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Satellite dwarf galaxies
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The high speed of stars and apparent presence of 'dark matter' in the satellite galaxies that orbit our Milky Way Galaxy presents a direct challenge to Newton's theory of gravitation, according to physicists from Germany, Austria and Australia. Professor Pavel Kroupa of the University of Bonn's Argelander-Institut fuer Astronomie (AlfA) will discuss the results of the team's two studies in a presentation on Wednesday 22nd April at the European Week of Astronomy and Space Science conference at the University of Hertfordshire.
Together with scientists at the University of Vienna and the Australian National University in Canberra, the AlfA team looked at the small dwarf galaxies that orbit the Milky Way. Some of these contain only a few thousand stars and so are relatively faint and difficult to find. Standard cosmological models predict the presence of hundreds of these companions around most of the larger galaxies, but up to now only 30 have been observed around the Milky Way.

Source: Royal Astronomical Society

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TDG VCC 2062
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Title: VCC 2062: an old Tidal Dwarf Galaxy in the Virgo Cluster?
Authors: Pierre-Alain Duc, Jonathan Braine, Ute Lisenfeld, Elias Brinks, Mederic Boquien

Numerical simulations predict the existence of old Tidal Dwarf Galaxies (TDGs) that would have survived several Gyr after the collision lying at their origin. Such survivors, which would by now have become independent relaxed galaxies, would be ideal laboratories, if nearby enough, to tackle a number of topical issues, including the distribution of Dark Matter in and around galaxies. However finding old dwarf galaxies with a confirmed tidal origin is an observational challenge. A dwarf galaxy in the Virgo Cluster, VCC 2062, exhibits several unusual properties that are typical of a galaxy made out of recycled material. We discuss whether it may indeed be a TDG. We analysed multi-wavelength observations of VCC 2062, including an IRAM CO map, an optical spectrum of its HII regions, GALEX ultraviolet and archival broad-band and narrow-band optical images as well as a VLA HI datacube, originally obtained as part of the VIVA project. VCC 2062 appears to be the optical, low surface brightness counterpart of a kinematically detached, rotating condensation that formed within an HI tail apparently physically linked to the disturbed galaxy NGC 4694. In contrast to its faint optical luminosity, VCC 2062 is characterised by strong CO emission and a high oxygen abundance more typical of spiral disks. Its dynamical mass however, is that of a dwarf galaxy. VCC 2062 was most likely formed within a pre-enriched gaseous structure expelled from a larger galaxy as a result of a tidal interaction. The natural provider for the gaseous tail is NGC 4694 or rather a former companion which subsequently has been accreted by the massive galaxy. According to that scenario, VCC 2062 has been formed by a past tidal encounter. Since its parent galaxies have most probably already totally merged, it qualifies as an old TDG.

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RE: Tidal Dwarf Galaxies
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Title: Spitzer Observations of Tidal Dwarf Galaxies
Authors: Sarah J. U. Higdon, James L. Higdon

We present Spitzer observations of Tidal Dwarf Galaxies (TDGs) in three interacting systems: NGC 5291, Arp105 and Stephan's Quintet. The spectra show bright emission from polyaromatic hydrocarbons (PAHs), nebular lines and warm molecular hydrogen, characteristic of recent episodes of star formation. The PAH emission that falls in the IRAC 8.0 micron band leads to the TDGs having an extremely red IRAC colour, with [4.5] - [8.0] > 3. The emission from PAHs is characterized by a model with mainly neutral 100-C PAH atoms.

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Minimum Amount of Stars
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Title: The Minimum Amount of Stars a Galaxy Will Form
Authors: Bradley E. Warren, Helmut Jerjen, Bärbel S. Koribalski

We present an analysis of the atomic hydrogen and stellar properties of 38 late-type galaxies in the local Universe covering a wide range of HI mass-to-light ratios (M_HI/L_B), stellar luminosities, and surface brightnesses. From these data we have identified an upper envelope for the M_HI/L_B as a function of galaxy luminosity. This implies an empirical relation between the minimum amount of stars a galaxy will form and its initial baryonic mass. While the stellar mass of a galaxy seems to be only loosely connected to its baryonic mass, the latter quantity is strongly linked to the galaxy's dynamical mass as it is observed in the baryonic Tully-Fisher relation. We find that dwarf irregular galaxies with generally high M_HI/L_B-ratios follow the same trend as defined by lower M_HI/L_B giant galaxies, but are underluminous for their rotation velocity to follow the trend in a stellar mass Tully-Fisher relation, suggesting that the baryonic mass of the dwarf galaxies is normal but they have failed to produced a sufficient amount of stars. Finally, we present a three dimensional equivalent to the morphology-density relation which shows that high M_HI/L_B galaxies preferentially evolve and/or survive in low-density environments. We conclude that an isolated galaxy with a shallow dark matter potential can retain a large portion of its baryonic matter in the form of gas, only producing a minimum quantity of stars necessary to maintain a stable gas disk.

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The Fate of Dwarf Galaxies in Clusters
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Title: The Fate of Dwarf Galaxies in Clusters and the Origin of Intracluster Stars. I. Isolated Clusters
Authors: Paramita Barai, William Brito, Hugo Martel

The main goal of this paper is to compare the relative importance of destruction by tides, vs. destruction by mergers, in order to assess if tidal destruction of dwarf galaxies in clusters is a viable scenario for explaining the origin of intracluster stars. We have designed a simple algorithm for simulating the evolution of isolated clusters. The distribution of galaxies in the cluster is evolved using a direct gravitational N-body algorithm combined with a subgrid treatment of physical processes such as mergers, tidal disruption, and galaxy harassment. Using this algorithm, we have performed a total of 227 simulations. Our main results are (1) destruction of dwarf galaxies by mergers dominates over destruction by tides, and (2) the destruction of dwarf galaxies by tides is sufficient to explain the observed intracluster light in clusters.

