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| [[Image:Messier75.jpg|thumb|right|300px|Fig. 1. A hypercompact stellar system would look very much like a [[star cluster]], as shown here. However the stars would orbit much faster than in an ordinary cluster due to the gravitational force from the [[supermassive black hole]] at the center.
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| A '''hypercompact stellar system''' ('''HCSS''') is a dense [[star cluster|cluster of stars]] around a [[supermassive black hole]] that has been ejected from the center of its host [[galaxy]]. Stars that are close to the black hole at the time of the ejection will remain bound to the black hole after it leaves the galaxy, forming the HCSS.
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| The term "hypercompact" refers to the fact HCSSs are small in size compared with ordinary star clusters of similar [[luminosity]]. This is because the gravitational force from the supermassive black hole keeps the stars moving in very tight orbits about the center of the cluster.
| | If you are looking for a specific plugin, then you can just search for the name of the plugin. Affilo - Theme is the guaranteed mixing of wordpress theme that Mark Ling use for his internet marketing career. PSD files are incompatible to browsers and are suppose to be converted into wordpress compatible files so that it opens up in browser. Out of the various designs of photography identified these days, sports photography is preferred most, probably for the enjoyment and enjoyment associated with it. By using this method one can see whether the theme has the potential to become popular or not and is their any scope of improvement in the theme. <br><br>If you have any sort of inquiries concerning where and ways to use [http://www.hipsterpunk.com/u/shq52/ backup plugin], you can contact us at the web-site. These folders as well as files have to copied and the saved. The higher your blog ranks on search engines, the more likely people will find your online marketing site. We also help to integrate various plug-ins to expand the functionalities of the web application. This is identical to doing a research as in depth above, nevertheless you can see various statistical details like the number of downloads and when the template was not long ago updated. Once you've installed the program you can quickly begin by adding content and editing it with features such as bullet pointing, text alignment and effects without having to do all the coding yourself. <br><br>This gives a clearer picture that online shoppers are familiar with the WP ecommerce system. s cutthroat competition prevailing in the online space won. If Gandhi was empowered with a blogging system, every event in his life would have been minutely documented so that it could be recounted to the future generations. Every single Theme might be unique, providing several alternatives for webpage owners to reap the benefits of in an effort to instantaneously adjust their web page appear. Customization of web layout is easy due to the availability of huge selection of templates. <br><br>The disadvantage is it requires a considerable amount of time to set every thing up. Quttera - Quttera describes itself as a 'Saa - S [Software as a Service] web-malware monitoring and alerting solution for websites of any size and complexity. Thus it is difficult to outrank any one of these because of their different usages. If you choose a blog then people will be able to post articles on your site and people will be able to make comments on your posts (unless you turn comments off). Where from they are coming, which types of posts are getting top traffic and many more. <br><br>Yet, overall, less than 1% of websites presently have mobile versions of their websites. Visit our website to learn more about how you can benefit. Offshore Wordpress development services from a legitimate source caters dedicated and professional services assistance with very simplified yet technically effective development and designing techniques from experienced professional Wordpress developer India. In addition, Word - Press design integration is also possible. Customers within a few seconds after visiting a site form their opinion about the site. |
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| No HCSS has yet been found, although they are believed to be fairly common. Finding an HCSS would confirm the theory of [[binary black hole#Black hole merger recoil|gravitational wave recoil]], and would prove that supermassive black holes can exist outside of galaxies.
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| ==Properties==
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| Astronomers believe that supermassive black holes (SMBHs) can be ejected from the centers of galaxies by gravitational wave recoil. This happens when two SMBHs in a binary system coalesce, after losing energy in the form of [[gravitational wave]]s. Because the gravitational waves are not emitted [[isotropic]]ally, some [[momentum]] is imparted to the coalescing black holes, and they feel a recoil, or "kick," at the moment of coalescence. [[Numerical relativity|Computer simulations]] suggest that the kick can be as large as 10000 kilometers per second,<ref name="Healy09">{{Citation
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| | last1 = Healy | first1 = J.
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| | last2 = Herrmann | first2 = F.
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| | last3 = Shoemaker | first3 = D. M.
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| | last4 = Laguna | first4 = P.
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| | last5 = Matzner | first5 = R. A.
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| | title = Superkicks in Hyperbolic Encounters of Binary Black Holes
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| | journal = Physical Review Letters
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| | volume = 102
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| | issue = 4
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| | pages = 041101–041105
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| | bibcode = 2009PhRvL.102d1101H
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| | year = 2009
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| | doi = 10.1103/PhysRevLett.102.041101
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| | last6 = Matzner
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| | first6 = Richard|arxiv = 0807.3292 }}
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| </ref>
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| which exceeds [[escape velocity]] from the centres of even the most massive galaxies.<ref name="MMFHH04">{{Citation
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| | last1 = Merritt | first1 = D.
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| | author-link = David Merritt
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| | last2 = Milosavljevic | first2 = M.
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| | last3 = Favata | first3 = M.
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| | last4 = Hughes | first4 = S. A.
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| | last5 = Holz | first5 = D. E.
