The nuclear cluster of the Milky Way: our primary testbed for the interaction of a dense star cluster with a massive black hole

AbstractBecause of its nearness to Earth, the centre of the Milky Way is the only galaxy nucleus in which we can study the characteristics, distribution, kinematics, and dynamics of the stars on milli-parsec scales. We have accurate and precise measurements of the Galactic centre's central black hole, Sagittarius A*, and can study its interaction with the surrounding nuclear star cluster in detail. This contribution aims at providing a concise overview of our current knowledge about the Milky Way's central black hole and nuclear star cluster, at highlighting the observational challenges and limitations, and at discussing some of the current key areas of investigation.

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The Milky Way’s nuclear star cluster: Old, metal-rich, and cuspy

Astronomy and Astrophysics ◽

10.1051/0004-6361/201936688 ◽

2020 ◽

Vol 641 ◽

pp. A102 ◽

Cited By ~ 4

Author(s):

R. Schödel ◽

F. Nogueras-Lara ◽

E. Gallego-Cano ◽

B. Shahzamanian ◽

A. T. Gallego-Calvente ◽

...

Keyword(s):

Black Hole ◽

Star Formation ◽

Milky Way ◽

Star Cluster ◽

Star Formation History ◽

Central Black Hole ◽

Formation History ◽

History Of ◽

Nuclear Cluster ◽

Sgr A

Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars. Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*. Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are ≥50% complete down to Ks ≈ 19 within a projected radius of 5″ around Sgr A*. We used H-band images to derive extinction maps. Results. We provide Ks photometry for roughly 39 000 stars and H-band photometry for ∼11 000 stars within a field of about 40″ × 40″, centred on Sgr A*. In addition, we provide Ks photometry of ∼3000 stars in a very deep central field of 10″ × 10″, centred on Sgr A*. We find that the Ks luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between Ks = 15−19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster’s mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way.

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The (quite dark) stellar cluster around the supermassive black hole Sagittarius A* at the center of the Milky Way

Proceedings of the International Astronomical Union ◽

10.1017/s1743921308017663 ◽

2007 ◽

Vol 3 (S245) ◽

pp. 207-210

Author(s):

Rainer Schödel ◽

A. Eckart

Keyword(s):

Black Hole ◽

Adaptive Optics ◽

Near Infrared ◽

Milky Way ◽

Infrared Imaging ◽

Current Theory ◽

Star Cluster ◽

Massive Black Hole ◽

Stellar Cluster ◽

Sagittarius A

AbstractHigh-resolution seeing limited and adaptive optics near-infrared imaging observations of the stellar cluster within about one parsec of the massive black hole Sagittarius A* allow us to obtain a detailed picture of the structure of the nuclear star cluster of the Milky Way. We find that the stellar number counts and the diffuse light of the unresolved stellar population can be described very well by a stellar density function in the form of a broken-power law. This agrees well with theoretical predictions on the structure of a dynamically relaxed star cluster around a massive black hole. However, the cusp slope is found to be too shallow, which may be related to mixing of different stellar populations and continuous star formation, phenomena that are not taken into account by current theory. Mass densities larger than 107 solar masses per pc3 are reached within 0.1 pc of the central black hole. Intriguingly, up to several tens of percent of the total cluster mass in the central parsec may be in the form of dark stellar remnants.

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First Conclusive Evidence for a Massive Black Hole in the Center of the Milky Way

Black Holes: Theory and Observation - Lecture Notes in Physics ◽

10.1007/978-3-540-49535-2_3 ◽

2003 ◽

pp. 60-68

Author(s):

Andreas Eckart ◽

Reinhard Genzel

Keyword(s):

Black Hole ◽

Milky Way ◽

Conclusive Evidence ◽

Massive Black Hole

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Very high redshift quasars and the rapid emergence of super-massive black holes

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa2276 ◽

2020 ◽

Cited By ~ 2

Author(s):

Pavel Kroupa ◽

Ladislav Subr ◽

Tereza Jerabkova ◽

Long Wang

Keyword(s):

Black Hole ◽

Massive Star ◽

High Redshift ◽

Host Galaxy ◽

Massive Black Hole ◽

First Stars ◽

Massive Black Holes ◽

Nuclear Cluster ◽

Gas Accretion ◽

Very High

Abstract The observation of quasars at very high redshift such as Pōniuā’ena is a challenge for models of super-massive black hole (SMBH) formation. This work presents a study of SMBH formation via known physical processes in star-burst clusters formed at the onset of the formation of their hosting galaxy. While at the early stages hyper-massive star-burst clusters reach the luminosities of quasars, once their massive stars die, the ensuing gas accretion from the still forming host galaxy compresses its stellar black hole (BH) component to a compact state overcoming heating from the BH–BH binaries such that the cluster collapses, forming a massive SMBH-seed within about a hundred Myr. Within this scenario the SMBH–spheroid correlation emerges near-to-exactly. The highest-redshift quasars may thus be hyper-massive star-burst clusters or young ultra-compact dwarf galaxies (UCDs), being the precursors of the SMBHs that form therein within about 200 Myr of the first stars. For spheroid masses ≲ 109.6 M⊙ a SMBH cannot form and instead only the accumulated nuclear cluster remains. The number evolution of the quasar phases with redshift is calculated and the possible problem of missing quasars at very high redshift is raised. SMBH-bearing UCDs and the formation of spheroids are discussed critically in view of the high redshift observations. A possible tension is found between the high star-formation rates (SFRs) implied by downsizing and the observed SFRs, which may be alleviated within the IGIMF theory and if the downsizing times are somewhat longer.

