). DBZ acknowledges support from Australian Research Council grant DP220102254. We thank Lorenzo Spina for sharing their open cluster catalog. EO wants to remember Jill Bechtold here. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has used data acquired at the Anglo-Australian Telescope.

Improvements for AI systems

The AI system can perform spectroscopic confirmation of star cluster candidates by fitting a model based on synthetic template spectra with 16 free parameters including effective temperature (Teff), surface gravity (log g), metallicity ([Fe/H]) and magnesium abundance ([Mg/Fe]).
The system can determine the orbital properties of star clusters by using the gala package to compute the orbits of the six star clusters and compare them to other MW globular clusters, calculating the integrals of motion, E and Lz, approximately conserved quantity L⊥ (Massari et al. 2019) and the orbital pericenter (rperi), apocenter (rapo), and eccentricity.
The AI can perform membership identification for star clusters using a two-parameter Gaussian likelihood function to compare the cluster likelihood to total likelihood for each star: pmember = Lcluster/L, allowing it to distinguish between cluster and MW foreground stars.
The system can analyze the spatial distribution of star clusters by modeling them with a Plummer distribution (Plummer 1911) and a King profile (King 1962) to infer structural parameters like core radius, tidal radius, and half-light radii for both globular and open clusters.
The system can compare the orbital properties of star clusters to known merger events by examining the energy, angular momentum in the z-direction, and the angular momentum in the perpendicular direction, allowing it to identify candidates for mergers like LMS-1/Wukong and/or Helmi streams merger events.
The system can trace the Galactic metallicity gradient using open clusters by comparing their properties to literature trends, specifically noting that the properties of Garro 01 are consistent with the Galactic metallicity gradient as traced by open clusters.

Abstract

We present Magellan/M2FS spectroscopy of four recently discovered Milky Way star clusters (Gran 3/Patchick 125, Gran 4, Garro 01, LP 866) and two newly discovered open clusters (Gaia 9, Gaia 10) at low Galactic latitudes. We measure line-of-sight velocities and stellar parameters ([Fe/H],, T eff, [Mg/Fe]) from high resolution spectroscopy centered on the Mg triplet and identify 20-80 members per star cluster. We determine the kinematics and chemical properties of each cluster and measure the systemic proper motion and orbital properties by utilizing Gaia astrometry. We find Gran 3 to be an old, metal-poor (mean metallicity of [Fe/H]=-1.84) globular cluster located in the Galactic bulge on a retrograde orbit. Gran 4 is an old, metal-poor ([Fe/H]=-1.84) globular cluster with a halo-like orbit that happens to be passing through the Galactic plane. The orbital properties of Gran 4 are consistent with the proposed LMS-1/Wukong and/or Helmi streams merger events. Garro 01 is metal-rich ([Fe/H]=-0.30) and on a near circular orbit in the outer disk but its classification as an open cluster or globular cluster is ambiguous.. Gaia 9 and Gaia 10 are among the most distant known open clusters at R GC about 18, 21.2 kpc and most metal-poor with [Fe/H] -0.50,-0.46 for Gaia 9 and Gaia 10, respectively. LP 866 is a nearby, metal-rich open cluster ([Fe/H] =+0.1). The discovery and confirmation of multiple star clusters in the Galactic plane shows the power of Gaia astrometry and the star cluster census remains incomplete.