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Ultra-Compact Dwarf galaxies
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Many tiny galaxies could host mammoth black holes

Little galaxies can pack a mighty punch. Astronomers detected and carefully weighed the black holes residing at the centres of a pair of extremely compact galaxies, finding to their surprise that they make up a large fraction of the mass of the host galaxies themselves.
These "ultra-compact dwarf galaxies" are now the second and third ones known to house such gargantuan black holes, confirming that the first one - discovered by the same group in 2014 - was not an anomaly.

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Hiding in Plain Sight: Undergraduates Discover the Densest Galaxies Known

Two undergraduates at San José State University have discovered two galaxies that are the densest known. Similar to ordinary globular star clusters but a hundred to a thousand times brighter, the new systems have properties intermediate in size and luminosity between galaxies and star clusters
The first system discovered by the investigators, M59-UCD3, has a width two hundred times smaller than our own Milky Way Galaxy and a stellar density 10,000 times larger than that in the neighbourhood of the Sun.
The stellar density of the second system, M85-HCC1, is higher still: about a million times that of the Solar neighbourhood. Both systems belong to the new class of galaxies known as ultracompact dwarfs (UCDs).

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Title: Sowing the seeds of massive black holes in small galaxies: Young clusters as the building blocks of Ultra-Compact-Dwarf Galaxies
Authors: Pau Amaro-Seoane, Symeon Konstantinidis, Marc Dewi Freitag, M. Coleman Miller, Frederic A. Rasio

Interacting galaxies often have complexes of hundreds of young stellar clusters of individual masses ~ 10^{4-6} solar masses in regions that are a few hundred parsecs across. These cluster complexes interact dynamically, and their coalescence is a candidate for the origin of some ultracompact dwarf galaxies (UCDs). Individual clusters with short relaxation times are candidates for the production of intermediate-mass black holes of a few hundred solar masses, via runaway stellar collisions prior to the first supernovae in a cluster. It is therefore possible that a cluster complex hosts multiple intermediate-mass black holes that may be ejected from their individual clusters due to mergers or binary processes, but bound to the complex as a whole. Here we explore the dynamical interaction between initially free-flying massive black holes and clusters in an evolving cluster complex. We find that, after hitting some clusters, it is plausible that the massive black hole will be captured in an ultracompact dwarf forming near the center of the complex. In the process, the hole typically triggers electromagnetic flares via stellar disruptions, and is also likely to be a prominent source of gravitational radiation for the advanced ground-based detectors LIGO and VIRGO. We also discuss other implications of this scenario, notably that the central black hole could be considerably larger than expected in other formation scenarios for ultracompact dwarfs.

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RE: Ultra-Compact Dwarf (UCDs) galaxies
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Ultra-compact dwarf galaxies are bright star clusters

Astronomy & Astrophysics is publishing the results of a detailed investigation of how many 'ultra-compact dwarf galaxies' (UCDs) can be found in nearby galaxy clusters. UCDs were recognised as a populous and potentially distinct class of stellar systems about a decade ago. But they are still mysterious objects that are characterised by a compact morphology (30-300 light-years in size) and high masses (more than one million solar masses). More generally, their properties (e.g., their size, shape, or luminosity) are similar to those of both star clusters and dwarf galaxies. Several hundred UCDs have been found to date. Two main formation channels for these puzzling objects have been proposed so far. UCDs might either be very massive star clusters or be 'normal' dwarf galaxies transformed by tidal effects.
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Title: The specific frequencies of ultra-compact dwarf galaxies
Authors: S. Mieske, M. Hilker and I. Misgeld

Context. One formation channel discussed for ultra-compact dwarf galaxies (UCDs) is that of massive star clusters, and the other main scenario is that of tidally transformed dwarf galaxies.
Aims. We aim at quantifying the specific frequency of UCDs in a range of environments and at relating this to the frequency of star clusters and potential progenitor dwarf galaxies. Are the frequencies of UCDs consistent with being the bright tail of the globular cluster luminosity function (GCLF)?
Methods. We propose a definition for the specific frequency of UCDs, S_N,UCD = N_UCD10^0.4(MV,host - MV,0)cw. The parameter MV,0 is the zero point of the definition, chosen such that the specific frequency of UCDs is the same as those of globular clusters, SN,GC, if UCDs follow a simple extrapolation of the GCLF. Considering UCDs as compact stellar systems with MV < -10.25 mag (mass above ~2 x 10^6 solar masses), it is MV,0 = -20 mag. The parameter cw is a correction term to take the dependence of the GCLF width sigma on the host galaxy luminosity into account. We apply our definition of S_N,UCD to results of spectroscopic UCD searches in the Fornax, Hydra, and Centaurus galaxy clusters, two Hickson compact groups, and the Local Group. This includes a large database of 180 confirmed UCDs in Fornax. Results. We find that the specific frequencies derived for UCDs match those of GCs very well, to within 10-50%. The ratio is 1.00 ± 0.44 for the four environments Fornax, Hydra, Centaurus, and Local Group, which have SN,GC values. This good match also holds for individual giant galaxies in Fornax and in the Fornax intracluster-space. The error ranges of the derived UCD specific frequencies in the various environments then imply that not more than ~50% of UCDs were formed from dwarf galaxies. We show that such a scenario would require lesseq90% of primordial dwarfs in galaxy cluster centers ( Conclusions. We conclude that the number counts of UCDs are fully consistent with them being the bright tail of the GC population. From a statistical point of view there is no need to invoke an additional formation channel.

