The connection between stellar and nuclear clusters: Can an IMBH be sitting at the heart of the Milky Way?

AbstractA vast number of observed galactic nuclei are known to harbour a central supermassive black hole (SMBH). In their early lifetime, these systems might have witnessed the strong interaction between the SMBH and massive star clusters formed in the inner galactic regions. Due to the strong tidal field exerted from the SMBH, clusters are likely to undergo tidal disruption, releasing their stars all around the SMBH, and possibly driving the formation of a nuclear cluster (NC). This mechanism can contribute to populate galactic nuclei with intermediate-mass black holes (IMBH). Interactions with the central SMBH can lead to the formation of tight massive BH binaries (MBBH) that undergo coalescence via gravitational waves (GW) emission. We discuss this mechanism in the context of the Milky Way centre, exploring the possibility that SgrA*, the Galactic SMBH, has an IMBH companion.

Download Full-text

The accreted nuclear clusters of the Milky Way

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3407 ◽

2020 ◽

Vol 500 (2) ◽

pp. 2514-2524

Author(s):

Joel Pfeffer ◽

Carmela Lardo ◽

Nate Bastian ◽

Sara Saracino ◽

Sebastian Kamann

Keyword(s):

Globular Clusters ◽

Milky Way ◽

Dwarf Galaxy ◽

Star Clusters ◽

Host Galaxy ◽

The Galaxy ◽

Nuclear Clusters ◽

The Common ◽

Massive Clusters ◽

Peculiar Case

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.

Download Full-text

The search for multiple populations in Magellanic Clouds clusters – V. Correlation between cluster age and abundance spreads

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stz1596 ◽

2019 ◽

Vol 487 (4) ◽

pp. 5324-5334 ◽

Cited By ~ 21

Author(s):

S Martocchia ◽

E Dalessandro ◽

C Lardo ◽

I Cabrera-Ziri ◽

N Bastian ◽

...

Keyword(s):

Significant Role ◽

Massive Star ◽

Milky Way ◽

Star Clusters ◽

Magellanic Clouds ◽

Key Factors ◽

Strong Function ◽

Combining Data ◽

Chemical Anomalies ◽

Cluster Age

ABSTRACT In our HST photometric survey, we have been searching for multiple stellar populations (MPs) in Magellanic Clouds (MCs) massive star clusters which span a significant range of ages (∼1.5–11 Gyr). In the previous papers of the series, we have shown that the age of the cluster represents one of the key factors in shaping the origin of the chemical anomalies. Here, we present the analysis of four additional clusters in the MCs, namely Lindsay 38, Lindsay 113, NGC 2121, and NGC 2155, for which we recently obtained new UV HST observations. These clusters are more massive than ∼104 M⊙ and have ages between ∼2.5 and ∼6 Gyr, i.e. located in a previously unexplored region of the cluster age/mass diagram. We found chemical anomalies, in the form of N spreads, in three out of four clusters in the sample, namely in NGC 2121, NGC 2155, and Lindsay 113. By combining data from our survey and HST photometry for three additional clusters in the Milky Way (namely 47 Tuc, M15, and NGC 2419), we show that the extent of the MPs in the form of N spread is a strong function of age, with older clusters having larger N spreads with respect to the younger ones. Hence, we confirm that cluster age plays a significant role in the onset of MPs.

Download Full-text

The MEGaN project – I. Missing formation of massive nuclear clusters and tidal disruption events by star clusters–massive black hole interactions

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/stx1586 ◽

2017 ◽

Vol 471 (1) ◽

pp. 478-490 ◽

Cited By ~ 17

Author(s):

M. Arca-Sedda ◽

R. Capuzzo-Dolcetta

Keyword(s):

Black Hole ◽

Star Clusters ◽

Tidal Disruption ◽

Massive Black Hole ◽

Nuclear Clusters

Download Full-text

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.

Download Full-text

Aligning nuclear cluster orbits with an active galactic nucleus accretion disc

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3004 ◽

2020 ◽

Vol 499 (2) ◽

pp. 2608-2616 ◽

Cited By ~ 1

Author(s):

Gaia Fabj ◽

Syeda S Nasim ◽

Freddy Caban ◽

K E Saavik Ford ◽

Barry McKernan ◽

...

