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.

We describe the various Hα morphologies of Virgo Cluster and isolated spiral galaxies, and associate the Hα morphologies with the types of environmental interactions which have altered the cluster galaxies. The spatial distributions of Hα and R-band emission are used to divide the star formation morphologies of the 52 Virgo Cluster spirals into several categories: normal (37%), anemic (6%), enhanced (6%), and (spatially) truncated (52%). Truncated galaxies are further subdivided based on their inner star formation rates into truncated/normal (37%), truncated/compact (6%), truncated/anemic (8%), and truncated/enhanced (2%). The fraction of anemic galaxies is relatively small (6-13%) in both environments, suggesting that starvation is not a major factor in the reduced star formation rates of Virgo spirals. The majority of Virgo spiral galaxies have their Hα disks truncated (52%), whereas truncated Hα disks are rarer in isolated galaxies (12%). Most of the Hα-truncated galaxies have relatively undisturbed stellar disks and normal-to-slightly enhanced inner disk star formation rates, suggesting that ICM-ISM stripping is the main mechanism causing the reduced star formation rates of Virgo spirals. Several of the truncated galaxies are peculiar, with enhanced central star formation rates, disturbed stellar disks, and bar-like distributions of luminous HII complexes inside the central 1 kpc, but no star formation beyond, suggesting recent tidal interactions or minor mergers have also influenced their morphology. Two highly-inclined Hα-truncated spirals have numerous extraplanar HII regions, and are likely in an active phase of ICM-ISM stripping. Several spirals have one-sided Hα enhancements at the outer edge of their truncated Hα disks, suggesting modest and local enhancements in their star formation rates due to ICM-ISM interactions. Low-velocity tidal interactions and perhaps outer cluster HI accretion seem to be the triggers for enhanced global star formation in four Virgo galaxies. These results indicate that most Virgo spiral galaxies experience ICM-ISM stripping, many experience significant tidal effects, and many experience both.