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RE: Tidal Dwarf Galaxies
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Title: Search for Tidal Dwarf Galaxies Candidates in a Sample of Ultraluminous Infrared Galaxies
Authors: A. Monreal-Ibero (1), L. Colina (2), S. Arribas (2), M. Garcia-Marin (2) ((1) Astrophysikalisches Institut Potsdam, (2) Instituto de Estructura de la Materia (CSIC))

Star-forming small galaxies made out of collisional debris have been found in a variety of merging systems. So far only a few of them are known in ULIRGs although they show clear signs of interactions. Whether external star formation may take place in such objects is an open question. The aim of this paper is to identify and characterise the physical and kinematic properties of the external star forming regions in a sample of ULIRGs, including TDG candidates, using optical IFS and high angular resolution HST imaging. We have found that the presence of external star-forming regions is common with 12 objects being identified in 5 ULIRGs. These regions show a large range of dynamical mass up to 1x10^{10} M_sun, with average sizes of ~750 pc. In addition, the line ratios, metallicities and H\alpha equivalent widths are typical of young bursts of star formation (age ~ 5-8 Myr), and similar to those of other TDG candidates. Their extinction corrected H\alpha luminosities lead to masses for the young stellar component of ~2x10^6 - 7x10^8 M_sun. The likelihood of survival of these regions as TDGs is discussed based on their structural and kinematic properties. Most of these systems follow the relation between effective radius and velocity dispersion found for globular clusters and Ellipticals, which suggests they are stable against internal motions. The stability against forces from the parent galaxy have been studied and a comparison of the data with the predictions of dynamical evolutionary models is also performed. Five regions out of twelve show High-Medium or High likelihood of survival. Our best candidate, which satisfy all the utilized criteria, is located in the advanced merger IRAS15250+3609 and presents a velocity field decoupled from the relatively distant parent galaxy.

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Title: The early evolution of tidal dwarf galaxies
Authors: S. Recchi (1,2), Ch. Theis (2), P. Kroupa (3), G. Hensler (2) ((1) INAF - Osservatorio Astronomico di Trieste, Italy, (2) Institute of Astronomy, Vienna University, Austria, (3) Argelander Institute for Astronomy, Bonn, Germany)

Context: Dwarf galaxies can arise from self-gravitating structures emerging from tidal tails. What fraction of the known dwarf galaxies in the Local Universe can have this origin is still a matter of debate.
Aims: In our effort to understand the origin and evolution of tidal dwarf galaxies and their correspondence with local objects, the first step is to understand how these galaxies (which are supposed to have a limited amount of dark matter) react to the feedback of the ongoing star formation.
Methods: We make use of 2-D chemodynamical calculations in order to study the early evolution of isolated, dark matter-free dwarf galaxies. We present models in which feedback parameters are varied. We also compare the results with dark matter-dominated dwarf galaxy models.
Results: All the considered models show that the star formation proceeds for more than 300 Myr, therefore dwarf galaxies without large dark matter halos are not necessarily quickly destroyed. The chemical evolution of these objects is consistent with the main chemical properties of the dSphs of the Local Group. Models with large dark matter halos show results consistent with models free of dark matter, indicating that the distribution of gas is more important than the depth of the potential well in determining the global behaviour of dSph-sized dwarf galaxies.

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Astronomers believe they may have found a significant portion of the universe's "missing" matter.
The mysterious dark matter may be giving invisible heft to small galaxies formed during galactic collisions, a new study says.
Dark matter is an invisible form of matter that does not give off or reflect light yet accounts for the vast majority of mass in the universe.
Scientists measured the mass of three so-called recycled dwarf galaxies near a massive galaxy that was recently in a collision.
The dwarfs appear to be more than twice as heavy as their visible stars and gas, indicating that they hold a type of dark matter.

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NGC5291 Tidal Dwarf Galaxies
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Astronomers studying dwarf galaxies formed from the debris of a collision of larger galaxies found the dwarfs much more massive than expected, and think the additional material is "missing mass" that theorists said should not be present in this kind of dwarf galaxy.

ngc5291_lab
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Composite radio/optical/ultraviolet image of NGC 5291 and its surroundings, including the debris propelled outward by collision with another galaxy. Blue is atomic Hydrogen observed with the VLA; white is optical; red is ultraviolet (Galex satellite). Red labels mark the dwarf galaxies studied in this research.
CREDIT: P-A Duc, CEA-CNRS/NRAO/AUI/NSF/NASA.

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Baby galaxies growing from the debris of a galactic traffic accident have been hiding a lot of extra matter, new observations suggest.
If so, galaxies like our own could contain vast quantities of invisible gas that outweigh their stars and other visible material and these gas stores could represent some of the "missing" normal matter astronomers have been puzzling over.
Measurements of the afterglow of the big bang suggest mysterious dark matter was about six times more abundant than ordinary matter in the early universe. That ratio should remain the same through time, but observations of all the stars, gas and dust in the nearby, modern universe can account for only about a quarter as much ordinary, or "baryonic", matter as expected
 
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