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| | title = Consequences of Gravitational Radiation Recoil
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| | journal = The Astrophysical Journal
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| | volume = 607
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| | issue = 1
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| | pages = L9–L12
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| | bibcode = 2004ApJ...607L...9M
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| | year = 2004
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| | doi = 10.1086/421551|arxiv = astro-ph/0402057 }}
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| </ref>
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| Stars that are orbiting around the SMBH at the moment of the kick will be dragged along with the SMBH, providing their orbital velocity exceeds the kick velocity {{math|''V<sub>k</sub>''}}. This is what determines the size of the HCSS: its radius is roughly the radius of the orbit that has the same velocity around the SMBH as the kick velocity, or
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| :<math>R=\frac{GM}{V_k^2}</math>
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| where {{math|''M''}} is the [[mass]] of the SMBH and {{math|''G''}} the [[gravitational constant]]. The size {{math|''R''}} works out to be roughly one-half [[parsec]] (pc) (two [[light year]]s) for a kick of 1000 km/s and a SMBH mass of 100 million [[solar mass]]es. The largest HCSSs would have sizes of about 20 pc, roughly the same as a large [[globular cluster]], and the smallest would be about a thousandth of a parsec across, smaller than any known star cluster.<ref name="MSK09">{{Citation
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| | last1 = Merritt | first1 = D.
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| | author-link = David Merritt
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| | last2 = Schnittman | first2 = J. D.
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| | last3 = Komossa | first3 = S.
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| | title = Hypercompact Stellar Systems Around Recoiling Supermassive Black Holes
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| | journal = The Astrophysical Journal
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| | volume = 699
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| | issue = 2
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| | pages = 1690–1710
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| | bibcode = 2009ApJ...699.1690M
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| | year = 2009
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| | doi = 10.1088/0004-637X/699/2/1690|arxiv = 0809.5046 }}
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| </ref> | |
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| The number of stars that remain bound to the SMBH after the kick depends both on {{math|''V<sub>k</sub>''}}, and on how densely the stars were clustered about the SMBH before the kick. A number of arguments suggest that the total stellar mass would be roughly 0.1% of the mass of the SMBH or less.<ref name=MSK09/> The biggest HCSSs would carry perhaps a few million stars, making them comparable in luminosity to a [[globular cluster]] or [[Dwarf galaxy#Ultra Compact Dwarfs|ultra-compact dwarf galaxy]].
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| Aside from being very compact, the main difference between an HCSS and an ordinary star cluster is the much greater mass of the HCSS, due to the SMBH at its centre. The SMBH itself is dark and undetectable, but its gravity causes the stars to move at much higher velocities than in an ordinary star cluster. Normal star clusters have internal velocities of a few kilometers per second, while in an HCSS, essentially all the stars are moving faster than {{math|''V<sub>k</sub>''}}, i.e. hundreds or thousands of kilometers per second.
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| If the kick velocity is less than the [[escape velocity]] from the galaxy, the SMBH will fall back toward the galaxy nucleus, oscillating many times through the galaxy before finally coming to rest.<ref name="GM08">{{Citation
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| | last1 = Gualandris | first1 = A.
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| | last2 = Merritt | first2 = D.
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| | author2-link = David Merritt
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| | title = Ejection of Supermassive Black Holes from Galaxy Cores
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| | journal = The Astrophysical Journal
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| | volume = 678
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| | issue = 2
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| | pages = 780–796
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| | year = 2008
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| | bibcode = 2008ApJ...678..780G
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| | doi = 10.1086/586877|arxiv = 0708.0771 }}
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| </ref> In this case, the HCSS would only exist as a distinct object for a relatively short time, of order hundreds of millions of years, before disappearing back into the galaxy nucleus. During this time the HCSS would be difficult to detect since it would be superposed on or behind the galaxy. | |
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| Even if an HCSS escapes from its host galaxy, it will remain bound to the [[galaxy group|group]] or [[galaxy cluster|cluster]] that contains the galaxy, since the escape velocity from a cluster of galaxies is much larger than that from a single galaxy. When observed, the HCSS will be moving more slowly than {{math|''V<sub>k</sub>''}}, since it will have climbed out through the gravitational [[potential well]] of the galaxy and/or cluster.
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| The stars in an HCSS would be similar to the types of stars that are observed in galactic nuclei. This would make the stars in an HCSS more [[stellar metallicity|metal-rich]] and younger than the stars in a typical globular cluster.<ref name=MSK09/>
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| ==Finding a HCSS==
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| Since the black hole at the center of the HCSS is essentially invisible, an HCSS would look very similar to a faint cluster of stars (see figure). Determining that an observed star cluster is a HCSS requires measuring the orbital velocities of the stars in the cluster via their [[Doppler shift]]s and verifying that they are moving much faster than expected for stars in an ordinary star cluster. This is a challenging observation to make because an HCSS would be relatively faint, requiring many hours of exposure time even on a [[List of largest optical reflecting telescopes|10m class]] telescope.