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The Nuclear Star Cluster of the Milky Way: Star Formation, Stellar Collisions and Central Dark Mass

Symposium - International Astronomical Union ◽

10.1017/s0074180900001418 ◽

1996 ◽

Vol 174 ◽

pp. 81-90

Author(s):

R. Genzel

Keyword(s):

Star Formation ◽

Near Infrared ◽

Milky Way ◽

Infrared Imaging ◽

Star Cluster ◽

Structure Evolution ◽

Late Type ◽

Massive Black Hole ◽

Imaging And Spectroscopy ◽

Central Parsec

High resolution near-infrared imaging and spectroscopy now gives detailed information about the structure, evolution and mass distribution in the nuclear star cluster of the Milky Way. The central parsec is powered by a cluster of luminous and helium rich, blue supergiants/Wolf-Rayet stars. The most likely scenario for the formation of the massive stars is a star formation burst a few million years ago at which time a dense gas cloud may have fallen into the center. The stellar density in the ∼ 0.3 pc radius central core is high enough that collisions with main sequence stars destroy the largest late type giant stars. Radial velocity measurements for about 300 early and late type stars between 0.1 and 5pc radius from the dynamic center now strongly favor the existence of a central dark mass of 2.5 − 3.3 × 106M⊙ (density (109M⊙pc−3, M/L2μm) ∼ 100M⊙/L⊙) within 0.1pc of the dynamic center. This central dark mass cannot be a cluster of neutron stars. It is either a compact cluster of stellar black holes or, most likely, a single massive black hole.

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The Core Radius of a Star Cluster Containing a Massive Black Hole

Publications of the Astronomical Society of Japan ◽

10.1093/pasj/59.3.l11 ◽

2007 ◽

Vol 59 (3) ◽

pp. L11-L14 ◽

Cited By ~ 23

Author(s):

Douglas C. Heggie ◽

Piet Hut ◽

Shin Mineshige ◽

Junichiro Makino ◽

Holger Baumgardt

Keyword(s):

Black Hole ◽

Star Cluster ◽

Core Radius ◽

Massive Black Hole ◽

The Core

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Potential Black Hole Seeding of the Spiral Galaxy NGC 4424 via an Infalling Star Cluster

The Astrophysical Journal ◽

10.3847/1538-4357/ac235b ◽

2021 ◽

Vol 923 (2) ◽

pp. 146

Author(s):

Alister W. Graham ◽

Roberto Soria ◽

Bogdan C. Ciambur ◽

Benjamin L. Davis ◽

Douglas A. Swartz

Keyword(s):

Black Hole ◽

Spiral Galaxy ◽

Near Infrared ◽

High Energy ◽

Star Cluster ◽

The Body ◽

Stellar Structure ◽

Massive Black Hole ◽

X Ray ◽

Low Mass

Abstract Galaxies can grow through their mutual gravitational attraction and subsequent union. While orbiting a regular high-surface-brightness galaxy, the body of a low-mass galaxy can be stripped away. However, the stellar heart of the infalling galaxy, if represented by a tightly bound nuclear star cluster, is more resilient. From archival Hubble Space Telescope images, we have discovered a red, tidally stretched star cluster positioned ∼5″ (∼400 pc in projection) from, and pointing toward the center of, the post-merger spiral galaxy NGC 4424. The star cluster, which we refer to as “Nikhuli,” has a near-infrared luminosity of (6.88 ± 1.85) × 106 L ⊙,F160W and likely represents the nucleus of a captured/wedded galaxy. Moreover, from our Chandra X-ray Observatory image, Nikhuli is seen to contain a high-energy X-ray point source, with L 0.5 − 8 keV = 6.31 − 3.77 + 7.50 × 10 38 erg s−1 (90% confidence). We argue that this is more likely to be an active massive black hole than an X-ray binary. Lacking an outward-pointing comet-like appearance, the stellar structure of Nikhuli favors infall rather than the ejection from a gravitational-wave recoil event. A minor merger with a low-mass early-type galaxy may have sown a massive black hole, aided an X-shaped pseudobulge, and be sewing a small bulge. The stellar mass and the velocity dispersion of NGC 4424 predict a central black hole of (0.6–1.0) × 105 M ⊙, similar to the expected intermediate-mass black hole in Nikhuli, and suggestive of a black hole supply mechanism for bulgeless late-type galaxies. We may potentially be witnessing black hole seeding by capture and sinking, with a nuclear star cluster the delivery vehicle.

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