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Dark Galaxy
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According to this simulation, VirgoHI 21 would be in fact a banal gas cloud escaping from its galaxy relationship - M99 - following a telescoping at high speed with another galaxy - M98.
© CNRS/CEA..



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VirgoHI 21
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Qu'est-ce qu'une galaxie ? A priori, des étoiles, du gaz, un peu de poussière et surtout un halo de matière « noire » invisible englobant le tout. La découverte il y a quelques années de VirgoHI 21, une galaxie dite « sombre » dépourvue d'étoiles, a suscité beaucoup d'intérêt auprès des chercheurs, troublés par un tel spécimen que les modèles ne prévoyaient pas. Une équipe du laboratoire AIM (CEA, Université Paris Diderot, CNRS)(1) vient de montrer, à l'aide de simulations numériques, que VirgoHI 21 n'était en fait probablement qu'un banal débris d'une collision passée entre deux galaxies massives. Ainsi disparait le prototype de galaxie sombre massive. Ces résultats seront publiés dans Astrophysical Journal de février 2008.

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Dark Galaxies
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Title: Tidal Debris posing as Dark Galaxies
Authors: Pierre-Alain Duc, Frederic Bournaud, Elias Brinks

Debris sent into the intergalactic medium during tidal collisions can tell us about several fundamental properties of galaxies, in particular their missing mass, both in the form of cosmological Dark Matter and so-called Lost Baryons. High velocity encounters, which are common in clusters of galaxies, are able to produce faint tidal debris that may appear as star-less, free floating HI clouds. These may be mistaken for Dark Galaxies, a putative class of gaseous, dark matter dominated, objects which for some reason never managed to form stars. VirgoHI21 is by far the most spectacular and most discussed Dark Galaxy candidate so far detected in HI surveys. We show here that it is most likely made out of material expelled 750 Myr ago from the nearby spiral galaxy NGC 4254 during its fly--by at about 1000 km/s by a massive intruder. Our numerical model of the collision is able to reproduce the main characteristics of the system: in particular the absence of stars, and its prominent velocity gradient. Originally attributed to the gas being in rotation within a massive dark matter halo, we find it instead to be consistent with a combination of simple streaming motion plus projection effects (Duc & Bournaud, 2007). We discuss several ways to identify a tidal origin in a Dark Galaxy candidate and illustrate the method using another HI system in Virgo, VCC 2062, which is most likely a Tidal Dwarf Galaxy (Duc et al., 2007). Now, whereas tidal debris should not contain any dark matter from the halo of their parent galaxies, it may exhibit missing mass in the form of dark baryons, unaccounted for by classical observations, as recently found in the collisional ring of NGC 5291 (Bournaud et al., 2007) and probably in the TDG VCC 2062. These "Lost Baryons" must originally have been located in the disks of their parent galaxies.

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RE: Dark Galaxy
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Title: ALFALFA: HI Cosmology in the Local Universe
Authors: Riccardo Giovanelli

 For the last 25 years, the 21 cm line has been used productively to investigate the large-scale structure of the Universe, its peculiar velocity field and the measurement of cosmic parameters. In February 2005 a blind HI survey that will cover 7074 square degrees of the high latitude sky was started at Arecibo, using the 7-beam feed L-band feed array (ALFA). Known as the Arecibo Legacy Fast ALFA (ALFALFA) Survey, the program is producing a census of HI-bearing objects over a cosmologically significant volume of the local Universe. With respect to previous blind HI surveys, ALFALFA offers an improvement of about one order of magnitude in sensitivity, 4 times the angular resolution, 3 times the spectral resolution, and 1.6 times the total bandwidth of HIPASS. ALFALFA can detect 7 X 10**4 D**2 solar masses of HI, where D is the source distance in Mpc. As of mid 2007, 44% of the survey observations and 15% of the source extraction are completed. We discuss the status of the survey and present a few preliminary results, in particular with reference to the proposed "dark galaxy" VirgoHI21.

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Title: The ALFALFA Search for (Almost) Dark Galaxies across the HI Mass Function
Authors: Martha P. Haynes

The Arecibo Legacy Fast ALFA (ALFALFA) survey is a second generation blind extragalactic HI survey currently in progress which is exploiting Arecibo's superior sensitivity, angular resolution and digital technology to derive a census of the local HI universe over a cosmologically significant volume. As of the time of this meeting, some 4500 good quality extragalactic HI line sources have been identified in about 15% of the final survey area. ALFALFA is detecting HI masses as low as 10**6 solar masses and as large as 10**10.8 solar masses with positional accuracies typically better than 20", allowing immediate identification of the most probable optical counterparts. Only 3% of all extragalactic HI sources and less than 1% of detections with HI mass 10**9.5 solar masses cannot be identified with a stellar component. Because ALFALFA is far from complete, the discussion here focuses on limitations of past surveys that ALFALFA will overcome because of its greater volume, sensitivity and reduced susceptibility to source confusion and on a sampling of illustrative preliminary results. First ALFALFA results already suggest, in agreement with previous studies, that there does not appear to be a cosmologically significant population of optically dark but HI rich galaxies. ALFALFA promises a wealthy dataset for the exploration of many issues in near-field cosmology and galaxy evolution studies, setting the stage for their extension to higher redshifts in the future with the Square Kilometre Array (SKA).

