We investigate the role of ram pressure stripping in the Virgo cluster using N-body simulations. Radial orbits within the Virgo cluster's gravitational potential are modeled and analyzed with respect to ram pressure stripping. The N-body model consists of 10 000 gas cloud complexes which can have inelastic collisions. Ram pressure is modeled as an additional acceleration on the clouds located at the surface of the gas distribution in the direction of the galaxy's motion within the cluster. We made several simulations changing the orbital parameters in order to recover different stripping scenarios using realistic temporal ram pressure profiles. We investigate systematically the influence of the inclination angle between the disk and the orbital plane of the galaxy on the gas dynamics. We show that ram pressure can lead to a temporary increase of the central gas surface density. In some cases a considerable part of the total atomic gas mass (several 10 8 M ⊙ ) can fall back onto the galactic disk after the stripping event. A quantitative relation between the orbit parameters and the resulting Hi deficiency is derived containing explicitly the inclination angle between the disk and the orbital plane. The comparison between existing Hi observations and the results of our simulations shows that the Hi deficiency depends strongly on galaxy orbits. It is concluded that the scenario where ram pressure stripping is responsible for the observed Hi deficiency is consistent with all Hi 21cm observations in the Virgo cluster.

Galaxy

In a new H i imaging survey of Virgo galaxies (VIVA: VLA Imaging of Virgo galaxies in Atomic gas), we find seven spiral galaxies with long H i tails. The morphology varies, but all the tails are extended well beyond the optical radii on one side. These galaxies are found in intermediate-to 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 could have indeed formed the tails in all but two cases. At least three systems show H i truncation to within the stellar disk, providing evidence of 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 outward, making it more susceptible to the intracluster medium 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 H i 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 their neighbors, or a combination of both.

We present VLA H i observations at $20 00 ' 1.5 kpc resolution of the highly inclined, H i-deficient Virgo Cluster spiral galaxy NGC 4522, which is one of the clearest and nearest cases of ongoing intracluster mediuminterstellar medium (ICM-ISM) stripping. H i is abundant and spatially coincident with the stellar disk in the center, but beyond R = 3 kpc the H i distribution in the disk is sharply truncated, and the only H i is extraplanar and all on the northwest side. Forty percent of the total H i, corresponding to 1.5 ; 10 8 M , is extraplanar and has likely been removed from the galaxy disk by an ICM-ISM interaction. The kinematics and the morphology of the H i appear more consistent with ongoing stripping and less consistent with gas fall-back, which may occur long after peak pressure. Some of the extraplanar gas has line widths (FWZI) of 150 km s À1 , including a blueshifted tail of weaker emission, and much of the extraplanar gas exhibits a modest net blueshift with respect to the galaxy's disk rotational velocities, consistent with gas accelerated toward the mean cluster velocity. The southwest side of the galaxy has less H i in the disk but more H i in the halo, suggesting more effective gas removal on the side of the galaxy that is rotating into the ICM wind. In recent simulations of ICM-ISM interactions large surface densities of extraplanar gas like that in NGC 4522 are seen at relatively early stages of active stripping and not during later gas fall-back stages. The galaxy is 3:3 ' 800 kpc from M87, somewhat outside the region of strongest cluster X-ray emission. The ram pressure at this location, assuming a static smooth ICM and standard values for ICM density and galaxy velocity, appears inadequate to cause the observed stripping. We consider the possibility that the ram pressure is significantly stronger than standard values, because of large bulk motions and local density enhancements of the ICM gas, which may occur in a dynamic, shock-filled ICM experiencing subcluster merging. The H i and H distributions are similar, with both truncated in the disk at the same radius and H ii regions located throughout much of the extraplanar H i. This implies that the star-forming molecular ISM has been effectively stripped from the outer disk of the galaxy along with the H i. The inferred peak stripping rate of $10 M yr À1 is much larger than the galaxy's total star formation rate of $0.1 M yr À1 , implying that the rate of triggered star formation due to ICM pressure is presently minor compared with the rate of gas lost as a result of stripping.

We present the results of a new VLA H i Imaging survey of Virgo galaxies, the VLA Imaging survey of Virgo galaxies in Atomic gas (VIVA). The survey includes high-resolution H i data of 53 carefully selected late type galaxies (48 spirals and five irregular systems). The goal is to study environmental effects on H i 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 projected distances from M87, d 87 = 0.3-3.3 Mpc). Contrary to previous 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, H i mass, and deficiency. The H i observations were done in C short (CS) configuration of the VLA radio telescope, with a typical spatial resolution of 15 and a column density sensitivity of ≈ 3-5 × 10 19 cm −2 in 3σ per 10 km s −1 channel. The survey was supplemented with data of comparable quality from the NRAO archive, taken in CS or C configuration. In this paper, we present H i channel maps, total intensity maps, velocity fields, velocity dispersions, global/radial profiles, position-velocity diagrams and overlays of H i/1.4 GHz continuum maps on the optical images. We also present H i properties such as total flux (S H i ), H i mass (M H i ), linewidths (W 20 and W 50 ), velocity (V H i ), deficiency (def H i ), and size (D eff H i and D iso H i ), and describe the H i morphology and kinematics of individual galaxies in detail. The survey has revealed details of H i features that were never seen before. In this paper, we briefly discuss differences in typical H i morphology for galaxies in regions of different galaxy densities. We confirm that galaxies near the cluster core (d 87 0.5 Mpc) have H i disks that are smaller compared to their stellar disks (D H i /D 25 < 0.5). Most of these galaxies in the core also show gas displaced from the disk, which is either currently being stripped or falling back after a stripping event. At intermediate distances (d 87 ∼ 1 Mpc) from the center, we find a remarkable number of galaxies with long one-sided H i tails pointing away from M87. In a previous letter, we argue that these galaxies are recent arrivals, falling into the Virgo core for the first time. In the outskirts, we find many gas-rich galaxies, with gas disks extending far beyond their optical disks. Interestingly, we also find some galaxies with H i disks that are smaller compared to their stellar disks at large clustercentric distances.

