B. Vollmer

French National Centre for Scientific Research, Lutetia Parisorum, Île-de-France, France

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Publications (110)286.52 Total impact

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    ABSTRACT: We present the result of a new VLA HI Imaging survey of Virgo galaxies, VIVA (the VLA Imaging survey of Virgo galaxies in Atomic gas). The survey includes high resolution HI data of 53 carefully selected late type galaxies (48 spirals and 5 irregular systems). The goal is to study environmental effects on HI gas properties of cluster galaxies to understand which physical mechanisms affect galaxy evolution in different density regions, and to establish how far out the impact of the cluster reaches. As a dynamically young cluster, Virgo contains examples of galaxies experiencing a variety of environmental effects. Its nearness allows us to study each galaxy in great detail. We have selected Virgo galaxies with a range of star formation properties in low to high density regions (at the projected distance from M87, d_87=0.3-3.3 Mpc). Contrary to pr evious studies, more than half of the galaxies in the sample (~60%) are fainter than 12 mag in B_T. Overall, the selected galaxies represent the late type Virgo galaxies (S0/a to Sd/Irr) down to m_p<~14.6 fairly well in morphological type, systemic velocity, subcluster membership, HI mass and deficiency. In this paper (VIVA I: the atlas and the HI properties), we present HI maps and properties, and describe the HI morphology and kinematics of individual galaxies in detail (abbreviated). Comment: K band magnitudes for 6 galaxies in Table 3 have been corrected. One of the labels in Figure 8 is corrected and an omission in the acknowledgments has been added. The latter two were correct in the previous astro-ph version but are wrong in the journal version. A full resolution with the complete HI atlas can be downloaded at http://www.astro.yale.edu/viva/pub.html
    09/2009;
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    ABSTRACT: The SPECFIND catalogue of radio cross-identifications and spectra, which is available in VizieR (Ochsenbein et al. 2000), has been used to search for Gigahertz peaked source candidates. These sources were observed quasi-simultaneously with the Effelsberg 100-m radio telescope at 6 cm (4.85~GHz), 2.8~cm (10.45 GHz), and 9 mm (32 GHz). It turned out that this is an efficient procedure to discover new Gigahertz peaked sources, which are believed to be AGNs at the beginning of their radio evolution. A new SPECFIND V2.0 catalogue is presented. It contains the cross-identification of 87000 radio objects from 105 catalogues or 3.76 million radio sources. With an increase of 8% of available sources we increase the number of radio objects with associated radio spectra by 25%. Going from 20 catalogues in the previous SPECFIND release to 105 catalogues in the new release was only possible due to the development of three Virtual Observatory (VO) tools within the European VO-TECH project. These new tools (i) identify pertinent radio catalogues in the VO registry using Unified Content Descriptors (UCDs), (ii) extract relevant data, and (iii) normalize these for the determination of radio spectra.
    07/2009;
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    B. Vollmer
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    ABSTRACT: Based on a comparison of dynamical models with observations of the interstellar gas in 6 Virgo cluster spiral galaxies a first complete ram pressure stripping time sequence has been established. The observational characteristics of the different stages of ram pressure stripping are presented. The dynamical models yield the 3D velocity vectors of the galaxies, peak ram pressures, and times to peak ram pressure. In the case of a smooth, static, and spherical intracluster medium, peak ram pressure occurs during the galaxy's closest approach to the cluster center, i.e. when the galaxy's velocity vector is perpendicular to its distance vector from the cluster center (M 87). Assuming this condition the galaxy's present line-of-sight distance and its 3D position during peak ram pressure can be calculated. The linear orbital segments derived in this way together with the intracluster medium density distribution derived from X-ray observations give estimates of the ram pressure that are on average a factor of 2 higher than derived from the dynamical simulations for NGC 4501, NGC 4330, and NGC 4569. Resolving this discrepancy would require either a 2 times higher intracluster medium density than derived from X-ray observations, or a 2 times higher stripping efficiency than assumed by the dynamical models. Compared to NGC 4501, NGC 4330, and NGC 4569, NGC 4388 requires a still 2 times higher local intracluster medium density or a direction which is moderately different from that derived from the dynamical model. A possible scenario for the dynamical evolution of NGC 4438 and M 86 within the Virgo cluster is presented. Comment: 10 pages, 5 figures; accepted for publication in A&A
    Astronomy and Astrophysics 05/2009; · 5.08 Impact Factor
