The accreted nuclear clusters of the Milky Way

ABSTRACT A number of the massive clusters in the halo, bulge, and disc of the Galaxy are not genuine globular clusters (GCs) but instead are different beasts altogether. They are the remnant nuclear star clusters (NSCs) of ancient galaxies since accreted by the Milky Way. While some clusters are readily identifiable as NSCs and can be readily traced back to their host galaxy (e.g. M54 and the Sagittarius Dwarf galaxy), others have proven more elusive. Here, we combine a number of independent constraints, focusing on their internal abundances and overall kinematics, to find NSCs accreted by the Galaxy and trace them to their accretion event. We find that the true NSCs accreted by the Galaxy are: M54 from the Sagittarius Dwarf, ω Centari from Gaia-Enceladus/Sausage, NGC 6273 from Kraken, and (potentially) NGC 6934 from the Helmi Streams. These NSCs are prime candidates for searches of intermediate-mass black holes (BHs) within star clusters, given the common occurrence of galaxies hosting both NSCs and central massive BHs. No NSC appears to be associated with Sequoia or other minor accretion events. Other claimed NSCs are shown not to be such. We also discuss the peculiar case of Terzan 5, which may represent a unique case of a cluster–cluster merger.

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A deep view into the nucleus of the Sagittarius dwarf spheroidal galaxy: M54

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319007907 ◽

2019 ◽

Vol 14 (S351) ◽

pp. 47-50

Author(s):

M. Alfaro-Cuello ◽

N. Kacharov ◽

N. Neumayer ◽

A. Mastrobuono-Battisti ◽

N. Lützgendorf ◽

...

Keyword(s):

Metal Complex ◽

Detailed Analysis ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Star Clusters ◽

High Mass ◽

Data Set

AbstractNuclear star clusters hosted by dwarf galaxies exhibit similar characteristics to high-mass, metal complex globular clusters. This type of globular clusters could, therefore, be former nuclei from accreted galaxies. M54 resides in the photometric center of the Sagittarius dwarf galaxy, at a distance where resolving stars is possible. M54 offers the opportunity to study a nucleus before the stripping of their host by the tidal field effects of the Milky Way. We use a MUSE data set to perform a detailed analysis of over 6600 stars. We characterize the stars by metallicity, age, and kinematics, identifying the presence of three stellar populations: a young metal-rich (YMR), an intermediate-age metal-rich (IMR), and an old metal-poor (OMP). The evidence suggests that the OMP population is the result of accretion of globular clusters in the center of the host, while the YMR population was born in-situ in the center of the OMP population.

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Nuclear angular momentum of early-type galaxies hosting nuclear star clusters

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833954 ◽

2019 ◽

Vol 629 ◽

pp. A44 ◽

Cited By ~ 2

Author(s):

Mariya Lyubenova ◽

Athanassia Tsatsi

Keyword(s):

Black Holes ◽

Angular Momentum ◽

Adaptive Optics ◽

Globular Clusters ◽

Milky Way ◽

Star Clusters ◽

Galactic Nuclei ◽

Host Galaxy ◽

Early Type ◽

Stellar Kinematics

Context. Nucleation is a common phenomenon in all types of galaxies and at least 70% of them host nuclear star clusters (NSCs) in their centres. Many of the NSCs co-habit with supermassive black holes and follow similar scaling relations with host galaxy properties. Unlike black holes, NSCs, preserve the signature of their evolutionary path imprinted onto their kinematics and stellar populations. Thus their study provides us with important information about the formation of galactic nuclei. Aims. In this paper we explored the angular momentum of the nuclei of six intermediate mass (9.7 > log(Mdyn/M⊙) > 10.6) early-type galaxies that host NSCs and are located in the Fornax cluster. Our goal was to derive a link between the nuclear angular momentum and the proposed formation scenarios of NSCs. Methods. We used adaptive optics assisted IFU observations with VLT/SINFONI to derive the spatially resolved stellar kinematics of the galaxy nuclei. We measured their specific stellar angular momenta λRe, and compared these with Milky Way globular clusters (GCs) and N-body simulations of NSC formation. Results. We found that all studied nuclei exhibit varied stellar kinematics. Their λRe and ellipticities are similar to Milky Way GCs. Five out of six galaxy nuclei are consistent with the λRe − ϵe of simulated NSCs embedded in a contaminating nuclear bulge that have formed via the in-spiralling and merging of GCs. Conclusion. It has previously been suggested that the NSCs in higher mass galaxies, such as those studied in this paper, form via dissipational sinking of gas onto the galactic nuclei with hints that some might also involve the merger of GCs. In this work we show that we cannot exclude the pure GC merging scenario as a viable path for the formation of NSCs.

