Nuclear star cluster
A nuclear star cluster (NSC) or compact stellar nucleus (sometimes called young stellar nucleus) is a star cluster with high density and high luminosity near the center of mass of many galaxies, including the Milky Way.[1][2]
NSCs are the central massive objects of fainter, low-mass galaxies where supermassive black holes (SMBHs) often co-exist or are not present. In the most massive galaxies, NSCs are entirely absent. Some galaxies, including the Milky Way, are known to contain both a NSC and a SMBH of comparable mass.[3] The co-existence of massive black holes and dense nuclear star clusters was quantified in 2009.[2] Following this, a scaling relation between the mass of the central supermassive black hole and the mass of the nuclear star cluster was discovered in 2016.[4] This relation is important for understanding the co-evolution of these central massive objects and for predicting rates of extreme mass-ratio inspirals (EMRIs) detectable by future space-based gravitational wave observatories such as LISA.
Properties
[edit]Nuclear star clusters are found in most galaxies that can be resolved sufficiently:[5]
- at least 50% of all early spiral galaxies (types Sa-Sc)
- at least 75% of all late spiral galaxies (types Scd-Sm)
- at least 70% of all spheroidal galaxies (types S0 and E).
NSCs are the densest known star clusters in the Universe. With apparent magnitudes between -14 and -10 mag in the infrared, they are on average 40 times brighter than globular clusters, although their effective radii are not larger than 2 to 5 parsecs. With a dynamic mass of 106 to 108 solar masses, they are at the upper end of the values reached by globular clusters.[5]
The majority of nuclear star clusters contain a mix of old (at least one billion years old) and young stellar populations and show signs of star formation within the last 100 million years.[3]
Formation
[edit]Although the mechanisms behind their formation are not entirely known, hypotheses provide four possibilities:[6][7]
- Nuclear star clusters originate somewhere else and are captured by a central black hole.
- Nuclear star clusters are due to an incidence of gas at some distance from the center of the galaxy.
- A combination of the above possibilities whereby the gravitational potential of a trapped object, such as the nucleus of a dwarf galaxy, triggers new star formation by incident gas near the galactic center.
- Nuclear star clusters are created by merging star clusters with subsequent migration to the galactic center due to dynamical friction with background stars.[8]
Relationship with globular clusters
[edit]Because nuclear star clusters occur in most galaxy species, they should still be present in the halo of the resulting galaxy after the fusion of galaxies. This is a hypothesis for the formation of globular clusters. Thus, globular clusters could be the remains of nuclear star clusters excluded from gas incidence, in which no new star formation occurs.[9]
According to other hypotheses, however, the nuclear star clusters could be the result of a fusion of globular clusters captured by a supermassive black hole in the center of the galaxy and dynamically destroyed.[10]
References
[edit]- ↑ Schödel, R.; Merritt, D.; Eckart, A. (2009). "The nuclear star cluster of the Milky Way: proper motions and mass". Astronomy & Astrophysics. 502 (1): 91–111. arXiv:0902.3892. Bibcode:2009A&A...502...91S. doi:10.1051/0004-6361/200810922.
- 1 2 Graham, A. W.; Spitler, L. R. (2009). "Quantifying the coexistence of massive black holes and dense nuclear star clusters". Monthly Notices of the Royal Astronomical Society. 397 (4): 2148–2156. arXiv:0907.5250. Bibcode:2009MNRAS.397.2148G. doi:10.1111/j.1365-2966.2009.15118.x.
- 1 2 Nishiyama, S.; Schödel, R. (2012). "Young, Massive Star Candidates Detected throughout the Nuclear Star Cluster of the Milky Way". Astronomy & Astrophysics. 549: A57. arXiv:1210.6125. Bibcode:2012yCat..35490057N. doi:10.1051/0004-6361/201219773.
- ↑ Graham, A. W. (2016). "Black hole and nuclear cluster scaling relations: Mbh ∝ Mnc2.7±0.7". Proceedings of the International Astronomical Union. 312: 269–276. arXiv:1412.5715. Bibcode:2016IAUS..312..269G. doi:10.1017/S1743921315008017.
- 1 2 Boeker, T (2009). "Nuclear Star Clusters". Proceedings of the International Astronomical Union. 266: 58–63. arXiv:0910.4863. Bibcode:2010IAUS..266...58B. doi:10.1017/S1743921309990871.
- ↑ Antonini, F. (2012). "Origin and growth of nuclear star clusters around massive black holes". The Astrophysical Journal. 763 (1): 62. arXiv:1207.6589. Bibcode:2013ApJ...763...62A. doi:10.1088/0004-637X/763/1/62.
- ↑ Madigan, A.; Levin, Y.; Pfuhl, O.; Gillessen, S.; Genzel, R.; Perets, H. B. (2013). "On the origin of the B-stars in the Galactic center". The Astrophysical Journal. 784 (1): 23. arXiv:1305.1625. Bibcode:2014ApJ...784...23M. doi:10.1088/0004-637X/784/1/23.
- ↑ Capuzzo-Dolcetta, R. (1993). "The Evolution of the Globular Cluster System in a Triaxial Galaxy: Can a Galactic Nucleus Form by Globular Cluster Capture?". The Astrophysical Journal. 763 (1): 62. arXiv:astro-ph/9301006. Bibcode:1993ApJ...415..616C. doi:10.1086/173189.
- ↑ Scott, N.; Graham, A. (2012). "Shifting Fundamentals: Scaling Relations involving Nuclear Star Clusters and Supermassive Black Holes". The Astrophysical Journal. 763 (2): 76. arXiv:1205.5338. Bibcode:2013ApJ...763...76S. doi:10.1088/0004-637X/763/2/76.
- ↑ Capuzzo-Dolcetta, R. (2013). "Galactic Nuclear Cluster Formation Via Globular Cluster Mergers". Memorie della Societa Astronomica Italiana. 84: 167. arXiv:1301.2899. Bibcode:2013MmSAI..84..167C.