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    ABSTRACT: Ram pressure stripping of the multiphase ISM is studied in the perturbed Virgo cluster spiral galaxy NGC 4438. This galaxy underwent a tidal interaction ~100 Myr ago and is now strongly affected by ram pressure stripping. Deep VLA radio continuum observations at 6 and 20 cm are presented. We detect prominent extraplanar emission to the west of the galactic center, which extends twice as far as the other tracers of extraplanar material. The spectral index of the extraplanar emission does not steepen with increasing distance from the galaxy. This implies in situ re-acceleration of relativistic electrons. The comparison with multiwavelength observations shows that the magnetic field and the warm ionized interstellar medium traced by Halpha emission are closely linked. The kinematics of the northern extraplanar Halpha emission, which is ascribed to star formation, follow those of the extraplanar CO emission. In the western and southern extraplanar regions, the Halpha measured velocities are greater than those of the CO lines. We suggest that the ionized gas of this region is excited by ram pressure. The spatial and velocity offsets are consistent with a scenario where the diffuse ionized gas is more efficiently pushed by ram pressure stripping than the neutral gas. We suggest that the recently found radio-deficient regions compared to 24 mum emission are due to this difference in stripping efficiency. Comment: 8 pages, 6 figures, A&A, accepted for publication
    Astronomy and Astrophysics 01/2009; · 5.08 Impact Factor
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    ABSTRACT: The natural output of imaging spectroscopy in astronomy is a 3D data cube with two spatial and one frequency axis. The spectrum of each image pixel consists of an emission line which is Doppler-shifted by gas motions along the line of sight. These data are essential to understand the gas distribution and kinematics of the astronomical object. We propose a two-step method to extract coherent kinematic structures from the data cube. First, the spectra are decomposed into a sum of Gaussians using a Bayesian method to obtain an estimation of spectral lines. Second, we aim at tracking the estimated lines to get an estimation of the structures in the cube. The performance of the approach is evaluated on a real radio-astronomical observation.
    Image Analysis, 16th Scandinavian Conference, SCIA 2009, Oslo, Norway, June 15-18, 2009. Proceedings; 01/2009
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    ABSTRACT: IRAM 30m 12CO(1-0) and 12CO(2-1) HERA observations are presented for the ram-pressure stripped Virgo spiral galaxy NGC 4522. The CO emission is detected in the galactic disk and the extraplanar gas. The extraplanar CO emission follows the morphology of the atomic gas closely but is less extended. The CO maxima do not appear to correspond to regions where there is peak massive star formation as probed by Halpha emission. The presence of molecular gas is a necessary but not sufficient condition for star formation. Compared to the disk gas, the molecular fraction of the extraplanar gas is 30% lower and the star formation efficiency of the extraplanar gas is about 3 times lower. The comparison with an existing dynamical model extended by a recipe for distinguishing between atomic and molecular gas shows that a significant part of the gas is stripped in the form of overdense arm-like structures. It is argued that the molecular fraction depends on the square root of the total large-scale density. Based on the combination of the CO/Halpha and an analytical model, the total gas density is estimated to be about 4 times lower than that of the galactic disk. Molecules and stars form within this dense gas according to the same laws as in the galactic disk, i.e. they mainly depend on the total large-scale gas density. Star formation proceeds where the local large-scale gas density is highest. Given the complex 3D morphology this does not correspond to the peaks in the surface density. In the absence of a confining gravitational potential, the stripped gas arms will most probably disperse; i.e. the density of the gas will decrease and star formation will cease. Comment: 11 pages, 15 figures, A&A accepted for publication
    Astronomy and Astrophysics 09/2008; · 5.08 Impact Factor
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    ABSTRACT: We present the result of a new VLA H I imaging survey of Virgo galaxies, VIVA (VLA Imaging of Virgo in Atomic gas). The goal is to investigate the influence of the cluster on H I gas disks in different density regions. In order to sample various processes at work, we have carefully selected 48 spirals and 5 dwarfs/irregulars showing a range of star formation properties throughout the cluster. Overall, we confirm that galaxies near the cluster core (d_{M 87}<0.5 Mpc) are severely H I stripped while gas rich galaxies with extended H I disks are always found in the cluster outskirts (d_{M87}>1.0 Mpc). At intermediate distances from the cluster center however, our high resolution, high sensitivity H I data have revealed a range of H I features that are likely to represent different stripping stages through various effects. In particular, we find evidence for H I stripping due to the surrounding cluster gas even at the distances where the approximated intra-cluster medium (ICM) density is not high enough to affect galaxies. It appears that in some cases a dynamic ICM or a combination of tidal forces and ram-pressure are responsible for gas stripping at those locations. The survey result clearly shows that the impact of the cluster reaches quite far out from the cluster center.
    09/2008; 396:127.
