Cold gas & mergers: fundamental difference in HI properties of different types of radio galaxies?

We present results of a study of large-scale neutral hydrogen (HI) gas in nearby radio galaxies. We find that the early-type host galaxies of different types of radio sources (compact, FR-I and FR-II) appear to contain fundamentally different large-scale HI properties: enormous regular rotating disks and rings are present around the host galaxies of a significant fraction of low power compact radio sources, while no large-scale HI is detected in low power, edge-darkened FR-I radio galaxies. Preliminary results of a study of nearby powerful, edge-brightened FR-II radio galaxies show that these systems generally contain significant amounts of large-scale HI, often distributed in tail- or bridge-like structures, indicative of a recent galaxy merger or collision. Our results suggest that different types of radio galaxies may have a different formation history, which could be related to a difference in the triggering mechanism of the radio source. If confirmed by larger studies with the next generation radio telescopes, this would be in agreement with previous optical studies that suggest that powerful FR-II radio sources are likely triggered by galaxy mergers and collisions, while the lower power FR-I sources are fed in other ways (e.g. through the accretion of hot IGM). The giant HI disks/rings associated with some compact sources could - at least in some cases - be the relics of much more advanced mergers.Comment: 5 pages, 3 figures - to appear in PoS, "The Modern Radio Universe: From Planets to Dark Energy Conference", Manchester UK, eds: Beswick, Diamond & Schilizz

BUDHIES I: characterizing the environments in and around two clusters at z~0.2

We present the optical spectroscopy for the Blind Ultra Deep HI Environmental Survey (BUDHIES). With the Westerbork Synthesis Radio Telescope, BUDHIES has detected HI in over 150 galaxies in and around two Abell clusters at z~0.2. With the aim of characterizing the environments of the HI-detected galaxies, we obtained multi-fiber spectroscopy with the William Herschel Telescope. In this paper, we describe the spectroscopic observations, report redshifts and EW[OII] measurements for ~600 galaxies, and perform an environmental analysis. In particular, we present cluster velocity dispersion measurements for 5 clusters and groups in the BUDHIES volume, as well as a detailed substructure analysis.Comment: v2: Typos and small corrections after proofs added. 14 pages (plus small appendix), 12 figures. Accepted for publication in MNRAS. Adobe Acrobat Reader is required to correctly display the (3D) animated figures (Fig. 9). Full data tables and supporting videos are also available at the BUDHIES project website: http://www.astro.rug.nl/budhies

The Effect of Cluster Environment on Galaxy Evolution in the Pegasus I Cluster

We present neutral hydrogen observations of 54 galaxies in the Pegasus Cluster. The observations include single-dish H I measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as H I images obtained with the Very Large Array (VLA) for 10 of these. The Arecibo profiles reveal an overall H I deficiency in the cluster, with similar to 40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2-3. The HI morphology of so me galaxies shows that the HI disk is smaller than the optical disk and slightly offset from the stars. We find a correlation between HI deficiency and the ratio of the HI disk size to optical disk size. More HI deficient galaxies have relatively smaller HI disks, a configuration that is usually attributed to an interaction between the interstellar medium (ISM) of the galaxy and the hot intracluster med ium (ICM). Such a result is surprising since the Pegasus cluster has a low level of X-ray emission, and a low velocity dispersion. The low velocity dispersion, coupled with the lack of a dense hot ICM indicate that ram pressure stripping should not play a significant role in this environment. In addition, two of the galaxies, NGC7604 and NGC7648, are morphologically peculiar. Their peculiarities indicate contradictory scenarios of what is triggering their unusual star formation. Hα imaging, along with long-slit spectroscopy of NGC7648 reveal morphological features which point to a recent tidal interaction. On the other hand, Hα imaging of NGC7604 reveals a strong episode of star formation concentrated into an asymmetric arc, preferentially located on one side of the galaxy. VLA HI mapping shows the HI also highly concentrated into that region, suggestive of a ram pressure event. Our data indicate that ISM-ICM interactions may play a role in a wider variety of environments than suggested by simple ram pressure arguments

The Cosmically Depressed: Life, Sociology and Identity of Voids

We review and discuss aspects of Cosmic Voids that form the background for our Void Galaxy Survey (see accompanying paper by Stanonik et al.). Following a sketch of the general characteristics of void formation and evolution, we describe the influence of the environment on their development and structure and the characteristic hierarchical buildup of the cosmic void population. In order to be able to study the resulting tenuous void substructure and the galaxies populating the interior of voids, we subsequently set out to describe our parameter free tessellation-based watershed void finding technique. It allows us to trace the outline, shape and size of voids in galaxy redshift surveys. The application of this technique enables us to find galaxies in the deepest troughs of the cosmic galaxy distribution, and has formed the basis of our void galaxy program.Comment: 10 pages, 4 figures, proceedings "Galaxies in Isolation" (May 2009, Granada, Spain), eds. L. Verdes-Montenegro, ASP (this is a colour, extended and combined version; accompanying paper to Stanonik et al., arXiv:0909.2869, in same volume

HI imaging of Galaxy Clusters at z=0.2; a Pilot Survey of Abell 963 and Abell 2192

A pilot study with the powerful new backend of the Westerbork Synthesis Radio Telescope (WSRT) of two galaxy clusters at z=0.2 has revealed neutral hydrogen emission from 39 galaxies. The volume probed for each cluster is 1.7x10^4 Mpc^3, with spatial and velocity resolutions of 54x86 kpc^2 and 19.7 km/s, covering both clusters and the large scale structure in which they are embedded. The spatial distribution of the HI detected galaxies is very different for the two clusters. In Abell 963, most of the gas-rich galaxies are located to the northeast, at 1-3 Mpc from the cluster center in projection. Their velocities are slightly redshifted with respect to the cluster mean. This could be a gas-rich group falling in from the front. Abell 2192 is less massive and more diffuse, with the gas rich galaxies more uniformly spread over a large region around the cluster. The HI masses of the detected galaxies range from 5x10^9 to 4x10^10 Msun. Some HI-rich galaxies are spatially resolved and rudimentary rotation curves are derived, showing the prospect for Tully-Fisher studies of different galaxy populations in these environments. Only one galaxy is detected within a 1 Mpc radius from the center of the Butcher-Oemler cluster Abell 963, and none of the blue B-O galaxies which are all located within the central Mpc. The HI detected galaxies outside the central Mpc are of similar colour and magnitude as the non-detected B-O galaxies, indicating that the blue B-O galaxies are gas-poor compared to their counterparts in the field.Comment: 7 pages, 4 figures. to appear in the New Astronomy Review proceedings of "The Fate of Gas in Galaxies", held in Dwingeloo, July 200