The Next Generation Virgo Cluster Survey (NGVS) is a program that uses the 1 deg 2 MegaCam instrument on the Canada-France-Hawaii Telescope to carry out a comprehensive optical imaging survey of the Virgo cluster, from its core to its virial radius-covering a total area of 104 deg 2-in the u * griz bandpasses. Thanks to a dedicated data acquisition strategy and processing pipeline, the NGVS reaches a point-source depth of g ≈ 25.9 mag (10σ) and a surface brightness limit of μ g ∼ 29 mag arcsec −2 (2σ above the mean sky level), thus superseding all previous optical studies of this benchmark galaxy cluster. In this paper, we give an overview of the technical aspects of the survey, such as areal coverage, field placement, choice of filters, limiting magnitudes, observing strategies, data processing and calibration pipelines, survey timeline, and data products. We also describe the primary scientific topics of the NGVS, which include: the galaxy luminosity and mass functions; the color-magnitude relation; galaxy scaling relations; compact stellar systems; galactic nuclei; the extragalactic distance scale; the large-scale environment of the cluster and its relationship to the Local Supercluster; diffuse light and the intracluster medium; galaxy interactions and evolutionary processes; and extragalactic star clusters. In addition, we describe a number of ancillary programs dealing with "foreground" and "background" science topics, including the study of highinclination trans-Neptunian objects; the structure of the Galactic halo in the direction of the Virgo Overdensity and Sagittarius Stream; the measurement of cosmic shear, galaxy-galaxy, and cluster lensing; and the identification of distant galaxy clusters, and strong-lensing events.

We present a multi-wavelength study of NGC 4330, a highly-inclined spiral galaxy in the Virgo Cluster which is a clear example of strong, ongoing ICM-ISM ram pressure stripping. The HI has been removed from well within the undisturbed old stellar disk, to 50% -65% of R 25 . Multi-wavelength data (WIYN a BVR-Hα, VLA b 21-cm HI and radio continuum, and GALEX c NUV and FUV) reveal several one-sided extraplanar features likely caused by ram pressure at an intermediate disk-wind angle. At the leading edge of the interaction, the Hα and dust extinction curve sharply out of the disk in a remarkable and distinctive "upturn" feature that may be generally useful as a diagnostic indicator of active ram pressure. On the trailing side, the ISM is stretched out in a long tail which contains 10% of the galaxy's total HI emission, 6 -9% of its NUV-FUV emission, but only 2% of the Hα. The centroid of the HI tail is downwind of the UV/Hα tail, suggesting that the ICM wind has shifted most of the ISM downwind over the course of the past 10 -300 Myr. Along the major axis, the disk is highly asymmetric in the UV, but more symmetric in Hα and HI, also implying recent changes in the distributions of gas and star formation. The UV-optical colors indicate very different star formation histories for the leading and trailing sides of the galaxy. On the leading side, a strong gradient in the UV-optical colors of the gas-stripped disk suggests that it has taken 200-400 Myr to strip the gas from a radius of >8 to 5 kpc, but on the trailing side there is no age gradient. All our data suggest a scenario in which NGC 4330 is falling into cluster center for first time and has experienced a significant increase in ram pressure over the last 200-400 Myr.

We present optical, HI and radio continuum observations of the highly inclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for ram-pressure stripping and dense cloud ablation. VLA HI and radio continuum maps show a truncated gas disk and emission to the northwest of the main disk emission. In particular, the radio continuum emission is asymmetrically extended to the north and skewed to the west. The Hα image shows numerous HII complexes along the southern edge of the gas disk, possibly indicating star formation triggered by the ICM pressure. BVR images at 0. ′′ 5 resolution obtained with the WIYN Tip-Tilt Imager show a remarkable dust lane morphology: at half the optical radius, the dust lane of the galaxy curves up and out of the disk, matching the HI morphology. Large dust plumes extend upward for ∼ 1.5 kpc from luminous young star clusters at the SE edge of the truncated gas disk. These star clusters are very blue, indicating very little dust reddening, which suggests dust blown away by an ICM wind at the leading edge of the interaction. To the south of the main ridge of interstellar material, where the galaxy is relatively clean of gas and dust, we have discovered 1 kpc long linear dust filaments with a position angle that matches the extraplanar radio continuum tail; we interpret this angle as the projected ICM wind direction. One of the observed dust filaments has an HII region at its head. We interpret these dust filaments as large, dense clouds which were initially left behind as the lowdensity ISM is stripped, but are then ablated by the ICM wind. These results provide striking new evidence on the fate of molecular clouds in stripped cluster galaxies.