* Astronomy

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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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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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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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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