Keyword(s):

Active Galactic Nuclei ◽

Gamma Ray ◽

Galactic Nuclei ◽

Tidal Disruption ◽

Stellar Objects ◽

Binary Black Hole ◽

Disc Model ◽

Inclination Angles ◽

Nuclear Cluster ◽

Repeated Passage

ABSTRACT Active galactic nuclei (AGN) are powered by the accretion of discs of gas on to supermassive black holes (SMBHs). Stars and stellar remnants orbiting the SMBH in the nuclear star cluster (NSC) will interact with the AGN disc. Orbiters plunging through the disc experience a drag force and, through repeated passage, can have their orbits captured by the disc. A population of embedded objects in AGN discs may be a significant source of binary black hole mergers, supernovae, tidal disruption events, and embedded gamma-ray bursts. For two representative AGN disc models, we use geometric drag and Bondi–Hoyle–Littleton drag to determine the time to capture for stars and stellar remnants. We assume a range of initial inclination angles and semimajor axes for circular Keplerian prograde orbiters. Capture time strongly depends on the density and aspect ratio of the chosen disc model, the relative velocity of the stellar object with respect to the disc, and the AGN lifetime. We expect that for an AGN disc density $\rho \gtrsim 10^{-11}{\rm g\, cm^{-3}}$ and disc lifetime ≥1 Myr, there is a significant population of embedded stellar objects, which can fuel mergers detectable in gravitational waves with LIGO-Virgo and LISA.

Download Full-text

From nuclear clusters to halo globulars: Star clusters as basic galactic building blocks

Proceedings of the International Astronomical Union ◽

10.1017/s1743921306005035 ◽

2006 ◽

Vol 2 (S235) ◽

pp. 48-51

Author(s):

Richard de Grijs

Keyword(s):

Star Formation ◽

Globular Clusters ◽

Star Clusters ◽

Galactic Nuclei ◽

Building Blocks ◽

Dominant Mode ◽

Star Formation History ◽

Formation Mode ◽

Formation History ◽

Nuclear Clusters

AbstractI assess the similarities and differences between the star-formation modes in quiescent spiral galaxies versus those in violent starburst regions, including galactic nuclei. As opposed to the quiescent star-formation mode, current empirical evidence on the star-formation processes in the extreme, high-pressure environments induced by galaxy encounters strongly suggests that star cluster formation is an important and perhaps even the dominant mode of star formation in such starburst events. This implies that by using star clusters as unique diagnostic probes, we can trace a galaxy's most violent star formation history very well, at least for the past few Gyr. The sizes, luminosities, and mass estimates of the young massive star clusters are entirely consistent with what is expected for young Milky Way-type globular clusters (GCs). Recent evidence lends support to the scenario that GCs, which were once thought to be the oldest building blocks of galaxies, are still forming today.

Download Full-text

Calibrating the binary black hole population in nuclear star clusters through tidal disruption events

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa3493 ◽

2020 ◽

Vol 500 (4) ◽

pp. 4307-4318

Author(s):

Giacomo Fragione ◽

Rosalba Perna ◽

Abraham Loeb

Keyword(s):

Black Holes ◽

Black Hole ◽

Star Clusters ◽

Mass Function ◽

Intermediate Mass ◽

Tidal Disruption ◽

Scientific Question ◽

Binary Black Hole ◽

The Masses ◽

Optical Transient

ABSTRACT As the sensitivity of gravitational wave (GW) instruments improves and new networks start operating, hundreds of merging stellar-mass black holes (SBHs) and intermediate-mass black holes (IMBHs) are expected to be observed in the next few years. The origin and distribution of SBH and IMBH binaries in various dynamical environments is a fundamental scientific question in GW astronomy. In this paper, we discuss ways tidal disruption events (TDEs) may provide a unique electromagnetic window into the assembly and merger of binary SBHs and IMBHs in nuclear star clusters (NSCs). We discuss how the host NSC mass and density and the slope of the BH mass function set the orbital properties and the masses of the binaries that undergo a TDE. For typical NSC properties, we predict a TDE rate of ∼10−6–10−7 yr−1 per galaxy. The light curve of TDEs in NSCs could be interrupted and modulated by the companion BH on the orbital period of the binary. These should be readily detectable by optical transient surveys such as the Zwicky Transient Facility and LSST.

Download Full-text