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The Formation Chronology of Local Group Spiral Galaxies

Symposium - International Astronomical Union ◽

10.1017/s0074180900223401 ◽

2002 ◽

Vol 207 ◽

pp. 73-82

Author(s):

Ata Sarajedini

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Local Group ◽

Spiral Galaxies ◽

Dwarf Galaxy ◽

Additional Parameter ◽

Horizontal Branch ◽

The Galaxy ◽

Metal Abundance ◽

Cluster Age

The ‘Second Parameter Effect’ (2ndPE) has long been recognized as an important probe into the formation of spiral galaxies. The concept that the horizontal branch morphologies of globular clusters are primarily affected by metal abundance in the inner halo (RGC<8 kpc) of the Galaxy but require an additional parameter (probably cluster age) to explain their behavior in the outer halo (RGC > 8 kpc), suggests that the former experienced a rapid monotonic collapse while the latter underwent a slower chaotic formation scenario. As such, in the Milky Way, the so-called second parameter boundary is located at 8 kpc. We find that, in the other Local Group spirals — M31 and M33 — this boundary lies at ∼40 kpc and ∼0 kpc, respectively. We therefore speculate that the boundary delimiting rapid monotonic halo collapse from the chaotic accretion of dwarf galaxy fragments is inversely related to the mass of the spiral galaxy.

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Infrared surveys of Galactic star clusters

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309991062 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 203-210 ◽

Cited By ~ 1

Author(s):

Valentin D. Ivanov ◽

Maria Messineo ◽

Qingfeng Zhu ◽

Don Figer ◽

J. Borissova ◽

...

Keyword(s):

Surface Density ◽

Globular Clusters ◽

Visual Inspection ◽

Milky Way ◽

Star Clusters ◽

Detection Algorithm ◽

Point Sources ◽

The Galaxy ◽

Infrared Surveys ◽

Selection Of

AbstractMany attempts have been made to carry out a complete observational census of Milky Way star clusters based on recent near- and mid-infrared surveys. However, more clusters are still being discovered, indicating that existing catalogs are incomplete. We attempt to estimate the total number of supermassive (SM; Mcl ≥ 104 M⊙) clusters in the Galaxy, and to improve the yield from the automated cluster searches. Assuming that the ‘local’ census of SM clusters is complete, and that their surface density accross the disk follows that of the stars, we predict that the Milky Way contains ≥81 ± 21 SM clusters. We apply a cluster-detection algorithm to the 2mass Point Source Catalog after a preliminary color and/or magnitude selection of the point sources to improves the surface-density cluster-to-field contrast. Our algorithm identified 94 new candidates, and re-identified 34 known clusters. During the visual inspection, we detected an additional 41 new candidates, and re-identified 32 known objects. Preliminary characterization suggests that the new list may contain red-supergiant, open and globular clusters.

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Globular clusters in the stellar stream surrounding the Milky Way analogue NGC 5907

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3382 ◽

2019 ◽

Vol 491 (4) ◽

pp. 5693-5701 ◽

Cited By ~ 4

Author(s):

Adebusola B Alabi ◽

Duncan A Forbes ◽

Aaron J Romanowsky ◽

Jean P Brodie

Keyword(s):

Spiral Galaxy ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Young Star ◽

Star Cluster ◽

Wide Field ◽

Stellar Disc ◽

The Galaxy ◽

Specific Frequency

ABSTRACT We study the globular clusters (GCs) in the spiral galaxy NGC 5907 well-known for its spectacular stellar stream – to better understand its origin. Using wide-field Subaru/Suprime-Cam gri images and deep Keck/DEIMOS multi-object spectroscopy, we identify and obtain the kinematics of several GCs superimposed on the stellar stream and the galaxy disc. We estimate the total number of GCs in NGC 5907 to be 154 ± 44, with a specific frequency of 0.73 ± 0.21. Our analysis also reveals a significant, new population of young star cluster candidates found mostly along the outskirts of the stellar disc. Using the properties of the stream GCs, we estimate that the disrupted galaxy has a stellar mass similar to the Sagittarius dwarf galaxy accreted by the Milky Way, i.e. $\sim 10^8~\rm M_\odot$.