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    B. Vollmer, T. Beckert, R. Davies
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    ABSTRACT: Recent VLT SINFONI observations of the close environments (~30pc) of nearby AGNs have shown that thick gas tori and starbursts with ages between 10 and 150Myr are frequently found. By applying these observations to a previously established analytical model of clumpy accretion disks, we suggest an evolutionary sequence for starburst and AGN phases. Whereas the observed properties of the gas tell us about the current state of the torus, the starburst characteristics provide information on the history of the torus. In the suggested evolution, a torus passes through 3 different phases predetermined by an external mass accretion rate. Started by an initial, short, and massive gas infall, a turbulent and stellar wind-driven Q~1 disk is formed in which the starburst proceeds. Once the supernovae explode the intercloud medium is removed, leaving a massive, geometrically thick, collisional disk with a decreasing, but still high-mass accretion rate. When the mass accretion rate has significantly decreased, the collisional torus becomes thin and transparent as the circumnuclear disk in the Galactic center of the Milky Way. Variations on this scenario are possible either when there is a second short and massive gas infall, in which case the torus may switch back into the starburst mode, or when there is no initial short massive gas infall. All observed tori up to now have been collisional and thick. The observations show that this phase can last more than 100Myr. During this phase the decrease in the mass accretion rate within the torus is slow (a factor of 4 within 150Myr). The collisional tori also form stars, but with an efficiency of about 10% when compared to a turbulent disk. Comment: 14 pages, 4 figures, A&A accepted for publication
    Astronomy and Astrophysics 09/2008; · 5.08 Impact Factor
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    ABSTRACT: A sample of 214 radio sources, which were extracted from the SPECFIND catalog (Cat. VIII/74) and show an inverted radio spectrum, were observed quasi-simultaneously at 4.85, 10.45, and 32GHz with the 100-m Effelsberg radio telescope. Using the VLBA calibrator survey (VCS) we have investigated the parsec-scale morphology of the sources. Table 5: results of the observations. Table 6: properties of the sources. (2 data files).
    VizieR Online Data Catalog. 09/2008;
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    ABSTRACT: We have designed a new technique for the detection of Low Surface Brightness galaxies based on local background/source separation using Markovian analysis. This method helps to estimate smooth local variations of the background and therefore allows for determining source candidates as faint as LSB galaxies. For each source an average density profile is computed, the shape of which can help to sort out stars and bright objects. A list of LSB candidates is provided, for which position, profile and surface brightness are examined thoroughly. The results are very promising. This approach has been compared to the SExtractor source detection tool and to a previous original analysis by S. Sabatini et al. on the same INT image dataset of the Virgo Cluster. Detection rate, source selection criteria and calculation loop improvements are discussed.
    08/2008;
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    ABSTRACT: We present results of a new Very Large Array survey of 53 Virgo galaxies (48 spirals and 5 dwarf/irregular systems). The goal is to study how the H I gas properties are affected by the cluster environment. The survey covers galaxies in a wide range of densities from the center of the cluster to more than 3 Mpc from M 87. The gas is imaged down to a column-density sensitivity of a few times 1019cm-2. We find examples of gas stripping at all stages. Within ˜0.5 Mpc from M 87, most galaxies are severely H I stripped. The H I disks are truncated to well within the optical disks. While the H I looks asymmetric, the outer stellar disks look undisturbed. The fact that only the gas and not the stars has been stripped suggests that those galaxies have been affected by the hot and dense cluster gas. Interestingly we also find a few truncated disks at large projected distances from the center. Although some of these may have been stripped while crossing the cluster core, a detailed population-synthesis study of the outer disk of one of these shows that star formation was terminated recently. The time since stripping is too short for the galaxy to have traveled from the core to its current location. So at least one galaxy has lost its gas from the outer disk by another mechanism than ram-pressure stripping in the dense cluster core. At intermediate- to low-density regions (>0.6 Mpc) we find H I tails with various lengths. We find seven galaxies with long one-sided H I tails pointing away from M 87. The galaxies are at 0.6-1 Mpc from M 87. Since these galaxies are only mildly H I deficient and the tails point away from M 87, these galaxies are probably falling into the cluster for the first time on highly radial orbits. For all but two of the galaxies the estimated ram pressure at their location in the cluster would be sufficient to pull out the H I in the very outer disks. One galaxy also looks optically disturbed and a simulation suggests that a combination of ram pressure plus a tidal interaction has pulled out the tail. In the outskirts of the cluster we find several examples of tidally interacting galaxies. We possibly see evidence for some accretion of gas as well. Lastly, the merging of subclusters with Virgo can cause bulk motions of the ICM. We see one example of a galaxy far out that appears to be ram-pressure stripped by a dynamic ICM. In summary, our results show that galaxies are already affected in the low-density outer regions of the cluster through ram-pressure stripping and tidal interactions, or a combination of both.