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When is a group of stars not a galaxy?

Are there impostors lurking among the many millions of galaxies identified so far? No one can give a clear answer because there is as yet no formal definition of what a galaxy is. But a pair of astronomers are now putting the question of what defines a galaxy to a public vote, in the hope of reaching a consensus and avoiding the sort of controversy that surrounded Pluto being stripped of its status as a planet.
While a typical galaxy contains billions of stars, a number of tiny galaxies have been found in recent years that do not fit the classic picture and instead resemble the groups of stars known as star clusters. So which are they?

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Ultra Compact Dwarfs
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Title: What is a Galaxy? Cast your vote here...
Authors: Duncan Forbes (Swinburne, Melbourne), Pavel Kroupa (AIfA, Bonn)

Although originally classified as galaxies, Ultra Compact Dwarfs (UCDs) share many properties in common with globular star clusters. The debate on the origin and nature of UCDs, and the recently discovered ultra-faint dwarf spheroidal (dSph) galaxies which contain very few stars, has motivated us to ask the question 'what is a galaxy?' Our aim here is to promote further discussion of how to define a galaxy and, in particular, what separates it from a star cluster. Like most previous definitions, we adopt the requirement of a gravitationally bound stellar system as a minimum. In order to separate a dwarf galaxy from a globular cluster, we discuss other possible requirements, such as a minimum size, a long two-body relaxation time, a satellite system, the presence of complex stellar populations and non-baryonic dark matter. We briefly mention the implications of each of these definitions if they are adopted. Some special cases of objects with an ambiguous nature are also discussed. Finally, we give our favoured criteria, and in the spirit of a 'collective wisdom', invite readers to vote on their preferred definition of a galaxy via a dedicated website.

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RE: Ultra-Compact Dwarf (UCDs) galaxies
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In our Galaxy, we are used to the idea that even the nearest stars are light years away from the Sun. But a team of scientists led by Professor Pavel Kroupa of the University of Bonn think things were very different in the early Universe. In particular, Ultra Compact Dwarf galaxies (UCDs), a recently discovered class of object, may have had stars a hundred times closer together than in the solar neighbourhood, according to calculations made by team member and PhD student Joerg Dabringhausen and presented in a paper in Monthly Notices of the Royal Astronomical Society.
UCDs were discovered in 1999. Although they are still enormous by everyday standards, at about 60 light years across, they are less than 1/1000th the diameter of our own Galaxy, the Milky Way. (In more familiar units, a light year is about 10 million million km). Astronomers believe that UCDs were created when more normal galaxies collided in the early Universe. But oddly, UCDs clearly have more mass than the light from the stars they contain would imply.

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Title: UCDs - more massive than allowed?
Authors: M. Hilker (1), S. Mieske (1), H. Baumgardt (2), J. Dabringhausen (2) ((1) ESO/Garching; (2) AIfA/Bonn)

Dynamical mass estimates of ultra-compact dwarfs galaxies and massive globular clusters in the Fornax and Virgo clusters and around the giant elliptical Cen A have revealed some surprising results: 1) above about 10^6 M_sun the mass-to-light (M/L) ratio increases with the objects' mass; 2) some UCDs/massive GCs show high M/L values (4 to 6) that are not compatible with standard stellar population models; and 3) in the luminosity-velocity dispersion diagram, UCDs deviate from the well defined relation of `normal' GCs, being more in line with the Faber-Jackson relation of early-type galaxies. In this contribution, we present the observational evidences for high mass-to-light ratios of UCDs and discuss possible explanations for them.

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Deep Gemini imaging of the cluster of galaxies Hydra I (also known as Abell 1060) at a distance of 54 Mpc (176 million light-years) has revealed an abundance of massive luminous metal-rich globular clusters that appears to impersonate ultra-compact dwarf galaxies. This is the richest population of such massive stellar systems ever found in any nearby galaxy clusters. This new finding by Elizabeth M. H. Wehner and William E. Harris (McMaster University, Hamilton, Canada) indicates that these massive stellar clusters may be transition objects between Ultra-Compact Dwarf (UCDs) galaxies and the more familiar massive globular clusters.
UCDs are a recently discovered type of old stellar system too faint and compact to be called normal dwarf elliptical galaxies, but too big and bright to be called conventional globular clusters. Small numbers of them have been identified and studied in the nearby Virgo and Fornax clusters of galaxies over the past few years and have also been called Dwarf-Globular Transition Objects. But in the Hydra cluster the UCDs actually appear as a red and luminous extension to the sequence of most massive and metal-rich globular clusters. These UCDs appear to be closely associated with the Hydra central giant elliptical NGC 3311. Their masses are all above six million solar masses and extend to almost 30 million solar masses, making them very high-end globular clusters indeed if they were forced to fit into that category. Structurally they are very compact, for comparison, dwarf galaxies have scale lengths of about 300 parsecs, while these UCDs are at least ten-times smaller and thus hard to distinguish from globular clusters other than by their exceptionally high luminosities.

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