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The Next Generation Fornax Survey (NGFS): VII. A MUSE view of the nuclear star clusters in Fornax dwarf galaxies

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1261 ◽

2020 ◽

Vol 495 (2) ◽

pp. 2247-2264

Author(s):

Evelyn J Johnston ◽

Thomas H Puzia ◽

Giuseppe D’Ago ◽

Paul Eigenthaler ◽

Gaspar Galaz ◽

...

Keyword(s):

Star Formation ◽

Globular Clusters ◽

Dwarf Galaxies ◽

Star Clusters ◽

Stellar Populations ◽

Host Galaxy ◽

Wide Field ◽

The Core ◽

The Galaxy

ABSTRACT Clues to the formation and evolution of nuclear star clusters (NSCs) lie in their stellar populations. However, these structures are often very faint compared to their host galaxy, and spectroscopic analysis of NSCs is hampered by contamination of light from the rest of the system. With the introduction of wide-field integral field unit (IFU) spectrographs, new techniques have been developed to model the light from different components within galaxies, making it possible to cleanly extract the spectra of the NSCs and study their properties with minimal contamination from the light of the rest of the galaxy. This work presents the analysis of the NSCs in a sample of 12 dwarf galaxies in the Fornax Cluster observed with the Multi-Unit Spectroscopic Explorer (MUSE). Analysis of the stellar populations and star formation histories reveal that all the NSCs show evidence of multiple episodes of star formation, indicating that they have built up their mass further since their initial formation. The NSCs were found to have systematically lower metallicities than their host galaxies, which is consistent with a scenario for mass assembly through mergers with infalling globular clusters, whilst the presence of younger stellar populations and gas emission in the core of two galaxies is indicative of in-situ star formation. We conclude that the NSCs in these dwarf galaxies likely originated as globular clusters that migrated to the core of the galaxy that have built up their mass mainly through mergers with other infalling clusters, with gas-inflow leading to in-situ star formation playing a secondary role.

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Gaia results for star clusters and dwarf galaxies in the Milky Way

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319006793 ◽

2019 ◽

Vol 14 (S351) ◽

pp. 472-477

Author(s):

Davide Massari

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxies ◽

Dwarf Galaxy ◽

Dwarf Spheroidal Galaxies ◽

Orbital Parameters ◽

The Galaxy ◽

Almost All

AbstractThe second data release of the Gaia mission coupled with ground-based spectroscopic observations has allowed the determination of the orbital parameters for almost all of the Galactic globular clusters, as well as for the known dwarf spheroidal galaxies orbiting the Milky Way. Moreover, it has led to the discovery of dwarf galaxies that were accreted by the Galaxy long ago and that are now completely disrupted. By exploiting their dynamics in combination with the globular clusters age-metallicity relation, we investigated the clusters-to-dwarfs connection. We found that about 60 globulars likely formed in situ, and associated those that were accreted to the dwarf galaxy progenitor they likely formed in.

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The [α/Fe]–[Fe/H] relation in the E-MOSAICS simulations: its connection to the birth place of globular clusters and the fraction of globular cluster field stars in the bulge

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz3341 ◽

2019 ◽

Vol 491 (3) ◽

pp. 4012-4022 ◽

Cited By ~ 7

Author(s):

Meghan E Hughes ◽

Joel L Pfeffer ◽

Marie Martig ◽

Marta Reina-Campos ◽

Nate Bastian ◽

...