    07/2008; 395:364.
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    ABSTRACT: Gigahertz-Peaked Spectrum (GPS) sources are probably the precursors of local radio galaxies.Existing GPS source samples are small (<200). It is necessary to extend the availabe sample of the Gigahertz-Peaked Spectrum (GPS) and High Frequency Peaker (HFP) sources in order to study their nature with greater details and higher statistical significance. A sample of 214 radio sources, which were extracted from the SPECFIND catalog and show an inverted radio spectrum, were observed quasi-simultaneously at 4.85, 10.45, and 32GHz with the 100-m Effelsberg radio telescope. Using the VLBA calibrator survey (VCS) we have investigated the parsec-scale morphology of the sources. About 45% of the sources in our sample are classified as GPS or HFP candidates. We add 65 new GPS/HFP candidates to existing samples. We confirm the expected tendency that HFP are more compact on milliarcsecond scale than the 'classical' GPS sources, which peak at lower frequencies. The data mining of the SPECFIND database represents a promising tool for the discovery of new GPS/HFP sources. Comment: 16 pages, 7 figures, accepted for publication in A&A
    Astronomy and Astrophysics 06/2008; · 5.08 Impact Factor
  • 02/2008;
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    ABSTRACT: VIVA HI observations of the Virgo spiral galaxy NGC 4501 are presented. The HI disk is sharply truncated to the southwest, well within the stellar disk. A region of low surface-density gas, which is more extended than the main HI disk, is discovered northeast of the galaxy center. These data are compared to existing 6cm polarized radio continuum emission, Halpha, and optical broad band images. We observe a coincidence between the western HI and polarized emission edges, on the one hand, and a faint Halpha emission ridge, on the other. The polarized emission maxima are located within the gaps between the spiral arms and the faint Halpha ridge. Based on the comparison of these observations with a sample of dynamical simulations with different values for maximum ram pressure and different inclination angles between the disk and the orbital plane,we conclude that ram pressure stripping can account for the main observed characteristics. NGC 4501 is stripped nearly edge-on, is heading southwest, and is ~200-300 Myr before peak ram pressure, i.e. its closest approach to M87. The southwestern ridge of enhanced gas surface density and enhanced polarized radio-continuum emission is due to ram pressure compression. It is argued that the faint western Halpha emission ridge is induced by nearly edge-on ram pressure stripping. NGC 4501 represents an especially clear example of early stage ram pressure stripping of a large cluster-spiral galaxy. Comment: 22 pages, 25 figures, accepted for publication in A&A
    Astronomy and Astrophysics 01/2008; · 5.08 Impact Factor
  • Bernd Vollmer
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    ABSTRACT: The recent advances in understanding the effects of ram pressure on Virgo spiral galaxies is discussed. For a sample of 6 galaxies interaction diagnostics are made by a detailed comparison between observed and simulated gas distributions and velocity fields. Once the interaction is identified (gravitational, ram pressure or a mixture of both), we can estimate its parameters. The importance of a new diagnostic tool, the polarized radio continuum emission, is emphasized. This emission shows a characteristic distribution in cluster spiral galaxies. It directly traces gas compression or shear regions, which both can occur during an interaction of a cluster galaxy with its environment. A first complete time sequence for ram pressure stripping in the Virgo cluster is presented.
    12/2007: pages 253-260;
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    ABSTRACT: The CDS value-added bibliographic services, SIMBAD and VizieR, are updated daily. Most of the information comes from the astronomical literature and the update mechanism is different for different types of information. The semi-automated SIMBAD data flow is described and the synergy between astronomers, specialized librarians and computer engineers is discussed.
    09/2007; 377:43.
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    ABSTRACT: A new version of SIMBAD, SIMBAD4, has been developed at the CDS. Basically, everything that SIMBAD does today will be possible with the new version but not necessarily in the same way. The new features will concern the queries, flux, object types and hierarchical links.
    09/2007; 377:197.