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| The most promising places to look for HCSSs are [[galaxy cluster|cluster of galaxies]], for two reasons: (1) Most of the galaxies in a galaxy cluster are [[elliptical galaxy|elliptical galaxies]] which are believed to have formed through [[galaxy merger|mergers]]. A galaxy merger is a prerequisite for forming a binary SMBH, which is a prerequisite for a kick. (2) The escape velocity from a galaxy cluster is large enough that a HCSS would be retained even if it escaped from its host galaxy.
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| It has been estimated that the nearby [[Fornax cluster|Fornax]] and [[Virgo cluster|Virgo]] galaxy clusters may contain hundreds or thousands of HCSSs.<ref name=MSK09/> These galaxy clusters have been surveyed for compact galaxies and star clusters. It is possible that some of the objects picked up in these surveys were HCSSs that were misidentified as ordinary star clusters. A few of the compact objects in the surveys are known to have rather high internal velocities, but none appear to be massive enough to qualify as HCSSs.<ref name="Mieske08">{{Citation
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| | last = Mieske | first1 = S. et al.
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| | title = The nature of UCDs: Internal dynamics from an expanded sample and homogeneous database
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| | journal = Astronomy and Astrophysics
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| | volume = 487
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| | issue = 3
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| | pages = 921–935
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| | bibcode = 2008A&A...487..921M
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| | year = 2008
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| | doi = 10.1051/0004-6361:200810077
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| | last2 = Hilker
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| | first2 = M.
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| | last3 = Jordán
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| | first3 = A.
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| | last4 = Infante
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| | first4 = L.
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| | last5 = Kissler-Patig
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| | first5 = M.
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| | last6 = Rejkuba
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| | first6 = M.
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| | last7 = Richtler
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| | first7 = T.
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| | last8 = Côté
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| | first8 = P.
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| | last9 = Baumgardt
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| | first9 = H.|arxiv = 0806.0374 }}
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| </ref> | |
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| Another likely place to find a HCSS would be near the site of a recent [[galaxy merger]].
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| From time to time, the black hole at the center of an HCSS will disrupt a star that passes too close, producing a very luminous flare. A few such flares have been observed at the ''centers'' of galaxies, presumably caused by a star coming too close to the SMBH in the galaxy nucleus.<ref name="Komossa04">{{cite conference
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| | first = S.
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| | last = Komossa
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| | title = The Extremes of (X-ray) Variability Among Galaxies: Flares from Stars Tidally Disrupted by Supermassive Black Holes
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| | booktitle = The Interplay Among Black Holes, Stars and ISM in Galactic Nuclei
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| | url = http://journals.cambridge.org/action/displayAbstract?fromPage=online&aid=260531
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| | pages = 45–48
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| | year = 2004 }}
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| </ref> It has been estimated that a recoiling SMBH will disrupt about a dozen stars during the time it takes to escape from its galaxy.<ref name="KM08">{{Citation
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| | last1 = Komossa | first1 = S.
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| | last2 = Merritt | first2 = D.
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| | author2-link = David Merritt
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| | title = Tidal Disruption Flares from Recoiling Supermassive Black Holes
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| | journal = The Astrophysical Journal
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| | volume = 683
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| | issue = 1
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| | pages = L21–L24
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| | year = 2009
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| | bibcode = 2008ApJ...683L..21K
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| | doi = 10.1086/591420|arxiv = 0807.0223 }}
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| </ref> Since the lifetime of a flare is a few months, the chances of seeing such an event are small unless a large volume of space is surveyed. A star in a HCSS could also explode as a [[supernove|Type I]] ([[white dwarf]]) [[supernova]].<ref name=KM08/>
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| ==Importance==
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| Discovery of an HCSS would be important for several reasons.
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| * It would constitute proof that supermassive black holes can exist outside of galaxies.
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| * It would verify the [[numerical relativity#Recent developments|computer simulations]] that predict gravitational wave recoils of thousands of kilometers per second.
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| * Existence of HCSSs would imply that some galaxies do not have supermassive black holes at their centers. This would have important consequences for theories that link the growth of galaxies to the growth of supermassive black holes, and for [[M-sigma relation|empirical correlations]] between SMBH mass and galaxy properties.
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| * If many HCSSs could be discovered, it would be possible to reconstruct the [[probability distribution|distribution]] of kick velocities, which contains information about the merger history of galaxies, the masses and [[spin (physics)|spin]]s of binary black holes, etc.
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| ==See also==
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| *[[Stellar dynamics]]
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| *[[Numerical relativity]]
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| ==References==
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| {{reflist}}
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| ==External links==
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| * [http://news.nationalgeographic.com/news/2009/07/090710-new-black-hole-type.html New Type of Cosmic Object to Be Revealed?] ''National Geographic News'' article on hypercompact stellar systems.
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| * [http://space.newscientist.com/article/mg19926675.300-mangled-stars-could-reveal-ejected-black-holes.html?DCMP=ILC-hmts&nsref=news1_head_mg19926675.300 Mangled Stars Could Reveal Ejected Black Holes] ''New Scientist'' article on tidal disruption flares from recoiling black holes.
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| {{black holes}}
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| {{Stellar system}}
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| [[Category:Astrophysics]]
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| [[Category:Galaxies]]
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| [[Category:Star clusters]]
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| [[Category:Supermassive black holes]]
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