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VIRGOHI21
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The Hubble Space Telescope has failed to reveal the expected number of stars in the mysterious, galaxy-sized cloud of hydrogen known as VIRGOHI21. The research bolsters the idea that the gas cloud is the only known example of a 'dark galaxy' that never kick-started star birth.
Galaxies are thought to coalesce from normal, or baryonic, matter that has collected in clouds of hypothetical dark matter. But surveys have turned up fewer galaxies than expected, suggesting that for unknown reasons some galaxies are stillborn, and simply fail to form stars.

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VIRGOHI21 Dark galaxy
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Title: 21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster
Authors: Robert Minchin (1 and 2), Jonathan Davies (2), Michael Disney (2), Marco Grossi (2 and 3), Sabina Sabatini (4), Peter Boyce (2), Diego Garcia (2 and 5), Chris Impey (6), Christine Jordan (7), Robert Lang (2), Andrew Marble (6), Sarah Roberts (2), Wim van Driel (8) ((1) Arecibo Observatory, (2) Cardiff University, (3) Osservatorio Astrofisico di Arcetri, (4) Osservatorio Astronomico di Roma, (5) University of Bonn, (6) Steward Observatory, (7) Jodrell Bank Observatory, (8) Observatoire de Paris)

Many observations indicate that dark matter dominates the extra-galactic Universe, yet no totally dark structure of galactic proportions has ever been convincingly identified. Previously we have suggested that VIRGOHI 21, a 21-cm source we found in the Virgo Cluster using Jodrell Bank, was a possible dark galaxy because of its broad line-width (~200 km/s) unaccompanied by any visible gravitational source to account for it. We have now imaged VIRGOHI 21 in the neutral-hydrogen line and find what could be a dark, edge-on, spinning disk with the mass and diameter of a typical spiral galaxy. Moreover, VIRGOHI 21 has unquestionably been involved in an interaction with NGC 4254, a luminous spiral with an odd one-armed morphology, but lacking the massive interactor normally linked with such a feature. Numerical models of NGC 4254 call for a close interaction ~10^8 years ago with a perturber of ~10^11 solar masses. This we take as additional evidence for the massive nature of VIRGOHI 21 as there does not appear to be any other viable candidate. We have also used the Hubble Space Telescope to search for stars associated with the HI and find none down to an I band surface brightness limit of 31.1 +/- 0.2 mag/sq. arcsec.

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darkgalaxyVIrgo1
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VIRGOHI21 Dark galaxy
Credit: NAIC

VIRGOHI21 is a dark matter halo in the Virgo cluster. It is the size of a galaxy, but apparently contains no stars. It was detected through H-I emissions of neutral hydrogen (21 cm emissions) in the dark galaxy. This is the first discovery of the postulated dark matter clumps and galaxies anticipated by dark-matter theories.

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Dark Galaxy
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First results from the Arecibo Galaxy Environment Survey (AGES) suggest the discovery of a new dark galaxy. The AGES survey, which started in January 2006, is the most sensitive, large-scale survey of neutral hydrogen to date. Neutral hydrogen is found in most galaxies and it is a key tool in the search for dark galaxies as it can be detected even when there are no stars or other radiation sources to “shine a light” on matter.

The new candidate dark galaxy is located near NGC1156, an apparently isolated, irregularly-shaped galaxy found at the edge of the Aries constellation. The first observations in the AGES programme identified a number of new galaxies. One newly discovered source is approximately 153 million light-years from Earth and appears to be 200,000 light-years across. There is no obvious optical counterpart to the massive object.


Digitised Sky Survey image of the candidate dark galaxy.

"The new source showed up clearly in the AGES survey as it contains huge amounts of hydrogen gas but it was missed in all previous searches as it doesn’t appear to contain many bright stars. The interactions between hydrogen atoms in cosmic gas clouds are enough to stimulate light emission at the neutral hydrogen “fingerprint” wavelength of 21cm. In the first stage of the AGES campaign, we have used the Arecibo radio telescope to search at this wavelength, looking for galaxies that have remained hidden from astronomers in the past. We now need to follow up observations at other wavelengths and work out exactly how many stars this new galaxy may or may not contain" - Robbie Auld, who is presenting the results at the RAS National Astronomy Meeting in Leicester on 6th April

The AGES programme, which will last for four years, is led by Cardiff University’s Dr Jonathan Davies. In addition to the Arecibo radio telescope, AGES will use a network of ground-based and space-based telescopes to observe the sky in many different wavelengths. Among those used will be the UK Infrared Telescope in Hawaii, the GALEX ultraviolet space telescope, the Hubble Space Telescope.

The techniques used in AGES have already been used on a small scale and have led to the discovery of VIRGOHI21, the first galaxy to be detected with gas, large amounts of the mysterious dark matter but no visible stars. By discovering more objects like VIRGOHI21 scientist hope to answer one of the greatest cosmological questions: if, as theoreticians predict, matter in the Universe is mainly dark then where does is all reside? The AGES team hopes that the survey will reveal exactly how much matter is hidden in dark galaxies and determine whether current theories are correct.

Source RAS

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