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    ABSTRACT: Because of its proximity the Virgo Cluster is an excellent target for studying interactions of galaxies with the cluster environment. Both the high-velocity tidal interactions and effects of ram pressure stripping by the intracluster gas can be investigated. Optical and/or \ion{H}{i} observations do not always show effects of weak interactions between galaxies and their encounters with the cluster medium. For this reason we searched for possible anomalies in the magnetic field structure in Virgo Cluster spirals which could be attributed to perturbations in their gas distribution and kinematics. Five angularly large Virgo Cluster spiral galaxies (NGC 4501, NGC 4438, NGC 4535, NGC 4548 and NGC 4654) were the targets for a sensitive total power and polarization study using the 100-m radio telescope in Effelsberg at 4.85 GHz. For two objects polarization data at higher frequencies have been obtained allowing Faraday rotation analysis. Distorted magnetic field structures were identified in all galaxies. Interaction-induced magnetized outflows were found in NGC 4438 (due to nuclear activity) and NGC 4654 (a combination of tidal tails and ram pressure effects). Almost all objects (except the anaemic NGC 4548) exhibit distortions in polarized radio continuum attributable to influence of the ambient gas. For some galaxies they agree with observations of other species, but sometimes (NGC 4535) the magnetic field is the only tracer of the interaction with the cluster environment. The cluster environment clearly affects the evolution of the galaxies due to ram pressure and tidal effects. Magnetic fields provide a very long-lasting memory of past interactions. Therefore, they are a good tracer of weak interactions which are difficult to detect by other observations.
    Astronomy and Astrophysics 07/2007; · 5.08 Impact Factor
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    ABSTRACT: In a new HI imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo galaxies in Atomic gas), we find 7 spiral galaxies with long HI tails. The morphology varies but all the tails are extended well beyond the optical radii on one side. These galaxies are found in intermediate-low density regions (0.6-1 Mpc in projection from M87). The tails are all pointing roughly away from M87, suggesting that these tails may have been created by a global cluster mechanism. While the tidal effects of the cluster potential are too small, a rough estimate suggests that simple ram-pressure stripping indeed could have formed the tails in all but two cases. At least three systems show HI truncation to within the stellar disk, providing evidence for a gas-gas interaction. Although most of these galaxies do not appear disturbed optically, some have close neighbors, suggesting that tidal interactions may have moved gas outwards making it more susceptible to the ICM ram-pressure or viscosity. Indeed, a simulation study of one of the tail galaxies, NGC 4654, suggests that the galaxy is most likely affected by the combined effect of a gravitational interaction and ram-pressure stripping. We conclude that these one-sided HI tail galaxies have recently arrived in the cluster, falling in on highly radial orbits. It appears that galaxies begin to lose their gas already at intermediate distances from the cluster center through ram-pressure or turbulent viscous stripping and tidal interactions with neighbours, or a combination of both. Comment: 4 pages, 3 figures (including 1 plate), accepted for accepted for publication in ApJ Letter (vol. 659, L115), a version with full resolution Plate 1 is available from http://www.astro.umass.edu/~achung/astro-ph/viva_tail.pdf
    The Astrophysical Journal 03/2007; · 6.73 Impact Factor
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    ABSTRACT: Radio interferometry produces visibility data which can be processed flexibly to achieve higher resolution or better surface brightness sensitivity. These results are now within the reach of any astronomer as radio archives appear on-line and pipelines and other user-friendly data reduction tools become common. All raw VLA data are now available by ftp via a web form offering many search parameters. Images from the MERLIN archive, the NVSS, SUMSS and other surveys and many radio catalogues are already published electronically directly and via CDS. However a typical 1024×1024-pixel image covers less than one percent of the typical potential field of view. The ideal solution is to store calibrated visibility data and produce images or other products on demand to user specifications. Prototypes have been developed to do this via web interfaces (not requiring any specialised radio astronomy knowledge) for the MERLIN and ATCA archives.
    02/2007: pages 81-84;

Publication Stats

791 Citations
286.52 Total Impact Points

Institutions

  • 2013
    • French National Centre for Scientific Research
      • Observatoire astronomique de Strasbourg
      Lutetia Parisorum, Île-de-France, France
  • 2012
    • National Research Council Canada
      Ottawa, Ontario, Canada
  • 2011
    • Columbia University
      New York City, New York, United States
  • 2009
    • University of Strasbourg
      Strasburg, Alsace, France
  • 2007
    • Yale University
      • Department of Astronomy
      New Haven, Connecticut, United States
  • 2004
    • Université de Montpellier 1
      Montpelhièr, Languedoc-Roussillon, France
  • 2002–2003
    • Max Planck Institute for Radio Astronomy
      Bonn, North Rhine-Westphalia, Germany
  • 2001
    • Observatoire de Paris
      Lutetia Parisorum, Île-de-France, France
  • 1999–2000
    • Universität Heidelberg
      • Institute of Theoretical Physics
      Heidelberg, Baden-Wuerttemberg, Germany