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Element Abundances ◽

Field Star ◽

Wide Range ◽

The Galaxy ◽

High Fraction ◽

The Difference

ABSTRACT The α-element abundances of the globular cluster (GC) and field star populations of galaxies encode information about the formation of each of these components. We use the E-MOSAICS cosmological simulations of ∼L* galaxies and their GCs to investigate the [α/Fe]–[Fe/H] distribution of field stars and GCs in 25 Milky Way–mass galaxies. The [α/Fe]–[Fe/H] distribution of GCs largely follows that of the field stars and can also therefore be used as tracers of the [α/Fe]–[Fe/H] evolution of the galaxy. Due to the difference in their star formation histories, GCs associated with stellar streams (i.e. which have recently been accreted) have systematically lower [α/Fe] at fixed [Fe/H]. Therefore, if a GC is observed to have low [α/Fe] for its [Fe/H] there is an increased possibility that this GC was accreted recently alongside a dwarf galaxy. There is a wide range of shapes for the field star [α/Fe]–[Fe/H] distribution, with a notable subset of galaxies exhibiting bimodal distributions, in which the high [α/Fe] sequence is mostly comprised of stars in the bulge, a high fraction of which are from disrupted GCs. We calculate the contribution of disrupted GCs to the bulge component of the 25 simulated galaxies and find values between 0.3 and 14 per cent, where this fraction correlates with the galaxy’s formation time. The upper range of these fractions is compatible with observationally inferred measurements for the Milky Way, suggesting that in this respect the Milky Way is not typical of L*galaxies, having experienced a phase of unusually rapid growth at early times.

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Are we observing an NSC in course of formation in the NGC 4654 galaxy?

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stab458 ◽

2021 ◽

Vol 503 (1) ◽

pp. 594-602

Author(s):

R Schiavi ◽

R Capuzzo-Dolcetta ◽

I Y Georgiev ◽

M Arca-Sedda ◽

A Mastrobuono-Battisti

Keyword(s):

Central Zone ◽

Effective Radius ◽

Host Galaxy ◽

Galactic Centre ◽

Future Evolution ◽

Orbital Parameters ◽

Relative Orbit ◽

The Future ◽

The Galaxy ◽

Massive Clusters

ABSTRACT We use direct N-body simulations to explore some possible scenarios for the future evolution of two massive clusters observed towards the centre of NGC 4654, a spiral galaxy with mass similar to that of the Milky Way. Using archival HST data, we obtain the photometric masses of the two clusters, M = 3 × 105 M⊙ and M = 1.7 × 106 M⊙, their half-light radii, Reff ∼ 4 pc and Reff ∼ 6 pc, and their projected distances from the photometric centre of the galaxy (both <22 pc). The knowledge of the structure and separation of these two clusters (∼24 pc) provides a unique view for studying the dynamics of a galactic central zone hosting massive clusters. Varying some of the unknown cluster orbital parameters, we carry out several N-body simulations showing that the future evolution of these clusters will inevitably result in their merger. We find that, mainly depending on the shape of their relative orbit, they will merge into the galactic centre in less than 82 Myr. In addition to the tidal interaction, a proper consideration of the dynamical friction braking would shorten the merging times up to few Myr. We also investigate the possibility to form a massive nuclear star cluster (NSC) in the centre of the galaxy by this process. Our analysis suggests that for low-eccentricity orbits, and relatively long merger times, the final merged cluster is spherical in shape, with an effective radius of few parsecs and a mass within the effective radius of the order of $10^5\, \mathrm{M_{\odot }}$. Because the central density of such a cluster is higher than that of the host galaxy, it is likely that this merger remnant could be the likely embryo of a future NSC.

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Chemical Abundances of Seven Outer Halo M31 Globular Clusters from the Pan-Andromeda Archaeological Survey

Proceedings of the International Astronomical Union ◽

10.1017/s1743921316011200 ◽

2016 ◽

Vol 11 (S321) ◽

pp. 10-12

Author(s):

Charli M. Sakari

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Large Magellanic Cloud ◽

Archaeological Survey ◽

Chemical Abundances ◽

Light Spectra ◽

Sigma Level ◽

Stellar Streams ◽

Detailed Chemical

AbstractObservations of stellar streams in M31’s outer halo suggest that M31 is actively accreting several dwarf galaxies and their globular clusters (GCs). Detailed abundances can chemically link clusters to their birth environments, establishing whether or not a GC has been accreted from a satellite dwarf galaxy. This talk presents the detailed chemical abundances of seven M31 outer halo GCs (with projected distances from M31 greater than 30 kpc), as derived from high-resolution integrated-light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)—this talk presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] < -1.5), α-enhanced (though they are possibly less alpha-enhanced than Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three GCs (H10, H23, and PA17) are more metal-rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way GCs, and other M31 clusters, H10 and PA17 have moderately-low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17’s high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud.

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